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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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glib: Portability and Utility
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<div class="CHAPTER">
<h1>
<a name="CHA-GLIB">glib: Portability and Utility</a>
</h1>
<div class="TOC">
<dl>
<dt>
<b>Table of Contents</b>
</dt>
<dt>
<a href="cha-glib.html#Z24">Basics</a>
</dt>
<dt>
<a href="z29.html">Data Structures</a>
</dt>
<dt>
<a href="z35.html">Other Features</a>
</dt>
</dl>
</div>
<p>
glib is a C portability and utility library for UNIX-like
systems and Windows. This chapter covers some of the most
commonly-used library features in GTK+ and Gnome
applications. glib is simple, and the concepts are
familiar; so we'll move quickly. For more complete coverage
of glib, see <tt class="FILENAME">glib.h</tt> or the free
glib reference manual that comes with the library. (By the
way: don't be afraid of using the glib, GTK+, or Gnome
header files; they are very clean and easy to read, and are
handy as a quick reference. For that matter, don't be
afraid to look at the source code, if you have very
specific questions about the implementation.)
</p>
<p>
glib's various facilities are intended to have a consistent
interface; the coding style is semi-object-oriented, and
identifiers are prefixed with "g" to create a kind of
namespace.
</p>
<p>
glib has a single header file, <tt class="FILENAME">
glib.h</tt>.
</p>
<div class="SECT1">
<h1 class="SECT1">
<a name="Z24">Basics</a>
</h1>
<p>
glib provides substitutes for many standard and
commonly-used C language constructs. This section
describes glib's fundamental type definitions, macros,
memory allocation routines, and string utility functions.
</p>
<div class="SECT2">
<h2 class="SECT2">
<a name="Z25">Type Definitions</a>
</h2>
<p>
Rather than using C's standard types (<span class=
"STRUCTNAME">int</span>, <span class="STRUCTNAME">
long</span>, etc.) glib defines its own. These serve a
variety of purposes. For example, <span class=
"STRUCTNAME">gint32</span> is guaranteed to be 32 bits
wide, something no standard C type can ensure. <span
class="STRUCTNAME">guint</span> is simply easier to
type than <span class="STRUCTNAME">unsigned</span>. A
few of the typedefs exist only for consistency; for
example, <span class="STRUCTNAME">gchar</span> is
always equivalent to the standard <span class=
"STRUCTNAME">char</span>.
</p>
<p>
The following primitive types are defined by glib:
</p>
<ul>
<li>
<p>
<span class="STRUCTNAME">gint8</span>, <span class=
"STRUCTNAME">guint8</span>, <span class=
"STRUCTNAME">gint16</span>, <span class=
"STRUCTNAME">guint16</span>, <span class=
"STRUCTNAME">gint32</span>, <span class=
"STRUCTNAME">guint32</span>, <span class=
"STRUCTNAME">gint64</span>, <span class=
"STRUCTNAME">guint64</span>---these give you
integers of a guaranteed size. Not all platforms
provide 64-bit integers; if a platform has them,
glib will define <tt class="FUNCTION">
G_HAVE_GINT64</tt>. (If it isn't obvious, the <span
class="STRUCTNAME">guint</span> types are unsigned,
the <span class="STRUCTNAME">gint</span> types are
signed.)
</p>
</li>
<li>
<p>
<span class="STRUCTNAME">gboolean</span> is useful
to make your code more readable, since C has no
<span class="STRUCTNAME">bool</span> type.
</p>
</li>
<li>
<p>
<span class="STRUCTNAME">gchar</span>, <span class=
"STRUCTNAME">gshort</span>, <span class=
"STRUCTNAME">glong</span>, <span class=
"STRUCTNAME">gint</span>, <span class="STRUCTNAME">
gfloat</span>, <span class="STRUCTNAME">
gdouble</span> are purely cosmetic.
</p>
</li>
<li>
<p>
<span class="STRUCTNAME">gpointer</span> may be
more convenient to type than <span class=
"STRUCTNAME">void *</span>. <span class=
"STRUCTNAME">gconstpointer</span> gives you <span
class="STRUCTNAME">const void*</span>. (<span
class="STRUCTNAME">const gpointer</span> will <i
class="EMPHASIS">not</i> do what you typically mean
it to; spend some time with a good book on C if you
don't see why.)
