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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="sect1" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="xfunc-c"></a>32.9.C-Language Functions</h2></div></div></div>
<a name="id706106"></a><p>    User-defined functions can be written in C (or a language that can
    be made compatible with C, such as C++).  Such functions are
    compiled into dynamically loadable objects (also called shared
    libraries) and are loaded by the server on demand.  The dynamic
    loading feature is what distinguishes &#8220;<span class="quote">C language</span>&#8221; functions
    from &#8220;<span class="quote">internal</span>&#8221; functions [mdash ] the actual coding conventions
    are essentially the same for both.  (Hence, the standard internal
    function library is a rich source of coding examples for user-defined
    C functions.)
   </p>
<p>    Two different calling conventions are currently used for C functions.
    The newer &#8220;<span class="quote">version 1</span>&#8221; calling convention is indicated by writing
    a <code class="literal">PG_FUNCTION_INFO_V1()</code> macro call for the function,
    as illustrated below.  Lack of such a macro indicates an old-style
    (&#8220;<span class="quote">version 0</span>&#8221;) function.  The language name specified in <code class="command">CREATE FUNCTION</code>
    is <code class="literal">C</code> in either case.  Old-style functions are now deprecated
    because of portability problems and lack of functionality, but they
    are still supported for compatibility reasons.
   </p>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="xfunc-c-dynload"></a>32.9.1.Dynamic Loading</h3></div></div></div>
<a name="id706185"></a><p>    The first time a user-defined function in a particular
    loadable object file is called in a session,
    the dynamic loader loads that object file into memory so that the
    function can be called.  The <code class="command">CREATE FUNCTION</code>
    for a user-defined C function must therefore specify two pieces of
    information for the function: the name of the loadable
    object file, and the C name (link symbol) of the specific function to call
    within that object file.  If the C name is not explicitly specified then
    it is assumed to be the same as the SQL function name.
   </p>
<p>    The following algorithm is used to locate the shared object file
    based on the name given in the <code class="command">CREATE FUNCTION</code>
    command:

    </p>
<div class="orderedlist"><ol type="1">
<li><p>       If the name is an absolute path, the given file is loaded.
      </p></li>
<li><p>       If the name starts with the string <code class="literal">$libdir</code>,
       that part is replaced by the <span class="productname">PostgreSQL</span> package
        library directory
       name, which is determined at build time.<a name="id706257"></a>
      </p></li>
<li><p>       If the name does not contain a directory part, the file is
       searched for in the path specified by the configuration variable
       <a href="runtime-config-client.html#guc-dynamic-library-path">dynamic_library_path</a>.<a name="id706280"></a>
      </p></li>
<li><p>       Otherwise (the file was not found in the path, or it contains a
       non-absolute directory part), the dynamic loader will try to
       take the name as given, which will most likely fail.  (It is
       unreliable to depend on the current working directory.)
      </p></li>
</ol></div>
<p>

    If this sequence does not work, the platform-specific shared
    library file name extension (often <code class="filename">.so</code>) is
    appended to the given name and this sequence is tried again.  If
    that fails as well, the load will fail.
   </p>
<p>    The user ID the <span class="productname">PostgreSQL</span> server runs
    as must be able to traverse the path to the file you intend to
    load.  Making the file or a higher-level directory not readable
    and/or not executable by the <span class="systemitem">postgres</span>
    user is a common mistake.
   </p>
<p>    In any case, the file name that is given in the
    <code class="command">CREATE FUNCTION</code> command is recorded literally
    in the system catalogs, so if the file needs to be loaded again
    the same procedure is applied.
   </p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>     <span class="productname">PostgreSQL</span> will not compile a C function
     automatically.  The object file must be compiled before it is referenced
     in a <code class="command">CREATE
     FUNCTION</code> command.  See <a href="xfunc-c.html#dfunc" title="32.9.6.Compiling and Linking Dynamically-Loaded Functions">Section32.9.6, &#8220;Compiling and Linking Dynamically-Loaded Functions&#8221;</a> for additional
     information.
    </p>
</div>
<p>    After it is used for the first time, a dynamically loaded object
    file is retained in memory.  Future calls in the same session to
    the function(s) in that file will only incur the small overhead of
    a symbol table lookup.  If you need to force a reload of an object
    file, for example after recompiling it, use the <a href="sql-load.html">LOAD</a> command or begin a
    fresh session.
   </p>
<p>    It is recommended to locate shared libraries either relative to
    <code class="literal">$libdir</code> or through the dynamic library path.
    This simplifies version upgrades if the new installation is at a
    different location.  The actual directory that
    <code class="literal">$libdir</code> stands for can be found out with the
    command <code class="literal">pg_config --pkglibdir</code>.
   </p>
<p>    Before <span class="productname">PostgreSQL</span> release 7.2, only
    exact absolute paths to object files could be specified in
    <code class="command">CREATE FUNCTION</code>.  This approach is now deprecated
    since it makes the function definition unnecessarily unportable.
    It's best to specify just the shared library name with no path nor
    extension, and let the search mechanism provide that information
    instead.
   </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="xfunc-c-basetype"></a>32.9.2.Base Types in C-Language Functions</h3></div></div></div>
<a name="id706448"></a><p>     To know how to write C-language functions, you need to know how
     <span class="productname">PostgreSQL</span> internally represents base
     data types and how they can be passed to and from functions.
     Internally, <span class="productname">PostgreSQL</span> regards a base
     type as a &#8220;<span class="quote">blob of memory</span>&#8221;.  The user-defined
     functions that you define over a type in turn define the way that
     <span class="productname">PostgreSQL</span> can operate on it.  That
     is, <span class="productname">PostgreSQL</span> will only store and
     retrieve the data from disk and use your user-defined functions
     to input, process, and output the data.
    </p>
<p>     Base types can have one of three internal formats:

     </p>
<div class="itemizedlist"><ul type="disc">
<li><p>        pass by value, fixed-length
       </p></li>
<li><p>        pass by reference, fixed-length
       </p></li>
<li><p>        pass by reference, variable-length
       </p></li>
</ul></div>
<p>
    </p>
<p>     By-value  types  can  only be 1, 2, or 4 bytes in length
     (also 8 bytes, if <code class="literal">sizeof(Datum)</code> is 8 on your machine).
     You should be careful 
     to define your types such that  they  will  be  the  same  
     size (in bytes) on all architectures.  For example, the 
     <code class="literal">long</code> type is dangerous because  it  
     is 4 bytes on some machines and 8 bytes on others, whereas 
     <code class="type">int</code>  type  is  4  bytes  on  most  
     Unix machines.  A reasonable implementation of  
     the  <code class="type">int4</code>  type  on  Unix
     machines might be:
     
</p>
<pre class="programlisting">/* 4-byte integer, passed by value */
typedef int int4;</pre>
<p>
    </p>
<p>     On  the  other hand, fixed-length types of any size may
     be passed by-reference.  For example, here is a  sample
     implementation of a <span class="productname">PostgreSQL</span> type:
     
</p>
<pre class="programlisting">/* 16-byte structure, passed by reference */
typedef struct
{
    double  x, y;
} Point;</pre>
<p>

     Only  pointers  to  such types can be used when passing
     them in and out of <span class="productname">PostgreSQL</span> functions.
     To return a value of such a type, allocate the right amount of
     memory with <code class="literal">palloc</code>, fill in the allocated memory,
     and return a pointer to it.  (Also, if you just want to return the
     same value as one of your input arguments that's of the same data type,
     you can skip the extra <code class="literal">palloc</code> and just return the
     pointer to the input value.)
    </p>
<p>     Finally, all variable-length types must also be  passed
     by  reference.   All  variable-length  types must begin
     with a length field of exactly 4 bytes, and all data to
     be  stored within that type must be located in the memory 
     immediately  following  that  length  field.   The
     length field contains the total length of the structure,
     that is,  it  includes  the  size  of  the  length  field
     itself.
    </p>
<div class="warning" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Warning</h3>
<p>      <span class="emphasis"><em>Never</em></span> modify the contents of a pass-by-reference input
      value.  If you do so you are likely to corrupt on-disk data, since
      the pointer you are given may well point directly into a disk buffer.
      The sole exception to this rule is explained in
      <a href="xaggr.html" title="32.10.User-Defined Aggregates">Section32.10, &#8220;User-Defined Aggregates&#8221;</a>.
     </p>
</div>
<p>     As an example, we can define the type <code class="type">text</code> as
     follows:

</p>
<pre class="programlisting">typedef struct {
    int4 length;
    char data[1];
} text;</pre>
<p>