</p>
</li>
</ul>
</div>
<div class="SECT2">
<h2 class="SECT2">
<a name="Z26">Frequently Used Macros</a>
</h2>
<p>
glib defines a number of familiar macros used in many C
programs, shown in <a href=
"cha-glib.html#ML-SIMPLEMACROS">Figure 1</a>. All of
these should be self-explanatory. <tt class="FUNCTION">
MIN()</tt>/<tt class="FUNCTION">MAX()</tt> return the
smaller or larger of their arguments. <tt class=
"FUNCTION">ABS()</tt> returns the absolute value of its
argument. <tt class="FUNCTION">CLAMP(x, low, high)</tt>
means <span class="STRUCTNAME">x</span>, unless <span
class="STRUCTNAME">x</span> is outside the range [<span
class="STRUCTNAME">low</span>, <span class=
"STRUCTNAME">high</span>]; if <span class="STRUCTNAME">
x</span> is below the range, <span class="STRUCTNAME">
low</span> is returned; if <span class="STRUCTNAME">
x</span> is above the range, <span class="STRUCTNAME">
high</span> is returned. In addition to the macros
shown in <a href="cha-glib.html#ML-SIMPLEMACROS">Figure
1</a>, <tt class="FUNCTION">TRUE</tt>/<tt class=
"FUNCTION">FALSE</tt>/<tt class="FUNCTION">NULL</tt>
are defined as the usual <span class="STRUCTNAME">
1</span>/<span class="STRUCTNAME">0</span>/<span class=
"STRUCTNAME">((void*)0)</span>.
</p>
<div class="FIGURE">
<a name="ML-SIMPLEMACROS"></a>
<div class="FUNCSYNOPSIS">
<a name="ML-SIMPLEMACROS.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <glib.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
MAX</tt></code>(<tt class=
"PARAMETER"><i>a</i></tt>, <tt class="PARAMETER">
<i>b</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
MIN</tt></code>(<tt class=
"PARAMETER"><i>a</i></tt>, <tt class="PARAMETER">
<i>b</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
ABS</tt></code>(<tt class=
"PARAMETER"><i>x</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
CLAMP</tt></code>(<tt class=
"PARAMETER"><i>x</i></tt>, <tt class="PARAMETER">
<i>low</i></tt>, <tt class="PARAMETER"><i>
high</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 1. Familiar C Macros</b>
</p>
</div>
<p>
There are also many macros unique to glib, such as the
portable <span class="STRUCTNAME">
gpointer</span>-to-<span class="STRUCTNAME">gint</span>
and <span class="STRUCTNAME">gpointer</span>-to-<span
class="STRUCTNAME">guint</span> conversions shown in <a
href="cha-glib.html#ML-POINTERINT">Figure 2</a>.
</p>
<p>
Most of glib's data structures are designed to store a
<span class="STRUCTNAME">gpointer</span>. If you want
to store pointers to dynamically allocated objects,
this is the right thing. However, sometimes you want to
store a simple list of integers without having to
dynamically allocate them. Though the C standard does
not strictly guarantee it, it is possible to store a
<span class="STRUCTNAME">gint</span> or <span class=
"STRUCTNAME">guint</span> in a <span class=
"STRUCTNAME">gpointer</span> variable on the wide range
of platforms glib has been ported to; in some cases, an
intermediate cast is required. The macros in <a href=
"cha-glib.html#ML-POINTERINT">Figure 2</a> abstract the
presence of the cast.
</p>
<p>
Here's an example:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
gint my_int;
gpointer my_pointer;
my_int = 5;
my_pointer = GINT_TO_POINTER(my_int);
printf("We are storing %d\n", GPOINTER_TO_INT(my_pointer));
</pre>
</td>
</tr>
</table>
<p>
Be careful, though; these macros allow you to store an
integer in a pointer, but storing a pointer in an
integer will <i class="EMPHASIS">not</i> work. To do
that portably, you must store the pointer in a <span
class="STRUCTNAME">long</span>. (It's undoubtedly a bad
idea to do so, however.)
</p>
<div class="FIGURE">
<a name="ML-POINTERINT"></a>
<div class="FUNCSYNOPSIS">
<a name="ML-POINTERINT.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <glib.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
GINT_TO_POINTER</tt></code>(<tt class=
"PARAMETER"><i>p</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
GPOINTER_TO_INT</tt></code>(<tt class=
"PARAMETER"><i>p</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
GUINT_TO_POINTER</tt></code>(<tt class=
"PARAMETER"><i>p</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
GPOINTER_TO_UINT</tt></code>(<tt class=
"PARAMETER"><i>p</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 2. Macros for storing integers in
pointers</b>
</p>
</div>
</div>
<div class="SECT2">
<h2 class="SECT2">
<a name="Z27">Debugging Macros</a>
</h2>
<p>
glib has a nice set of macros you can use to enforce
invariants and preconditions in your code. GTK+ uses
these liberally---one of the reasons it's so stable and
easy to use. They all disappear when you define <tt
class="FUNCTION">G_DISABLE_CHECKS</tt> or <tt class=
"FUNCTION">G_DISABLE_ASSERT</tt>, so there's no
performance penalty in production code. Using these
liberally is a very, very good idea. You'll find bugs
much faster if you do. You can even add assertions and
checks whenever you find a bug to be sure the bug
doesn't reappear in future versions---this complements
a regression suite. Checks are especially useful when
the code you're writing will be used as a black box by
other programmers; users will immediately know when and
how they've misused your code.
</p>
<p>
Of course you should be very careful to ensure your
code isn't subtly dependent on debug-only statements to
function correctly. Statements that will disappear in
production code should <i class="EMPHASIS">never</i>
have side effects.
</p>
<div class="FIGURE">
<a name="ML-PRECONDITION"></a>
<div class="FUNCSYNOPSIS">
<a name="ML-PRECONDITION.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <glib.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
g_return_if_fail</tt></code>(<tt class=
"PARAMETER"><i>condition</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
g_return_val_if_fail</tt></code>(<tt class=
"PARAMETER"><i>condition</i></tt>, <tt class=
"PARAMETER"><i>retval</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 3. Precondition Checks</b>
</p>
</div>
<p>
<a href="cha-glib.html#ML-PRECONDITION">Figure 3</a>
shows glib's precondition checks. <tt class="FUNCTION">
g_return_if_fail()</tt> prints a warning and
immediately returns from the current function if <span
class="STRUCTNAME">condition</span> is <span class=
"STRUCTNAME">FALSE</span>. <tt class="FUNCTION">
g_return_val_if_fail()</tt> is similar but allows you
to return some <span class="STRUCTNAME">retval</span>.
These macros are incredibly useful---if you use them
liberally, especially in combination with GTK+'s
runtime type checking, you'll halve the time you spend
looking for bad pointers and type errors.
</p>
<p>
Using these functions is simple; here's an example from
the glib hash table implementation:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
void
g_hash_table_foreach (GHashTable *hash_table,
GHFunc func,
gpointer user_data)
{
GHashNode *node;
gint i;
g_return_if_fail (hash_table != NULL);
g_return_if_fail (func != NULL);
for (i = 0; i < hash_table->size; i++)
for (node = hash_table->nodes[i]; node; node = node->next)
(* func) (node->key, node->value, user_data);
}
</pre>
</td>
</tr>
</table>
<p>
Without the checks, passing <span class="STRUCTNAME">
NULL</span> as a parameter to this function would
result in a mysterious segmentation fault. The person
using the library would have to figure out where the
error occurred with a debugger, and maybe even dig in
to the glib code to see what was wrong. With the
checks, they'll get a nice error message telling them
that <span class="STRUCTNAME">NULL</span> arguments are
not allowed.
</p>
<div class="FIGURE">
<a name="ML-ASSERTIONS"></a>
<div class="FUNCSYNOPSIS">
<a name="ML-ASSERTIONS.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <glib.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
g_assert</tt></code>(<tt class=
"PARAMETER"><i>condition</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
g_assert_not_reached</tt></code>(void);</code>
</p>
</div>
<p>
<b>Figure 4. Assertions</b>
</p>
</div>
<p>
glib also has more traditional assertion macros, shown
in <a href="cha-glib.html#ML-ASSERTIONS">Figure 4</a>.