     Obviously,  the  data  field declared here is not long enough to hold
     all possible strings.  Since it's impossible to declare a variable-size
     structure in <acronym class="acronym">C</acronym>, we rely on the knowledge that the
     <acronym class="acronym">C</acronym> compiler won't range-check array subscripts.  We
     just allocate the necessary amount of space and then access the array as
     if it were declared the right length.  (This is a common trick, which
     you can read about in many textbooks about C.)
    </p>
<p>     When manipulating 
     variable-length types, we must  be  careful  to  allocate  
     the  correct amount  of memory and set the length field correctly.
     For example, if we wanted to  store  40  bytes  in  a <code class="structname">text</code>
     structure, we might use a code fragment like this:

</p>
<pre class="programlisting">#include "postgres.h"
...
char buffer[40]; /* our source data */
...
text *destination = (text *) palloc(VARHDRSZ + 40);
destination-&gt;length = VARHDRSZ + 40;
memcpy(destination-&gt;data, buffer, 40);
...</pre>
<p>

     <code class="literal">VARHDRSZ</code> is the same as <code class="literal">sizeof(int4)</code>, but
     it's considered good style to use the macro <code class="literal">VARHDRSZ</code>
     to refer to the size of the overhead for a variable-length type.
    </p>
<p>     <a href="xfunc-c.html#xfunc-c-type-table" title="Table32.1.Equivalent C Types for Built-In SQL Types">Table32.1, &#8220;Equivalent C Types for Built-In SQL Types&#8221;</a> specifies which C type
     corresponds to which SQL type when writing a C-language function
     that uses a built-in type of <span class="productname">PostgreSQL</span>.
     The &#8220;<span class="quote">Defined In</span>&#8221; column gives the header file that
     needs to be included to get the type definition.  (The actual
     definition may be in a different file that is included by the
     listed file.  It is recommended that users stick to the defined
     interface.)  Note that you should always include
     <code class="filename">postgres.h</code> first in any source file, because
     it declares a number of things that you will need anyway.
    </p>
<div class="table">
<a name="xfunc-c-type-table"></a><p class="title"><b>Table32.1.Equivalent C Types for Built-In SQL Types</b></p>
<div class="table-contents"><table summary="Equivalent C Types for Built-In SQL Types" border="1">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>          SQL Type
         </th>
<th>          C Type
         </th>
<th>          Defined In
         </th>
</tr></thead>
<tbody>
<tr>
<td><code class="type">abstime</code></td>
<td><code class="type">AbsoluteTime</code></td>
<td><code class="filename">utils/nabstime.h</code></td>
</tr>
<tr>
<td><code class="type">boolean</code></td>
<td><code class="type">bool</code></td>
<td>
<code class="filename">postgres.h</code> (maybe compiler built-in)</td>
</tr>
<tr>
<td><code class="type">box</code></td>
<td><code class="type">BOX*</code></td>
<td><code class="filename">utils/geo_decls.h</code></td>
</tr>
<tr>
<td><code class="type">bytea</code></td>
<td><code class="type">bytea*</code></td>
<td><code class="filename">postgres.h</code></td>
</tr>
<tr>
<td><code class="type">"char"</code></td>
<td><code class="type">char</code></td>
<td>(compiler built-in)</td>
</tr>
<tr>
<td><code class="type">character</code></td>
<td><code class="type">BpChar*</code></td>
<td><code class="filename">postgres.h</code></td>
</tr>
<tr>
<td><code class="type">cid</code></td>
<td><code class="type">CommandId</code></td>
<td><code class="filename">postgres.h</code></td>
</tr>
<tr>
<td><code class="type">date</code></td>
<td><code class="type">DateADT</code></td>
<td><code class="filename">utils/date.h</code></td>
</tr>
<tr>
<td>
<code class="type">smallint</code> (<code class="type">int2</code>)</td>
<td>
<code class="type">int2</code> or <code class="type">int16</code>
</td>
<td><code class="filename">postgres.h</code></td>
</tr>
<tr>
<td><code class="type">int2vector</code></td>
<td><code class="type">int2vector*</code></td>
<td><code class="filename">postgres.h</code></td>
</tr>
<tr>
<td>
<code class="type">integer</code> (<code class="type">int4</code>)</td>
<td>
<code class="type">int4</code> or <code class="type">int32</code>
</td>
<td><code class="filename">postgres.h</code></td>
</tr>
<tr>
<td>
<code class="type">real</code> (<code class="type">float4</code>)</td>
<td><code class="type">float4*</code></td>
<td><code class="filename">postgres.h</code></td>
</tr>
<tr>
<td>
<code class="type">double precision</code> (<code class="type">float8</code>)</td>
<td><code class="type">float8*</code></td>
<td><code class="filename">postgres.h</code></td>
</tr>
<tr>
<td><code class="type">interval</code></td>
<td><code class="type">Interval*</code></td>
<td><code class="filename">utils/timestamp.h</code></td>
</tr>
<tr>
<td><code class="type">lseg</code></td>
<td><code class="type">LSEG*</code></td>
<td><code class="filename">utils/geo_decls.h</code></td>
</tr>
<tr>
<td><code class="type">name</code></td>
<td><code class="type">Name</code></td>
<td><code class="filename">postgres.h</code></td>
</tr>
<tr>
<td><code class="type">oid</code></td>
<td><code class="type">Oid</code></td>
<td><code class="filename">postgres.h</code></td>
</tr>
<tr>
<td><code class="type">oidvector</code></td>
<td><code class="type">oidvector*</code></td>
<td><code class="filename">postgres.h</code></td>
</tr>
<tr>
<td><code class="type">path</code></td>
<td><code class="type">PATH*</code></td>
<td><code class="filename">utils/geo_decls.h</code></td>
</tr>
<tr>
<td><code class="type">point</code></td>
<td><code class="type">POINT*</code></td>
<td><code class="filename">utils/geo_decls.h</code></td>
</tr>
<tr>
<td><code class="type">regproc</code></td>
<td><code class="type">regproc</code></td>
<td><code class="filename">postgres.h</code></td>
</tr>
<tr>
<td><code class="type">reltime</code></td>
<td><code class="type">RelativeTime</code></td>
<td><code class="filename">utils/nabstime.h</code></td>
</tr>
<tr>
<td><code class="type">text</code></td>
<td><code class="type">text*</code></td>
<td><code class="filename">postgres.h</code></td>
</tr>
<tr>
<td><code class="type">tid</code></td>
<td><code class="type">ItemPointer</code></td>
<td><code class="filename">storage/itemptr.h</code></td>
</tr>
<tr>
<td><code class="type">time</code></td>
<td><code class="type">TimeADT</code></td>
<td><code class="filename">utils/date.h</code></td>
</tr>
<tr>
<td><code class="type">time with time zone</code></td>
<td><code class="type">TimeTzADT</code></td>
<td><code class="filename">utils/date.h</code></td>
</tr>
<tr>
<td><code class="type">timestamp</code></td>
<td><code class="type">Timestamp*</code></td>
<td><code class="filename">utils/timestamp.h</code></td>
</tr>
<tr>
<td><code class="type">tinterval</code></td>
<td><code class="type">TimeInterval</code></td>
<td><code class="filename">utils/nabstime.h</code></td>
</tr>
<tr>
<td><code class="type">varchar</code></td>
<td><code class="type">VarChar*</code></td>
<td><code class="filename">postgres.h</code></td>
</tr>
<tr>
<td><code class="type">xid</code></td>
<td><code class="type">TransactionId</code></td>
<td><code class="filename">postgres.h</code></td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><p>     Now that we've gone over all of the possible structures
     for base types, we can show some examples of real functions.
    </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id707272"></a>32.9.3.Calling Conventions Version 0 for C-Language Functions</h3></div></div></div>
<p>     We present the &#8220;<span class="quote">old style</span>&#8221; calling convention first [mdash ] although
     this approach is now deprecated, it's easier to get a handle on
     initially.  In the version-0 method, the arguments and result
     of the C function are just declared in normal C style, but being
     careful to use the C representation of each SQL data type as shown
     above.
    </p>
<p>     Here are some examples:

</p>
<pre class="programlisting">#include "postgres.h"
#include &lt;string.h&gt;

/* by value */
         
int
add_one(int arg)
{
    return arg + 1;
}

/* by reference, fixed length */

float8 *
add_one_float8(float8 *arg)
{
    float8    *result = (float8 *) palloc(sizeof(float8));

    *result = *arg + 1.0;
       
    return result;
}

Point *
makepoint(Point *pointx, Point *pointy)
{
    Point     *new_point = (Point *) palloc(sizeof(Point));

    new_point-&gt;x = pointx-&gt;x;
    new_point-&gt;y = pointy-&gt;y;
       
    return new_point;
}

/* by reference, variable length */

text *
copytext(text *t)
{
    /*
     * VARSIZE is the total size of the struct in bytes.
     */
    text *new_t = (text *) palloc(VARSIZE(t));
    VARATT_SIZEP(new_t) = VARSIZE(t);
    /*
     * VARDATA is a pointer to the data region of the struct.
     */
    memcpy((void *) VARDATA(new_t), /* destination */
           (void *) VARDATA(t),     /* source */
           VARSIZE(t)-VARHDRSZ);    /* how many bytes */
    return new_t;
}

text *
concat_text(text *arg1, text *arg2)
{
    int32 new_text_size = VARSIZE(arg1) + VARSIZE(arg2) - VARHDRSZ;
    text *new_text = (text *) palloc(new_text_size);