<tt class="FUNCTION">g_assert()</tt> is basically
identical to <tt class="FUNCTION">assert()</tt>, but
responds to <span class="STRUCTNAME">
G_DISABLE_ASSERT</span> and behaves consistently across
all platforms. <tt class="FUNCTION">
g_assert_not_reached()</tt> is also provided; this is
an assertion which always fails. Assertions call <tt
class="FUNCTION">abort()</tt> to exit the program and
(if your environment supports it) dump a core file for
debugging purposes.
</p>
<p>
Fatal assertions should be used to check <i class=
"EMPHASIS">internal consistency</i> of a function or
library, while <tt class="FUNCTION">
g_return_if_fail()</tt> is intended to ensure sane
values are passed to the public interfaces of a program
module. That is, if an assertion fails, you typically
look for a bug in the module containing the assertion;
if a <tt class="FUNCTION">g_return_if_fail()</tt> check
fails, you typically look for the bug in the code which
invokes the module.
</p>
<p>
This code from glib's calendrical calculations module
shows the difference:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
GDate*
g_date_new_dmy (GDateDay day, GDateMonth m, GDateYear y)
{
GDate *d;
g_return_val_if_fail (g_date_valid_dmy (day, m, y), NULL);
d = g_new (GDate, 1);
d->julian = FALSE;
d->dmy = TRUE;
d->month = m;
d->day = day;
d->year = y;
g_assert (g_date_valid (d));
return d;
}
</pre>
</td>
</tr>
</table>
<p>
The precondition check at the beginning ensures the
user passes in reasonable values for the day, month and
year; the assertion at the end ensures that glib
constructed a sane object, given sane values.
</p>
<p>
<tt class="FUNCTION">g_assert_not_reached()</tt> should
be used to mark "impossible" situations; a common use
is to detect switch statements that don't handle all
possible values of an enumeration:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
switch (val)
{
case FOO_ONE:
break;
case FOO_TWO:
break;
default:
/* Invalid enumeration value */
g_assert_not_reached();
break;
}
</pre>
</td>
</tr>
</table>
<p>
All of the debugging macros print a warning using
glib's <tt class="FUNCTION">g_log()</tt> facility,
which means the warning includes the name of the
originating application or library, and you can
optionally install a replacement warning-printing
routine. For example, you might send all warnings to a
dialog box or log file instead of printing them on the
console.
</p>
</div>
<div class="SECT2">
<h2 class="SECT2">
<a name="GLIB-MEMORY">Memory</a>
</h2>
<p>
glib wraps the standard <tt class="FUNCTION">
malloc()</tt> and <tt class="FUNCTION">free()</tt> with
its own <span class="STRUCTNAME">g_</span> variants,
<tt class="FUNCTION">g_malloc()</tt> and <tt class=
"FUNCTION">g_free()</tt>, shown in <a href=
"cha-glib.html#FL-MEMORY">Figure 5</a>. These are nice
in several small ways:
</p>
<ul>
<li>
<p>
<tt class="FUNCTION">g_malloc()</tt> always returns
a <span class="STRUCTNAME">gpointer</span>, never a
<span class="STRUCTNAME">char*</span>, so there's
no need to cast the return value.
</p>
</li>
<li>
<p>
<tt class="FUNCTION">g_malloc()</tt> aborts the
program if the underlying <tt class="FUNCTION">
malloc()</tt> fails, so you don't have to check for
a <span class="STRUCTNAME">NULL</span> return
value.
</p>
</li>
<li>
<p>
<tt class="FUNCTION">g_malloc()</tt> gracefully
handles a <span class="STRUCTNAME">size</span> of
<span class="STRUCTNAME">0</span>, by returning
<span class="STRUCTNAME">NULL</span>.
</p>
</li>
<li>
<p>
<tt class="FUNCTION">g_free()</tt> will ignore any
<span class="STRUCTNAME">NULL</span> pointers you
pass to it.
</p>
</li>
</ul>
<p>
In addition to these minor conveniences, <tt class=
"FUNCTION">g_malloc()</tt> and <tt class="FUNCTION">
g_free()</tt> can support various kinds of memory
debugging and profiling. If you pass the <tt class=
"APPLICATION">--enable-mem-check</tt> option to glib's
configure script, the compiled <tt class="FUNCTION">
g_free()</tt> will warn you whenever you free the same
pointer twice. The <tt class="APPLICATION">
--enable-mem-profile</tt> option enables code which
keeps memory use statistics; when you call <tt class=
"FUNCTION">g_mem_profile()</tt> they are printed to the
console. Finally, you can define <span class=
"STRUCTNAME">USE_DMALLOC</span> and the glib memory
wrappers will use the <span class="STRUCTNAME">
MALLOC()</span>, etc. debugging macros available in <tt
class="FILENAME">dmalloc.h</tt> on some platforms.
</p>
<div class="FIGURE">
<a name="FL-MEMORY"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-MEMORY.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <glib.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">gpointer <tt class=
"FUNCTION">g_malloc</tt></code>(gulong <tt class=
"PARAMETER"><i>size</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">g_free</tt></code>(gpointer <tt class=
"PARAMETER"><i>mem</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">gpointer <tt class=
"FUNCTION">g_realloc</tt></code>(gpointer <tt
class="PARAMETER"><i>mem</i></tt>, gulong <tt
class="PARAMETER"><i>size</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">gpointer <tt class=
"FUNCTION">g_memdup</tt></code>(gconstpointer <tt
class="PARAMETER"><i>mem</i></tt>, guint <tt class=
"PARAMETER"><i>bytesize</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 5. glib memory allocation</b>
</p>
</div>
<p>
It's important to match <tt class="FUNCTION">
g_malloc()</tt> with <tt class="FUNCTION">
g_free()</tt>, plain <tt class="FUNCTION">malloc()</tt>
with <tt class="FUNCTION">free()</tt>, and (if you're
using C++) <span class="STRUCTNAME">new</span> with
<span class="STRUCTNAME">delete</span>. Otherwise bad
things can happen, since these allocators may use
different memory pools (and <span class="STRUCTNAME">
new</span>/<span class="STRUCTNAME">delete</span> call
constructors and destructors).
</p>
<p>
Of course there's a <tt class="FUNCTION">
g_realloc()</tt> equivalent to <tt class="FUNCTION">
realloc()</tt>. There's also a convenient <tt class=
"FUNCTION">g_malloc0()</tt> which fills allocated
memory with 0s, and <tt class="FUNCTION">
g_memdup()</tt> which returns a copy of <span class=
"STRUCTNAME">bytesize</span> bytes starting at <span
class="STRUCTNAME">mem</span>. <tt class="FUNCTION">
g_realloc()</tt> and <tt class="FUNCTION">
g_malloc0()</tt> will both accept a <span class=
"STRUCTNAME">size</span> of <span class="STRUCTNAME">
0</span>, for consistency with <span class=
"STRUCTNAME">g_malloc()</span>. However, <tt class=
"FUNCTION">g_memdup()</tt> will not.
</p>
<p>
If it isn't obvious: <tt class="FUNCTION">
g_malloc0()</tt> fills raw memory with unset bits, not
the value <span class="STRUCTNAME">0</span> for
whatever type you intend to put there. Occasionally
someone expects to get an array of floating point
numbers initialized to <span class="STRUCTNAME">
0.0</span>; this will <i class="EMPHASIS">not</i> work.
</p>
<p>
Finally, there are type-aware allocation macros, shown
in <a href="cha-glib.html#ML-G-NEW">Figure 6</a>. The
<span class="STRUCTNAME">type</span> argument to each
of these is the name of a type, and the <span class=
"STRUCTNAME">count</span> argument is the number of
<span class="STRUCTNAME">type</span>-size blocks to
allocate. These macros save you some typing and
multiplication, and are thus less error-prone. They
automatically cast to the target pointer type, so
attempting to assign the allocated memory to the wrong
kind of pointer should trigger a compiler warning. (If
you have warnings turned on, as a responsible
programmer should!)