    VARATT_SIZEP(new_text) = new_text_size;
    memcpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1)-VARHDRSZ);
    memcpy(VARDATA(new_text) + (VARSIZE(arg1)-VARHDRSZ),
           VARDATA(arg2), VARSIZE(arg2)-VARHDRSZ);
    return new_text;
}</pre>
<p>
    </p>
<p>     Supposing that the above code has been prepared in file
     <code class="filename">funcs.c</code> and compiled into a shared object,
     we could define the functions to <span class="productname">PostgreSQL</span>
     with commands like this:
     
</p>
<pre class="programlisting">CREATE FUNCTION add_one(integer) RETURNS integer
     AS '<em class="replaceable"><code>DIRECTORY</code></em>/funcs', 'add_one'
     LANGUAGE C STRICT;

-- note overloading of SQL function name "add_one"
CREATE FUNCTION add_one(double precision) RETURNS double precision
     AS '<em class="replaceable"><code>DIRECTORY</code></em>/funcs', 'add_one_float8'
     LANGUAGE C STRICT;

CREATE FUNCTION makepoint(point, point) RETURNS point
     AS '<em class="replaceable"><code>DIRECTORY</code></em>/funcs', 'makepoint'
     LANGUAGE C STRICT;
                         
CREATE FUNCTION copytext(text) RETURNS text
     AS '<em class="replaceable"><code>DIRECTORY</code></em>/funcs', 'copytext'
     LANGUAGE C STRICT;

CREATE FUNCTION concat_text(text, text) RETURNS text
     AS '<em class="replaceable"><code>DIRECTORY</code></em>/funcs', 'concat_text'
     LANGUAGE C STRICT;</pre>
<p>
    </p>
<p>     Here, <em class="replaceable"><code>DIRECTORY</code></em> stands for the
     directory of the shared library file (for instance the
     <span class="productname">PostgreSQL</span> tutorial directory, which
     contains the code for the examples used in this section).
     (Better style would be to use just <code class="literal">'funcs'</code> in the
     <code class="literal">AS</code> clause, after having added
     <em class="replaceable"><code>DIRECTORY</code></em> to the search path.  In any
     case, we may omit the system-specific extension for a shared
     library, commonly <code class="literal">.so</code> or
     <code class="literal">.sl</code>.)
    </p>
<p>     Notice that we have specified the functions as &#8220;<span class="quote">strict</span>&#8221;,
     meaning that
     the system should automatically assume a null result if any input
     value is null.  By doing this, we avoid having to check for null inputs
     in the function code.  Without this, we'd have to check for null values
     explicitly, by checking for a null pointer for each
     pass-by-reference argument.  (For pass-by-value arguments, we don't
     even have a way to check!)
    </p>
<p>     Although this calling convention is simple to use,
     it is not very portable; on some architectures there are problems
     with passing data types that are smaller than <code class="type">int</code> this way.  Also, there is
     no simple way to return a null result, nor to cope with null arguments
     in any way other than making the function strict.  The version-1
     convention, presented next, overcomes these objections.
    </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id707493"></a>32.9.4.Calling Conventions Version 1 for C-Language Functions</h3></div></div></div>
<p>     The version-1 calling convention relies on macros to suppress most
     of the complexity of passing arguments and results.  The C declaration
     of a version-1 function is always
</p>
<pre class="programlisting">Datum funcname(PG_FUNCTION_ARGS)</pre>
<p>
     In addition, the macro call
</p>
<pre class="programlisting">PG_FUNCTION_INFO_V1(funcname);</pre>
<p>
     must appear in the same source file.  (Conventionally. it's
     written just before the function itself.)  This macro call is not
     needed for <code class="literal">internal</code>-language functions, since
     <span class="productname">PostgreSQL</span> assumes that all internal functions
     use the version-1 convention.  It is, however, required for
     dynamically-loaded functions.
    </p>
<p>     In a version-1 function, each actual argument is fetched using a
     <code class="function">PG_GETARG_<em class="replaceable"><code>xxx</code></em>()</code>
     macro that corresponds to the argument's data type, and the
     result is returned using a
     <code class="function">PG_RETURN_<em class="replaceable"><code>xxx</code></em>()</code>
     macro for the return type.
     <code class="function">PG_GETARG_<em class="replaceable"><code>xxx</code></em>()</code>
     takes as its argument the number of the function argument to
     fetch, where the count starts at 0.
     <code class="function">PG_RETURN_<em class="replaceable"><code>xxx</code></em>()</code>
     takes as its argument the actual value to return.
    </p>
<p>     Here we show the same functions as above, coded in version-1 style:

</p>
<pre class="programlisting">#include "postgres.h"
#include &lt;string.h&gt;
#include "fmgr.h"

/* by value */

PG_FUNCTION_INFO_V1(add_one);
         
Datum
add_one(PG_FUNCTION_ARGS)
{
    int32   arg = PG_GETARG_INT32(0);

    PG_RETURN_INT32(arg + 1);
}

/* by reference, fixed length */

PG_FUNCTION_INFO_V1(add_one_float8);

Datum
add_one_float8(PG_FUNCTION_ARGS)
{
    /* The macros for FLOAT8 hide its pass-by-reference nature. */
    float8   arg = PG_GETARG_FLOAT8(0);

    PG_RETURN_FLOAT8(arg + 1.0);
}

PG_FUNCTION_INFO_V1(makepoint);

Datum
makepoint(PG_FUNCTION_ARGS)
{
    /* Here, the pass-by-reference nature of Point is not hidden. */
    Point     *pointx = PG_GETARG_POINT_P(0);
    Point     *pointy = PG_GETARG_POINT_P(1);
    Point     *new_point = (Point *) palloc(sizeof(Point));

    new_point-&gt;x = pointx-&gt;x;
    new_point-&gt;y = pointy-&gt;y;
       
    PG_RETURN_POINT_P(new_point);
}

/* by reference, variable length */

PG_FUNCTION_INFO_V1(copytext);

Datum
copytext(PG_FUNCTION_ARGS)
{
    text     *t = PG_GETARG_TEXT_P(0);
    /*
     * VARSIZE is the total size of the struct in bytes.
     */
    text     *new_t = (text *) palloc(VARSIZE(t));
    VARATT_SIZEP(new_t) = VARSIZE(t);
    /*
     * VARDATA is a pointer to the data region of the struct.
     */
    memcpy((void *) VARDATA(new_t), /* destination */
           (void *) VARDATA(t),     /* source */
           VARSIZE(t)-VARHDRSZ);    /* how many bytes */
    PG_RETURN_TEXT_P(new_t);
}

PG_FUNCTION_INFO_V1(concat_text);

Datum
concat_text(PG_FUNCTION_ARGS)
{
    text  *arg1 = PG_GETARG_TEXT_P(0);
    text  *arg2 = PG_GETARG_TEXT_P(1);
    int32 new_text_size = VARSIZE(arg1) + VARSIZE(arg2) - VARHDRSZ;
    text *new_text = (text *) palloc(new_text_size);

    VARATT_SIZEP(new_text) = new_text_size;
    memcpy(VARDATA(new_text), VARDATA(arg1), VARSIZE(arg1)-VARHDRSZ);
    memcpy(VARDATA(new_text) + (VARSIZE(arg1)-VARHDRSZ),
           VARDATA(arg2), VARSIZE(arg2)-VARHDRSZ);
    PG_RETURN_TEXT_P(new_text);
}</pre>
<p>
    </p>
<p>     The <code class="command">CREATE FUNCTION</code> commands are the same as
     for the version-0 equivalents.
    </p>
<p>     At first glance, the version-1 coding conventions may appear to
     be just pointless obscurantism.  They do, however, offer a number
     of improvements, because the macros can hide unnecessary detail.
     An example is that in coding <code class="function">add_one_float8</code>, we no longer need to
     be aware that <code class="type">float8</code> is a pass-by-reference type.  Another
     example is that the <code class="literal">GETARG</code> macros for variable-length types allow
     for more efficient fetching of &#8220;<span class="quote">toasted</span>&#8221; (compressed or
     out-of-line) values.
    </p>
<p>     One big improvement in version-1 functions is better handling of null
     inputs and results.  The macro <code class="function">PG_ARGISNULL(<em class="replaceable"><code>n</code></em>)</code>
     allows a function to test whether each input is null.  (Of course, doing
     this is only necessary in functions not declared &#8220;<span class="quote">strict</span>&#8221;.)
     As with the
     <code class="function">PG_GETARG_<em class="replaceable"><code>xxx</code></em>()</code> macros,
     the input arguments are counted beginning at zero.  Note that one
     should refrain from executing
     <code class="function">PG_GETARG_<em class="replaceable"><code>xxx</code></em>()</code> until
     one has verified that the argument isn't null.
     To return a null result, execute <code class="function">PG_RETURN_NULL()</code>;
     this works in both strict and nonstrict functions.
    </p>
<p>     Other options provided in the new-style interface are two
     variants of the
     <code class="function">PG_GETARG_<em class="replaceable"><code>xxx</code></em>()</code>
     macros. The first of these,
     <code class="function">PG_GETARG_<em class="replaceable"><code>xxx</code></em>_COPY()</code>,
     guarantees to return a copy of the specified argument that is
     safe for writing into. (The normal macros will sometimes return a
     pointer to a value that is physically stored in a table, which
     must not be written to. Using the
     <code class="function">PG_GETARG_<em class="replaceable"><code>xxx</code></em>_COPY()</code>
     macros guarantees a writable result.)
    The second variant consists of the
    <code class="function">PG_GETARG_<em class="replaceable"><code>xxx</code></em>_SLICE()</code>
    macros which take three arguments. The first is the number of the
    function argument (as above). The second and third are the offset and
    length of the segment to be returned. Offsets are counted from
    zero, and a negative length requests that the remainder of the
    value be returned. These macros provide more efficient access to
    parts of large values in the case where they have storage type
    &#8220;<span class="quote">external</span>&#8221;. (The storage type of a column can be specified using
    <code class="literal">ALTER TABLE <em class="replaceable"><code>tablename</code></em> ALTER
    COLUMN <em class="replaceable"><code>colname</code></em> SET STORAGE
    <em class="replaceable"><code>storagetype</code></em></code>. <em class="replaceable"><code>storagetype</code></em> is one of
    <code class="literal">plain</code>, <code class="literal">external</code>, <code class="literal">extended</code>,
     or <code class="literal">main</code>.)
    </p>
<p>     Finally, the version-1 function call conventions make it possible
     to return set results (<a href="xfunc-c.html#xfunc-c-return-set" title="32.9.10.Returning Sets from C-Language Functions">Section32.9.10, &#8220;Returning Sets from C-Language Functions&#8221;</a>) and
     implement trigger functions (<a href="triggers.html" title="Chapter33.Triggers">Chapter33, <i>Triggers</i></a>) and
     procedural-language call handlers (<a href="plhandler.html" title="Chapter46.Writing A Procedural Language Handler">Chapter46, <i>Writing A Procedural Language Handler</i></a>).  Version-1 code is also more
     portable than version-0, because it does not break restrictions
     on function call protocol in the C standard.  For more details
     see <code class="filename">src/backend/utils/fmgr/README</code> in the
     source distribution.
    </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id707920"></a>32.9.5.Writing Code</h3></div></div></div>
<p>     Before we turn to the more advanced topics, we should discuss
     some coding rules for <span class="productname">PostgreSQL</span>
     C-language functions.  While it may be possible to load functions
     written in languages other than C into
     <span class="productname">PostgreSQL</span>, this is usually difficult
     (when it is possible at all) because other languages, such as
     C++, FORTRAN, or Pascal often do not follow the same calling
     convention as C.  That is, other languages do not pass argument
     and return values between functions in the same way.  For this
     reason, we will assume that your C-language functions are
     actually written in C.
    </p>
<p>     The basic rules for writing and building C functions are as follows:

     </p>
<div class="itemizedlist"><ul type="disc">
<li><p>        Use <code class="literal">pg_config
        --includedir-server</code><a name="id707965"></a>
        to find out where the <span class="productname">PostgreSQL</span> server header
        files are installed on your system (or the system that your
        users will be running on).  This option is new with
        <span class="productname">PostgreSQL</span> 7.2.  For
        <span class="productname">PostgreSQL</span> 7.1 you should use the option
        <code class="option">--includedir</code>.  (<code class="command">pg_config</code>
        will exit with a non-zero status if it encounters an unknown
        option.)  For releases prior to 7.1 you will have to guess,
        but since that was before the current calling conventions were
        introduced, it is unlikely that you want to support those
        releases.
       </p></li>
<li><p>        When allocating memory, use the
        <span class="productname">PostgreSQL</span> functions
        <code class="function">palloc</code><a name="id708032"></a> and <code class="function">pfree</code><a name="id708045"></a>
        instead of the corresponding C library functions
        <code class="function">malloc</code> and <code class="function">free</code>.
        The memory allocated by <code class="function">palloc</code> will be
        freed automatically at the end of each transaction, preventing
        memory leaks.
       </p></li>
<li><p>        Always zero the bytes of your structures using
        <code class="function">memset</code>.  Without this, it's difficult to
        support hash indexes or hash joins, as you must pick out only
        the significant bits of your data structure to compute a hash.
        Even if you initialize all fields of your structure, there may be
        alignment padding (holes in the structure) that may contain
        garbage values.
       </p></li>
<li><p>        Most of the internal <span class="productname">PostgreSQL</span>
        types are declared in <code class="filename">postgres.h</code>, while
        the function manager interfaces
        (<code class="symbol">PG_FUNCTION_ARGS</code>, etc.)  are in
        <code class="filename">fmgr.h</code>, so you will need to include at
        least these two files.  For portability reasons it's best to
        include <code class="filename">postgres.h</code> <span class="emphasis"><em>first</em></span>,
        before any other system or user header files.  Including
        <code class="filename">postgres.h</code> will also include
        <code class="filename">elog.h</code> and <code class="filename">palloc.h</code>
        for you.
       </p></li>
<li><p>        Symbol names defined within object files must not conflict
        with each other or with symbols defined in the
        <span class="productname">PostgreSQL</span> server executable.  You
        will have to rename your functions or variables if you get
        error messages to this effect.
       </p></li>
<li><p>        Compiling and linking your code so that it can be dynamically
        loaded into <span class="productname">PostgreSQL</span> always
        requires special flags.  See <a href="xfunc-c.html#dfunc" title="32.9.6.Compiling and Linking Dynamically-Loaded Functions">Section32.9.6, &#8220;Compiling and Linking Dynamically-Loaded Functions&#8221;</a> for a
        detailed explanation of how to do it for your particular
        operating system.
       </p></li>
</ul></div>
<p>
    </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="dfunc"></a>32.9.6.Compiling and Linking Dynamically-Loaded Functions</h3></div></div></div>
<p>  Before you are able to use your
  <span class="productname">PostgreSQL</span> extension functions written in
  C, they must be compiled and linked in a special way to produce a
  file that can be dynamically loaded by the server.  To be precise, a
  <em class="firstterm">shared library</em> needs to be
  created.<a name="id708224"></a>