</p>
<div class="FIGURE">
<a name="ML-G-NEW"></a>
<div class="FUNCSYNOPSIS">
<a name="ML-G-NEW.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <glib.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
g_new</tt></code>(<tt class=
"PARAMETER"><i>type</i></tt>, <tt class=
"PARAMETER"><i>count</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
g_new0</tt></code>(<tt class=
"PARAMETER"><i>type</i></tt>, <tt class=
"PARAMETER"><i>count</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
g_renew</tt></code>(<tt class=
"PARAMETER"><i>type</i></tt>, <tt class=
"PARAMETER"><i>mem</i></tt>, <tt class="PARAMETER">
<i>count</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 6. Allocation macros</b>
</p>
</div>
</div>
<div class="SECT2">
<h2 class="SECT2">
<a name="Z28">String Handling</a>
</h2>
<p>
glib provides a number of functions for string
handling; some are unique to glib, and some solve
portability concerns. They all interoperate nicely with
the glib memory allocation routines.
</p>
<p>
For those interested in a better string than <span
class="STRUCTNAME">gchar*</span>, there's also a <span
class="STRUCTNAME">GString</span> type. It isn't
covered in this book, but documentation is available at
<a href="http://www.gtk.org/" target="_top">
http://www.gtk.org/</a>.
</p>
<div class="FIGURE">
<a name="FL-STREXT"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-STREXT.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <glib.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">gint <tt class=
"FUNCTION">g_snprintf</tt></code>(gchar* <tt class=
"PARAMETER"><i>buf</i></tt>, gulong <tt class=
"PARAMETER"><i>n</i></tt>, const gchar* <tt class=
"PARAMETER"><i>format</i></tt>, <tt class=
"PARAMETER"><i>...</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">gint <tt class=
"FUNCTION">g_strcasecmp</tt></code>(const gchar*
<tt class="PARAMETER"><i>s1</i></tt>, const gchar*
<tt class="PARAMETER"><i>s2</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">gint <tt class=
"FUNCTION">g_strncasecmp</tt></code>(const gchar*
<tt class="PARAMETER"><i>s1</i></tt>, const gchar*
<tt class="PARAMETER"><i>s2</i></tt>, guint <tt
class="PARAMETER"><i>n</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 7. Portability Wrappers</b>
</p>
</div>
<p>
<a href="cha-glib.html#FL-STREXT">Figure 7</a> shows
some substitutes glib provides for commonly-implemented
but unportable extensions to ANSI C.
</p>
<p>
One of the annoying things about C is that it provides
the crash-causing, security-hole-creating, generally
evil <tt class="FUNCTION">sprintf()</tt>, but the
relatively safe and widely implemented <tt class=
"FUNCTION">snprintf()</tt> is a vendor extension. <tt
class="FUNCTION">g_snprintf()</tt> wraps native <tt
class="FUNCTION">snprintf()</tt> on platforms that have
it, and provides an implementation on those that don't.
So you can say goodbye to <tt class="FUNCTION">
sprintf()</tt> forever. Even better: classically, <tt
class="FUNCTION">snprintf()</tt> does not guarantee
that it will <span class="STRUCTNAME">
NULL</span>-terminate the buffer it fills. <tt class=
"FUNCTION">g_snprintf()</tt> does.
</p>
<p>
<tt class="FUNCTION">g_strcasecmp()</tt> and <tt class=
"FUNCTION">g_strncasecmp()</tt> perform a
case-insensitive comparison of two strings, optionally
with a maximum length. <span class="STRUCTNAME">
strcasecmp()</span> is available on many platforms but
not universally, so using glib instead is advisable.
</p>
<p>
The functions in <a href="cha-glib.html#FL-STRMANIP">
Figure 8</a> modify a string in-place: the first two
convert the string to lowercase or uppercase,
respectively, while <tt class="FUNCTION">
g_strreverse()</tt> reverses its characters. <tt class=
"FUNCTION">g_strchug()</tt> and <tt class="FUNCTION">
g_strchomp()</tt> "chug" the string (remove leading
spaces), or "chomp" it (remove trailing spaces). These
last two return the string, in addition to modifying it
in-place; in some cases it may be convenient to use the
return value. There is a macro, <tt class="FUNCTION">
g_strstrip()</tt>, which combines both functions to
remove both leading and trailing spaces; it is used
just as the individual functions are.