 </p>
<p>  For information beyond what is contained in this section
  you should read the documentation of your
  operating system, in particular the manual pages for the C compiler,
  <code class="command">cc</code>, and the link editor, <code class="command">ld</code>.
  In addition, the <span class="productname">PostgreSQL</span> source code
  contains several working examples in the
  <code class="filename">contrib</code> directory.  If you rely on these
  examples you will make your modules dependent on the availability
  of the <span class="productname">PostgreSQL</span> source code, however.
 </p>
<p>  Creating shared libraries is generally analogous to linking
  executables: first the source files are compiled into object files,
  then the object files are linked together.  The object files need to
  be created as <em class="firstterm">position-independent code</em>
  (<acronym class="acronym">PIC</acronym>),<a name="id708288"></a> which
  conceptually means that they can be placed at an arbitrary location
  in memory when they are loaded by the executable.  (Object files
  intended for executables are usually not compiled that way.)  The
  command to link a shared library contains special flags to
  distinguish it from linking an executable (at least in theory
  [mdash ] on some systems the practice is much uglier).
 </p>
<p>  In the following examples we assume that your source code is in a
  file <code class="filename">foo.c</code> and we will create a shared library
  <code class="filename">foo.so</code>.  The intermediate object file will be
  called <code class="filename">foo.o</code> unless otherwise noted.  A shared
  library can contain more than one object file, but we only use one
  here.
 </p>
<div class="variablelist"><dl>
<dt><span class="term"><span class="systemitem">BSD/OS</span></span></dt>
<dd>
<p>      The compiler flag to create <acronym class="acronym">PIC</acronym> is
      <code class="option">-fpic</code>.  The linker flag to create shared
      libraries is <code class="option">-shared</code>.
</p>
<pre class="programlisting">gcc -fpic -c foo.c
ld -shared -o foo.so foo.o</pre>
<p>
      This is applicable as of version 4.0 of
      <span class="systemitem">BSD/OS</span>.
     </p>
</dd>
<dt><span class="term"><span class="systemitem">FreeBSD</span></span></dt>
<dd>
<p>      The compiler flag to create <acronym class="acronym">PIC</acronym> is
      <code class="option">-fpic</code>.  To create shared libraries the compiler
      flag is <code class="option">-shared</code>.
</p>
<pre class="programlisting">gcc -fpic -c foo.c
gcc -shared -o foo.so foo.o</pre>
<p>
      This is applicable as of version 3.0 of
      <span class="systemitem">FreeBSD</span>.
     </p>
</dd>
<dt><span class="term"><span class="systemitem">HP-UX</span></span></dt>
<dd>
<p>      The compiler flag of the system compiler to create
      <acronym class="acronym">PIC</acronym> is <code class="option">+z</code>.  When using
      <span class="application">GCC</span> it's <code class="option">-fpic</code>. The
      linker flag for shared libraries is <code class="option">-b</code>.  So
</p>
<pre class="programlisting">cc +z -c foo.c</pre>
<p>
      or
</p>
<pre class="programlisting">gcc -fpic -c foo.c</pre>
<p>
      and then
</p>
<pre class="programlisting">ld -b -o foo.sl foo.o</pre>
<p>
      <span class="systemitem">HP-UX</span> uses the extension
      <code class="filename">.sl</code> for shared libraries, unlike most other
      systems.
     </p>
</dd>
<dt><span class="term"><span class="systemitem">IRIX</span></span></dt>
<dd>
<p>      <acronym class="acronym">PIC</acronym> is the default, no special compiler
      options are necessary.  The linker option to produce shared
      libraries is <code class="option">-shared</code>.
</p>
<pre class="programlisting">cc -c foo.c
ld -shared -o foo.so foo.o</pre>
<p>
     </p>
</dd>
<dt><span class="term"><span class="systemitem">Linux</span></span></dt>
<dd>
<p>      The compiler flag to create <acronym class="acronym">PIC</acronym> is
      <code class="option">-fpic</code>.  On some platforms in some situations
      <code class="option">-fPIC</code> must be used if <code class="option">-fpic</code>
      does not work.  Refer to the GCC manual for more information.
      The compiler flag to create a shared library is
      <code class="option">-shared</code>.  A complete example looks like this:
</p>
<pre class="programlisting">cc -fpic -c foo.c
cc -shared -o foo.so foo.o</pre>
<p>
     </p>
</dd>
<dt><span class="term"><span class="systemitem">MacOS X</span></span></dt>
<dd>
<p>      Here is an example.  It assumes the developer tools are installed.
</p>
<pre class="programlisting">cc -c foo.c 
cc -bundle -flat_namespace -undefined suppress -o foo.so foo.o</pre>
<p>
     </p>
</dd>
<dt><span class="term"><span class="systemitem">NetBSD</span></span></dt>
<dd>
<p>      The compiler flag to create <acronym class="acronym">PIC</acronym> is
      <code class="option">-fpic</code>.  For <acronym class="acronym">ELF</acronym> systems, the
      compiler with the flag <code class="option">-shared</code> is used to link
      shared libraries.  On the older non-ELF systems, <code class="literal">ld
      -Bshareable</code> is used.
</p>
<pre class="programlisting">gcc -fpic -c foo.c
gcc -shared -o foo.so foo.o</pre>
<p>
     </p>
</dd>
<dt><span class="term"><span class="systemitem">OpenBSD</span></span></dt>
<dd>
<p>      The compiler flag to create <acronym class="acronym">PIC</acronym> is
      <code class="option">-fpic</code>.  <code class="literal">ld -Bshareable</code> is
      used to link shared libraries.
</p>
<pre class="programlisting">gcc -fpic -c foo.c
ld -Bshareable -o foo.so foo.o</pre>
<p>
     </p>
</dd>
<dt><span class="term"><span class="systemitem">Solaris</span></span></dt>
<dd>
<p>      The compiler flag to create <acronym class="acronym">PIC</acronym> is
      <code class="option">-KPIC</code> with the Sun compiler and
      <code class="option">-fpic</code> with <span class="application">GCC</span>.  To
      link shared libraries, the compiler option is
      <code class="option">-G</code> with either compiler or alternatively
      <code class="option">-shared</code> with <span class="application">GCC</span>.
</p>
<pre class="programlisting">cc -KPIC -c foo.c
cc -G -o foo.so foo.o</pre>
<p>
      or
</p>
<pre class="programlisting">gcc -fpic -c foo.c
gcc -G -o foo.so foo.o</pre>
<p>
     </p>
</dd>
<dt><span class="term"><span class="systemitem">Tru64 UNIX</span></span></dt>
<dd>
<p>      <acronym class="acronym">PIC</acronym> is the default, so the compilation command
      is the usual one.  <code class="command">ld</code> with special options is
      used to do the linking:
</p>
<pre class="programlisting">cc -c foo.c
ld -shared -expect_unresolved '*' -o foo.so foo.o</pre>
<p>
      The same procedure is used with GCC instead of the system
      compiler; no special options are required.
     </p>
</dd>
<dt><span class="term"><span class="systemitem">UnixWare</span></span></dt>
<dd>
<p>      The compiler flag to create <acronym class="acronym">PIC</acronym> is <code class="option">-K
      PIC</code> with the SCO compiler and <code class="option">-fpic</code>
      with <span class="productname">GCC</span>.  To link shared libraries,
      the compiler option is <code class="option">-G</code> with the SCO compiler
      and <code class="option">-shared</code> with
      <span class="productname">GCC</span>.
</p>
<pre class="programlisting">cc -K PIC -c foo.c
cc -G -o foo.so foo.o</pre>
<p>
      or
</p>
<pre class="programlisting">gcc -fpic -c foo.c
gcc -shared -o foo.so foo.o</pre>
<p>
     </p>
</dd>
</dl></div>
<div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Tip</h3>
<p>   If this is too complicated for you, you should consider using   
   <a href="http://www.gnu.org/software/libtool/" target="_top">   <span class="productname">GNU Libtool</span></a>,
   which hides the platform differences behind a uniform interface.
  </p>
</div>
<p>  The resulting shared library file can then be loaded into
  <span class="productname">PostgreSQL</span>.  When specifying the file name
  to the <code class="command">CREATE FUNCTION</code> command, one must give it
  the name of the shared library file, not the intermediate object file.
  Note that the system's standard shared-library extension (usually
  <code class="literal">.so</code> or <code class="literal">.sl</code>) can be omitted from
  the <code class="command">CREATE FUNCTION</code> command, and normally should
  be omitted for best portability.
 </p>
<p>  Refer back to <a href="xfunc-c.html#xfunc-c-dynload" title="32.9.1.Dynamic Loading">Section32.9.1, &#8220;Dynamic Loading&#8221;</a> about where the
  server expects to find the shared library files.
 </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="xfunc-c-pgxs"></a>32.9.7.Extension Building Infrastructure</h3></div></div></div>
<a name="id709032"></a><p>    If you are thinking about distributing your
    <span class="productname">PostgreSQL</span> extension modules, setting up a
    portable build system for them can be fairly difficult.  Therefore
    the <span class="productname">PostgreSQL</span> installation provides a build
    infrastructure for extensions, called <acronym class="acronym">PGXS</acronym>, so
    that simple extension modules can be built simply against an
    already installed server.  Note that this infrastructure is not
    intended to be a universal build system framework that can be used
    to build all software interfacing to <span class="productname">PostgreSQL</span>;
    it simply automates common build rules for simple server extension
    modules.  For more complicated packages, you need to write your
    own build system.
   </p>
<p>    To use the infrastructure for your extension, you must write a
    simple makefile.  In that makefile, you need to set some variables
    and finally include the global <acronym class="acronym">PGXS</acronym> makefile.
    Here is an example that builds an extension module named
    <code class="literal">isbn_issn</code> consisting of a shared library, an
    SQL script, and a documentation text file:
</p>
<pre class="programlisting">MODULES = isbn_issn
DATA_built = isbn_issn.sql
DOCS = README.isbn_issn

PGXS := $(shell pg_config --pgxs)
include $(PGXS)</pre>
<p>
    The last two lines should always be the same.  Earlier in the
    file, you assign variables or add custom
    <span class="application">make</span> rules.
   </p>
<p>    The following variables can be set:

    </p>
<div class="variablelist"><dl>
<dt><span class="term"><code class="varname">MODULES</code></span></dt>
<dd><p>        list of shared objects to be built from source file with same
        stem (do not include suffix in this list)
       </p></dd>
<dt><span class="term"><code class="varname">DATA</code></span></dt>
<dd><p>        random files to install into <code class="literal"><em class="replaceable"><code>prefix</code></em>/share/contrib</code>
       </p></dd>
<dt><span class="term"><code class="varname">DATA_built</code></span></dt>
<dd><p>        random files to install into
        <code class="literal"><em class="replaceable"><code>prefix</code></em>/share/contrib</code>,
        which need to be built first
       </p></dd>
<dt><span class="term"><code class="varname">DOCS</code></span></dt>
<dd><p>        random files to install under
        <code class="literal"><em class="replaceable"><code>prefix</code></em>/doc/contrib</code>
       </p></dd>
<dt><span class="term"><code class="varname">SCRIPTS</code></span></dt>
<dd><p>        script files (not binaries) to install into
        <code class="literal"><em class="replaceable"><code>prefix</code></em>/bin</code>
       </p></dd>
<dt><span class="term"><code class="varname">SCRIPTS_built</code></span></dt>
<dd><p>        script files (not binaries) to install into
        <code class="literal"><em class="replaceable"><code>prefix</code></em>/bin</code>,
        which need to be built first
       </p></dd>
<dt><span class="term"><code class="varname">REGRESS</code></span></dt>
<dd><p>        list of regression test cases (without suffix)
       </p></dd>
</dl></div>
<p>

    or at most one of these two:

    </p>
<div class="variablelist"><dl>
<dt><span class="term"><code class="varname">PROGRAM</code></span></dt>
<dd><p>        a binary program to build (list objects files in <code class="varname">OBJS</code>)
       </p></dd>
<dt><span class="term"><code class="varname">MODULE_big</code></span></dt>
<dd><p>        a shared object to build (list object files in <code class="varname">OBJS</code>)
       </p></dd>
</dl></div>
<p>

    The following can also be set:

    </p>
<div class="variablelist"><dl>
<dt><span class="term"><code class="varname">EXTRA_CLEAN</code></span></dt>
<dd><p>        extra files to remove in <code class="literal">make clean</code>
       </p></dd>
<dt><span class="term"><code class="varname">PG_CPPFLAGS</code></span></dt>
<dd><p>        will be added to <code class="varname">CPPFLAGS</code>
       </p></dd>
<dt><span class="term"><code class="varname">PG_LIBS</code></span></dt>
<dd><p>        will be added to <code class="varname">PROGRAM</code> link line
       </p></dd>
<dt><span class="term"><code class="varname">SHLIB_LINK</code></span></dt>
<dd><p>        will be added to <code class="varname">MODULE_big</code> link line
       </p></dd>
</dl></div>
<p>
   </p>
<p>    Put this makefile as <code class="literal">Makefile</code> in the directory
    which holds your extension. Then you can do
    <code class="literal">make</code> to compile, and later <code class="literal">make
    install</code> to install your module.  The extension is
    compiled and installed for the
    <span class="productname">PostgreSQL</span> installation that
    corresponds to the first <code class="command">pg_config</code> command
    found in your path.
   </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id709371"></a>32.9.8.Composite-Type Arguments in C-Language Functions</h3></div></div></div>
<p>     Composite types do not have a fixed layout like C structures.
     Instances of a composite type may contain null fields.  In
     addition, composite types that are part of an inheritance
     hierarchy may have different fields than other members of the
     same inheritance hierarchy.  Therefore,
     <span class="productname">PostgreSQL</span> provides a function
     interface for accessing fields of composite types from C.
    </p>
<p>     Suppose we want to write a function to answer the query

</p>
<pre class="programlisting">SELECT name, c_overpaid(emp, 1500) AS overpaid
    FROM emp
    WHERE name = 'Bill' OR name = 'Sam';</pre>
<p>

     Using call conventions version 0, we can define
     <code class="function">c_overpaid</code> as:
     
</p>
<pre class="programlisting">#include "postgres.h"
#include "executor/executor.h"  /* for GetAttributeByName() */

bool
c_overpaid(HeapTupleHeader t, /* the current row of emp */
           int32 limit)
{
    bool isnull;
    int32 salary;

    salary = DatumGetInt32(GetAttributeByName(t, "salary", &amp;isnull));
    if (isnull)
        return false;
    return salary &gt; limit;
}</pre>
<p>

     In version-1 coding, the above would look like this:

</p>
<pre class="programlisting">#include "postgres.h"
#include "executor/executor.h"  /* for GetAttributeByName() */

PG_FUNCTION_INFO_V1(c_overpaid);

Datum
c_overpaid(PG_FUNCTION_ARGS)
{
    HeapTupleHeader  t = PG_GETARG_HEAPTUPLEHEADER(0);
    int32            limit = PG_GETARG_INT32(1);
    bool isnull;
    Datum salary;

    salary = GetAttributeByName(t, "salary", &amp;isnull);
    if (isnull)
        PG_RETURN_BOOL(false);
    /* Alternatively, we might prefer to do PG_RETURN_NULL() for null salary. */

    PG_RETURN_BOOL(DatumGetInt32(salary) &gt; limit);
}</pre>
<p>
    </p>
<p>     <code class="function">GetAttributeByName</code> is the 
     <span class="productname">PostgreSQL</span> system function that
     returns attributes out of the specified row.  It has
     three arguments: the argument of type <code class="type">HeapTupleHeader</code> passed
     into 
     the  function, the name of the desired attribute, and a
     return parameter that tells whether  the  attribute
     is  null.   <code class="function">GetAttributeByName</code> returns a <code class="type">Datum</code>
     value that you can convert to the proper data type by using the
     appropriate <code class="function">DatumGet<em class="replaceable"><code>XXX</code></em>()</code>
     macro.  Note that the return value is meaningless if the null flag is
     set; always check the null flag before trying to do anything with the
     result.
    </p>
<p>     There is also <code class="function">GetAttributeByNum</code>, which selects
     the target attribute by column number instead of name.
    </p>
<p>     The following command declares the function
     <code class="function">c_overpaid</code> in SQL:

</p>
<pre class="programlisting">CREATE FUNCTION c_overpaid(emp, integer) RETURNS boolean
    AS '<em class="replaceable"><code>DIRECTORY</code></em>/funcs', 'c_overpaid'
    LANGUAGE C STRICT;</pre>
<p>