</p>
<div class="FIGURE">
<a name="FL-STRMANIP"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-STRMANIP.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <glib.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">g_strdown</tt></code>(gchar* <tt class=
"PARAMETER"><i>string</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">g_strup</tt></code>(gchar* <tt class=
"PARAMETER"><i>string</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">g_strreverse</tt></code>(gchar* <tt
class="PARAMETER"><i>string</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">gchar* <tt class=
"FUNCTION">g_strchug</tt></code>(gchar* <tt class=
"PARAMETER"><i>string</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">gchar* <tt class=
"FUNCTION">g_strchomp</tt></code>(gchar* <tt class=
"PARAMETER"><i>string</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 8. In-place string modifications</b>
</p>
</div>
<p>
<a href="cha-glib.html#FL-STRFORMATS">Figure 9</a>
shows a few more semi-standard functions glib wraps.
<tt class="FUNCTION">g_strtod</tt> is like <tt class=
"FUNCTION">strtod()</tt>---it converts string <span
class="STRUCTNAME">nptr</span> to a double---with the
exception that it will also attempt to convert the
double in the <tt class="APPLICATION">"C"</tt> locale
if it fails to convert it in the user's default locale.
<span class="STRUCTNAME">*endptr</span> is set to the
first unconverted character, i.e. any text after the
number representation. If conversion fails, <span
class="STRUCTNAME">*endptr</span> is set to <span
class="STRUCTNAME">nptr</span>. <span class=
"STRUCTNAME">endptr</span> may be <span class=
"STRUCTNAME">NULL</span>, causing it to be ignored.
</p>
<p>
<tt class="FUNCTION">g_strerror()</tt> and <tt class=
"FUNCTION">g_strsignal()</tt> are like their non-<span
class="STRUCTNAME">g_</span> equivalents, but portable.
(They return a string representation for an <span
class="STRUCTNAME">errno</span> or a signal number.)
</p>
<div class="FIGURE">
<a name="FL-STRFORMATS"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-STRFORMATS.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <glib.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">gdouble <tt class=
"FUNCTION">g_strtod</tt></code>(const gchar* <tt
class="PARAMETER"><i>nptr</i></tt>, gchar** <tt
class="PARAMETER"><i>endptr</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">gchar* <tt class=
"FUNCTION">g_strerror</tt></code>(gint <tt class=
"PARAMETER"><i>errnum</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">gchar* <tt class=
"FUNCTION">g_strsignal</tt></code>(gint <tt class=
"PARAMETER"><i>signum</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 9. String Conversions</b>
</p>
</div>
<p>
<a href="cha-glib.html#FL-STRDUP">Figure 10</a> shows
glib's rich array of functions for allocating strings.
Unsurprisingly, <tt class="FUNCTION">g_strdup()</tt>
and <tt class="FUNCTION">g_strndup()</tt> produce an
allocated copy of <span class="STRUCTNAME">str</span>
or the first <span class="STRUCTNAME">n</span>
characters of <span class="STRUCTNAME">str</span>. For
consistency with the glib memory allocation functions,
they return <span class="STRUCTNAME">NULL</span> if
passed a <span class="STRUCTNAME">NULL</span> pointer.
The <tt class="FUNCTION">printf()</tt> variants return
a formatted string. <tt class="FUNCTION">
g_strescape</tt> escapes any <span class="STRUCTNAME">
\</span> characters in its argument by inserting
another <span class="STRUCTNAME">\</span> before them,
returning the escaped string. <tt class="FUNCTION">
g_strnfill()</tt>returns a string of size <span class=
"STRUCTNAME">length</span> filled with <span class=
"STRUCTNAME">fill_char</span>.
</p>
<p>
<tt class="FUNCTION">g_strdup_printf()</tt> deserves a
special mention; it is a simpler way to handle this
common piece of code:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
gchar* str = g_malloc(256);
g_snprintf(str, 256, "%d printf-style %s", 1, "format");
</pre>
</td>
</tr>
</table>
<p>
Instead you could say this, and avoid having to figure
out the proper length of the buffer to boot:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
gchar* str = g_strdup_printf("%d printf-style %s", 1, "format");
</pre>
</td>
</tr>
</table>
<div class="FIGURE">
<a name="FL-STRDUP"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-STRDUP.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <glib.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">gchar* <tt class=
"FUNCTION">g_strdup</tt></code>(const gchar* <tt
class="PARAMETER"><i>str</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">gchar* <tt class=
"FUNCTION">g_strndup</tt></code>(const gchar* <tt
class="PARAMETER"><i>format</i></tt>, guint <tt
class="PARAMETER"><i>n</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">gchar* <tt class=
"FUNCTION">g_strdup_printf</tt></code>(const gchar*
<tt class="PARAMETER"><i>format</i></tt>, <tt
class="PARAMETER"><i>...</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">gchar* <tt class=
"FUNCTION">g_strdup_vprintf</tt></code>(const
gchar* <tt class="PARAMETER"><i>format</i></tt>,
va_list <tt class="PARAMETER"><i>
args</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">gchar* <tt class=
"FUNCTION">g_strescape</tt></code>(gchar* <tt
class="PARAMETER"><i>string</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">gchar* <tt class=
"FUNCTION">g_strnfill</tt></code>(guint <tt class=
"PARAMETER"><i>length</i></tt>, gchar <tt class=
"PARAMETER"><i>fill_char</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 10. Allocating Strings</b>
</p>
</div>
<p>
glib provides some convenient functions for
concatenating strings, shown in <a href=
"cha-glib.html#FL-STRCONCAT">Figure 11</a>. <tt class=
"FUNCTION">g_strconcat()</tt> returns a newly-allocated
string created by concatenating each of the strings in
the argument list. The last argument must be <span
class="STRUCTNAME">NULL</span>, so <tt class=
"FUNCTION">g_strconcat()</tt> knows when to stop. <tt
class="FUNCTION">g_strjoin()</tt> is similar, but <span
class="STRUCTNAME">separator</span> is inserted between
each string. If <span class="STRUCTNAME">
separator</span> is <span class="STRUCTNAME">
NULL</span>, no separator is used.
</p>
<div class="FIGURE">
<a name="FL-STRCONCAT"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-STRCONCAT.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <glib.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">gchar* <tt class=
"FUNCTION">g_strconcat</tt></code>(const gchar* <tt
class="PARAMETER"><i>string1</i></tt>, <tt class=
"PARAMETER"><i>...</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">gchar* <tt class=
"FUNCTION">g_strjoin</tt></code>(const gchar* <tt
class="PARAMETER"><i>separator</i></tt>, <tt class=
"PARAMETER"><i>...</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 11. Concatenating strings</b>
</p>
</div>
<p>
Finally, <a href="cha-glib.html#FL-STRVECTOR">Figure
12</a> summarizes a few routines which manipulate <span
class="STRUCTNAME">NULL</span>-terminated arrays of
strings. <tt class="FUNCTION">g_strsplit()</tt> breaks
<span class="STRUCTNAME">string</span> at each <span
class="STRUCTNAME">delimiter</span>, returning a
newly-allocated array. <tt class="FUNCTION">
g_strjoinv()</tt> concatenates each string in the array
with an optional <span class="STRUCTNAME">
separator</span>, returning an allocated string. <tt
class="FUNCTION">g_strfreev()</tt> frees each string in
the array and then the array itself.
</p>
<div class="FIGURE">
<a name="FL-STRVECTOR"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-STRVECTOR.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <glib.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">gchar** <tt class=
"FUNCTION">g_strsplit</tt></code>(const gchar* <tt
class="PARAMETER"><i>string</i></tt>, const gchar*
<tt class="PARAMETER"><i>delimiter</i></tt>, gint
<tt class="PARAMETER"><i>
max_tokens</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">gchar* <tt class=
"FUNCTION">g_strjoinv</tt></code>(const gchar* <tt
class="PARAMETER"><i>separator</i></tt>, gchar**
<tt class="PARAMETER"><i>
str_array</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">g_strfreev</tt></code>(gchar** <tt
class="PARAMETER"><i>str_array</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 12. Manipulating <span class="STRUCTNAME">
NULL</span>-terminated string vectors</b>
</p>
</div>
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