     Notice we have used <code class="literal">STRICT</code> so that we did not have to
     check whether the input arguments were NULL.
    </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id709538"></a>32.9.9.Returning Rows (Composite Types) from C-Language Functions</h3></div></div></div>
<p>     To return a row or composite-type value from a C-language
     function, you can use a special API that provides macros and
     functions to hide most of the complexity of building composite
     data types.  To use this API, the source file must include:
</p>
<pre class="programlisting">#include "funcapi.h"</pre>
<p>
    </p>
<p>     There are two ways you can build a composite data value (henceforth
     a &#8220;<span class="quote">tuple</span>&#8221;): you can build it from an array of Datum values,
     or from an array of C strings that can be passed to the input
     conversion functions of the tuple's column data types.  In either
     case, you first need to obtain or construct a <code class="structname">TupleDesc</code>
     descriptor for the tuple structure.  When working with Datums, you
     pass the <code class="structname">TupleDesc</code> to <code class="function">BlessTupleDesc</code>,
     and then call <code class="function">heap_form_tuple</code> for each row.  When working
     with C strings, you pass the <code class="structname">TupleDesc</code> to
     <code class="function">TupleDescGetAttInMetadata</code>, and then call
     <code class="function">BuildTupleFromCStrings</code> for each row.  In the case of a
     function returning a set of tuples, the setup steps can all be done
     once during the first call of the function.
    </p>
<p>     Several helper functions are available for setting up the needed
     <code class="structname">TupleDesc</code>.  The recommended way to do this in most
     functions returning composite values is to call
</p>
<pre class="programlisting">TypeFuncClass get_call_result_type(FunctionCallInfo fcinfo,
                                   Oid *resultTypeId,
                                   TupleDesc *resultTupleDesc)</pre>
<p>
     passing the same <code class="literal">fcinfo</code> struct passed to the calling function
     itself.  (This of course requires that you use the version-1
     calling conventions.)  <code class="varname">resultTypeId</code> can be specified
     as <code class="literal">NULL</code> or as the address of a local variable to receive the
     function's result type OID.  <code class="varname">resultTupleDesc</code> should be the
     address of a local <code class="structname">TupleDesc</code> variable.  Check that the
     result is <code class="literal">TYPEFUNC_COMPOSITE</code>; if so,
     <code class="varname">resultTupleDesc</code> has been filled with the needed
     <code class="structname">TupleDesc</code>.  (If it is not, you can report an error along
     the lines of &#8220;<span class="quote">function returning record called in context that
     cannot accept type record</span>&#8221;.)
    </p>
<div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Tip</h3>
<p>      <code class="function">get_call_result_type</code> can resolve the actual type of a
      polymorphic function result; so it is useful in functions that return
      scalar polymorphic results, not only functions that return composites.
      The <code class="varname">resultTypeId</code> output is primarily useful for functions
      returning polymorphic scalars.
     </p>
</div>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>      <code class="function">get_call_result_type</code> has a sibling
      <code class="function">get_expr_result_type</code>, which can be used to resolve the
      expected output type for a function call represented by an expression
      tree.  This can be used when trying to determine the result type from
      outside the function itself.  There is also
      <code class="function">get_func_result_type</code>, which can be used when only the
      function's OID is available.  However these functions are not able
      to deal with functions declared to return <code class="structname">record</code>, and
      <code class="function">get_func_result_type</code> cannot resolve polymorphic types,
      so you should preferentially use <code class="function">get_call_result_type</code>.
     </p>
</div>
<p>     Older, now-deprecated functions for obtaining
     <code class="structname">TupleDesc</code>s are
</p>
<pre class="programlisting">TupleDesc RelationNameGetTupleDesc(const char *relname)</pre>
<p>
     to get a <code class="structname">TupleDesc</code> for the row type of a named relation,
     and
</p>
<pre class="programlisting">TupleDesc TypeGetTupleDesc(Oid typeoid, List *colaliases)</pre>
<p>
     to get a <code class="structname">TupleDesc</code> based on a type OID. This can
     be used to get a <code class="structname">TupleDesc</code> for a base or
     composite type.  It will not work for a function that returns
     <code class="structname">record</code>, however, and it cannot resolve polymorphic
     types.
    </p>
<p>     Once you have a <code class="structname">TupleDesc</code>, call
</p>
<pre class="programlisting">TupleDesc BlessTupleDesc(TupleDesc tupdesc)</pre>
<p>
     if you plan to work with Datums, or
</p>
<pre class="programlisting">AttInMetadata *TupleDescGetAttInMetadata(TupleDesc tupdesc)</pre>
<p>
     if you plan to work with C strings.  If you are writing a function
     returning set, you can save the results of these functions in the
     <code class="structname">FuncCallContext</code> structure [mdash ] use the
     <code class="structfield">tuple_desc</code> or <code class="structfield">attinmeta</code> field
     respectively.
    </p>
<p>     When working with Datums, use
</p>
<pre class="programlisting">HeapTuple heap_form_tuple(TupleDesc tupdesc, Datum *values, bool *isnull)</pre>
<p>
     to build a <code class="structname">HeapTuple</code> given user data in Datum form.
    </p>
<p>     When working with C strings, use
</p>
<pre class="programlisting">HeapTuple BuildTupleFromCStrings(AttInMetadata *attinmeta, char **values)</pre>
<p>
     to build a <code class="structname">HeapTuple</code> given user data
     in C string form.  <code class="literal">values</code> is an array of C strings,
     one for each attribute of the return row. Each C string should be in
     the form expected by the input function of the attribute data
     type. In order to return a null value for one of the attributes,
     the corresponding pointer in the <em class="parameter"><code>values</code></em> array
     should be set to <code class="symbol">NULL</code>.  This function will need to
     be called again for each row you return.
    </p>
<p>     Once you have built a tuple to return from your function, it
     must be converted into a <code class="type">Datum</code>. Use
</p>
<pre class="programlisting">HeapTupleGetDatum(HeapTuple tuple)</pre>
<p>
     to convert a <code class="structname">HeapTuple</code> into a valid Datum.  This
     <code class="type">Datum</code> can be returned directly if you intend to return
     just a single row, or it can be used as the current return value
     in a set-returning function.
    </p>
<p>     An example appears in the next section.
    </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="xfunc-c-return-set"></a>32.9.10.Returning Sets from C-Language Functions</h3></div></div></div>
<p>     There is also a special API that provides support for returning
     sets (multiple rows) from a C-language function.  A set-returning
     function must follow the version-1 calling conventions.  Also,
     source files must include <code class="filename">funcapi.h</code>, as
     above.
    </p>
<p>     A set-returning function (<acronym class="acronym">SRF</acronym>) is called
     once for each item it returns.  The <acronym class="acronym">SRF</acronym> must
     therefore save enough state to remember what it was doing and
     return the next item on each call.
     The structure <code class="structname">FuncCallContext</code> is provided to help
     control this process.  Within a function, <code class="literal">fcinfo-&gt;flinfo-&gt;fn_extra</code>
     is used to hold a pointer to <code class="structname">FuncCallContext</code>
     across calls.
</p>
<pre class="programlisting">typedef struct
{
    /*
     * Number of times we've been called before
     * 
     * call_cntr is initialized to 0 for you by SRF_FIRSTCALL_INIT(), and
     * incremented for you every time SRF_RETURN_NEXT() is called.
     */
    uint32 call_cntr;

    /*
     * OPTIONAL maximum number of calls
     *
     * max_calls is here for convenience only and setting it is optional.
     * If not set, you must provide alternative means to know when the
     * function is done.
     */
    uint32 max_calls;

    /*
     * OPTIONAL pointer to result slot
     * 
     * This is obsolete and only present for backwards compatibility, viz,
     * user-defined SRFs that use the deprecated TupleDescGetSlot().
     */
    TupleTableSlot *slot;

    /*
     * OPTIONAL pointer to miscellaneous user-provided context information
     * 
     * user_fctx is for use as a pointer to your own data to retain
     * arbitrary context information between calls of your function.
     */
    void *user_fctx;

    /*
     * OPTIONAL pointer to struct containing attribute type input metadata
     * 
     * attinmeta is for use when returning tuples (i.e., composite data types)
     * and is not used when returning base data types. It is only needed
     * if you intend to use BuildTupleFromCStrings() to create the return
     * tuple.
     */
    AttInMetadata *attinmeta;

    /*
     * memory context used for structures that must live for multiple calls
     *
     * multi_call_memory_ctx is set by SRF_FIRSTCALL_INIT() for you, and used
     * by SRF_RETURN_DONE() for cleanup. It is the most appropriate memory
     * context for any memory that is to be reused across multiple calls
     * of the SRF.
     */
    MemoryContext multi_call_memory_ctx;

    /*
     * OPTIONAL pointer to struct containing tuple description
     *
     * tuple_desc is for use when returning tuples (i.e. composite data types)
     * and is only needed if you are going to build the tuples with
     * heap_form_tuple() rather than with BuildTupleFromCStrings().  Note that
     * the TupleDesc pointer stored here should usually have been run through
     * BlessTupleDesc() first.
     */
    TupleDesc tuple_desc;

} FuncCallContext;</pre>
<p>
    </p>
<p>     An <acronym class="acronym">SRF</acronym> uses several functions and macros that
     automatically manipulate the <code class="structname">FuncCallContext</code>
     structure (and expect to find it via <code class="literal">fn_extra</code>).  Use
</p>
<pre class="programlisting">SRF_IS_FIRSTCALL()</pre>
<p>
     to determine if your function is being called for the first or a
     subsequent time. On the first call (only) use
</p>
<pre class="programlisting">SRF_FIRSTCALL_INIT()</pre>
<p>
     to initialize the <code class="structname">FuncCallContext</code>. On every function call,
     including the first, use
</p>
<pre class="programlisting">SRF_PERCALL_SETUP()</pre>
<p>
     to properly set up for using the <code class="structname">FuncCallContext</code>
     and clearing any previously returned data left over from the
     previous pass.
    </p>
<p>     If your function has data to return, use
</p>
<pre class="programlisting">SRF_RETURN_NEXT(funcctx, result)</pre>
<p>
     to return it to the caller.  (<code class="literal">result</code> must be of type
     <code class="type">Datum</code>, either a single value or a tuple prepared as
     described above.)  Finally, when your function is finished
     returning data, use
</p>
<pre class="programlisting">SRF_RETURN_DONE(funcctx)</pre>
<p>
     to clean up and end the <acronym class="acronym">SRF</acronym>.
    </p>
<p>     The memory context that is current when the <acronym class="acronym">SRF</acronym> is called is
     a transient context that will be cleared between calls.  This means
     that you do not need to call <code class="function">pfree</code> on everything
     you allocated using <code class="function">palloc</code>; it will go away anyway.  However, if you want to allocate
     any data structures to live across calls, you need to put them somewhere
     else.  The memory context referenced by
     <code class="structfield">multi_call_memory_ctx</code> is a suitable location for any
     data that needs to survive until the <acronym class="acronym">SRF</acronym> is finished running.  In most
     cases, this means that you should switch into
     <code class="structfield">multi_call_memory_ctx</code> while doing the first-call setup.
    </p>
<p>     A complete pseudo-code example looks like the following:
</p>
<pre class="programlisting">Datum
my_set_returning_function(PG_FUNCTION_ARGS)
{
    FuncCallContext  *funcctx;
    Datum             result;
    MemoryContext     oldcontext;
    <em class="replaceable"><code>further declarations as needed</code></em>

    if (SRF_IS_FIRSTCALL())
    {
        funcctx = SRF_FIRSTCALL_INIT();
        oldcontext = MemoryContextSwitchTo(funcctx-&gt;multi_call_memory_ctx);
        /* One-time setup code appears here: */
        <em class="replaceable"><code>user code</code></em>
        <em class="replaceable"><code>if returning composite</code></em>
            <em class="replaceable"><code>build TupleDesc, and perhaps AttInMetadata</code></em>
        <em class="replaceable"><code>endif returning composite</code></em>
        <em class="replaceable"><code>user code</code></em>
        MemoryContextSwitchTo(oldcontext);
    }

    /* Each-time setup code appears here: */
    <em class="replaceable"><code>user code</code></em>
    funcctx = SRF_PERCALL_SETUP();
    <em class="replaceable"><code>user code</code></em>

    /* this is just one way we might test whether we are done: */
    if (funcctx-&gt;call_cntr &lt; funcctx-&gt;max_calls)
    {
        /* Here we want to return another item: */
        <em class="replaceable"><code>user code</code></em>
        <em class="replaceable"><code>obtain result Datum</code></em>
        SRF_RETURN_NEXT(funcctx, result);
    }
    else
    {
        /* Here we are done returning items and just need to clean up: */
        <em class="replaceable"><code>user code</code></em>
        SRF_RETURN_DONE(funcctx);
    }
}</pre>
<p>
    </p>
<p>     A complete example of a simple <acronym class="acronym">SRF</acronym> returning a composite type
     looks like:
</p>
<pre class="programlisting">PG_FUNCTION_INFO_V1(retcomposite);

Datum
retcomposite(PG_FUNCTION_ARGS)
{
    FuncCallContext     *funcctx;
    int                  call_cntr;
    int                  max_calls;
    TupleDesc            tupdesc;
    AttInMetadata       *attinmeta;

     /* stuff done only on the first call of the function */
     if (SRF_IS_FIRSTCALL())
     {
        MemoryContext   oldcontext;

        /* create a function context for cross-call persistence */
        funcctx = SRF_FIRSTCALL_INIT();

        /* switch to memory context appropriate for multiple function calls */
        oldcontext = MemoryContextSwitchTo(funcctx-&gt;multi_call_memory_ctx);

        /* total number of tuples to be returned */
        funcctx-&gt;max_calls = PG_GETARG_UINT32(0);

        /* Build a tuple descriptor for our result type */
        if (get_call_result_type(fcinfo, NULL, &amp;tupdesc) != TYPEFUNC_COMPOSITE)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("function returning record called in context "
                            "that cannot accept type record")));

        /*
         * generate attribute metadata needed later to produce tuples from raw
         * C strings
         */
        attinmeta = TupleDescGetAttInMetadata(tupdesc);
        funcctx-&gt;attinmeta = attinmeta;

        MemoryContextSwitchTo(oldcontext);
    }

    /* stuff done on every call of the function */
    funcctx = SRF_PERCALL_SETUP();

    call_cntr = funcctx-&gt;call_cntr;
    max_calls = funcctx-&gt;max_calls;
    attinmeta = funcctx-&gt;attinmeta;
 
    if (call_cntr &lt; max_calls)    /* do when there is more left to send */
    {
        char       **values;
        HeapTuple    tuple;
        Datum        result;

        /*
         * Prepare a values array for building the returned tuple.
         * This should be an array of C strings which will
         * be processed later by the type input functions.
         */
        values = (char **) palloc(3 * sizeof(char *));
        values[0] = (char *) palloc(16 * sizeof(char));
        values[1] = (char *) palloc(16 * sizeof(char));
        values[2] = (char *) palloc(16 * sizeof(char));

        snprintf(values[0], 16, "%d", 1 * PG_GETARG_INT32(1));
        snprintf(values[1], 16, "%d", 2 * PG_GETARG_INT32(1));
        snprintf(values[2], 16, "%d", 3 * PG_GETARG_INT32(1));

        /* build a tuple */
        tuple = BuildTupleFromCStrings(attinmeta, values);

        /* make the tuple into a datum */
        result = HeapTupleGetDatum(tuple);

        /* clean up (this is not really necessary) */
        pfree(values[0]);
        pfree(values[1]);
        pfree(values[2]);
        pfree(values);

        SRF_RETURN_NEXT(funcctx, result);
    }
    else    /* do when there is no more left */
    {
        SRF_RETURN_DONE(funcctx);
    }
}</pre>
<p>

     One way to declare this function in SQL is:
</p>
<pre class="programlisting">CREATE TYPE __retcomposite AS (f1 integer, f2 integer, f3 integer);

CREATE OR REPLACE FUNCTION retcomposite(integer, integer)
    RETURNS SETOF __retcomposite
    AS '<em class="replaceable"><code>filename</code></em>', 'retcomposite'
    LANGUAGE C IMMUTABLE STRICT;</pre>
<p>
     A different way is to use OUT parameters:
</p>
<pre class="programlisting">CREATE OR REPLACE FUNCTION retcomposite(IN integer, IN integer,
    OUT f1 integer, OUT f2 integer, OUT f3 integer)
    RETURNS SETOF record
    AS '<em class="replaceable"><code>filename</code></em>', 'retcomposite'
    LANGUAGE C IMMUTABLE STRICT;</pre>
<p>
     Notice that in this method the output type of the function is formally
     an anonymous <code class="structname">record</code> type.
    </p>
<p>     The directory <code class="filename">contrib/tablefunc</code> in the source
     distribution contains more examples of set-returning functions.
    </p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id710389"></a>32.9.11.Polymorphic Arguments and Return Types</h3></div></div></div>
<p>     C-language functions may be declared to accept and
     return the polymorphic types
     <code class="type">anyelement</code> and <code class="type">anyarray</code>.
     See <a href="extend-type-system.html#extend-types-polymorphic" title="32.2.5.Polymorphic Types">Section32.2.5, &#8220;Polymorphic Types&#8221;</a> for a more detailed explanation
     of polymorphic functions. When function arguments or return types
     are defined as polymorphic types, the function author cannot know
     in advance what data type it will be called with, or
     need to return. There are two routines provided in <code class="filename">fmgr.h</code>
     to allow a version-1 C function to discover the actual data types
     of its arguments and the type it is expected to return. The routines are
     called <code class="literal">get_fn_expr_rettype(FmgrInfo *flinfo)</code> and
     <code class="literal">get_fn_expr_argtype(FmgrInfo *flinfo, int argnum)</code>.
     They return the result or argument type OID, or <code class="symbol">InvalidOid</code> if the
     information is not available.
     The structure <code class="literal">flinfo</code> is normally accessed as
     <code class="literal">fcinfo-&gt;flinfo</code>. The parameter <code class="literal">argnum</code>
     is zero based.  <code class="function">get_call_result_type</code> can also be used
     as an alternative to <code class="function">get_fn_expr_rettype</code>.
    </p>
<p>     For example, suppose we want to write a function to accept a single
     element of any type, and return a one-dimensional array of that type:

</p>
<pre class="programlisting">PG_FUNCTION_INFO_V1(make_array);
Datum
make_array(PG_FUNCTION_ARGS)
{
    ArrayType  *result;
    Oid         element_type = get_fn_expr_argtype(fcinfo-&gt;flinfo, 0);
    Datum       element;
    int16       typlen;
    bool        typbyval;
    char        typalign;
    int         ndims;
    int         dims[MAXDIM];
    int         lbs[MAXDIM];

    if (!OidIsValid(element_type))
        elog(ERROR, "could not determine data type of input");

    /* get the provided element */
    element = PG_GETARG_DATUM(0);

    /* we have one dimension */
    ndims = 1;
    /* and one element */
    dims[0] = 1;
    /* and lower bound is 1 */
    lbs[0] = 1;

    /* get required info about the element type */
    get_typlenbyvalalign(element_type, &amp;typlen, &amp;typbyval, &amp;typalign);

    /* now build the array */
    result = construct_md_array(&amp;element, ndims, dims, lbs,
                                element_type, typlen, typbyval, typalign);

    PG_RETURN_ARRAYTYPE_P(result);
}</pre>
<p>
    </p>
<p>     The following command declares the function
     <code class="function">make_array</code> in SQL:

</p>
<pre class="programlisting">CREATE FUNCTION make_array(anyelement) RETURNS anyarray
    AS '<em class="replaceable"><code>DIRECTORY</code></em>/funcs', 'make_array'
    LANGUAGE C STRICT;</pre>
<p>

     Note the use of <code class="literal">STRICT</code>; this is essential
     since the code is not bothering to test for a null input.
    </p>
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