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<title>GCC warning options for numerical programs - GNU Scientific Library -- Reference Manual</title>
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<h3 class="section">A.4 GCC warning options for numerical programs</h3>
<p><a name="index-warning-options-2491"></a><a name="index-gcc-warning-options-2492"></a>
Writing reliable numerical programs in C requires great care. The
following GCC warning options are recommended when compiling numerical
programs:
<!-- Uninitialized variables, conversions to and from integers or from -->
<!-- signed to unsigned integers can all cause hard-to-find problems. For -->
<!-- many non-numerical programs compiling with @code{gcc}'s warning option -->
<!-- @code{-Wall} provides a good check against common errors. However, for -->
<!-- numerical programs @code{-Wall} is not enough. -->
<!-- If you are unconvinced take a look at this program which contains an -->
<!-- error that can occur in numerical code, -->
<!-- @example -->
<!-- #include <math.h> -->
<!-- #include <stdio.h> -->
<!-- double f (int x); -->
<!-- int main () -->
<!-- @{ -->
<!-- double a = 1.5; -->
<!-- double y = f(a); -->
<!-- printf("a = %g, sqrt(a) = %g\n", a, y); -->
<!-- return 0; -->
<!-- @} -->
<!-- double f(x) @{ -->
<!-- return sqrt(x); -->
<!-- @} -->
<!-- @end example -->
<!-- @noindent -->
<!-- This code compiles cleanly with @code{-Wall} but produces some strange -->
<!-- output, -->
<!-- @example -->
<!-- bash$ gcc -Wall tmp.c -lm -->
<!-- bash$ ./a.out -->
<!-- a = 1.5, sqrt(a) = 1 -->
<!-- @end example -->
<!-- @noindent -->
<!-- Note that adding @code{-ansi} does not help here, since the program does -->
<!-- not contain any invalid constructs. What is happening is that the -->
<!-- prototype for the function @code{f(int x)} is not consistent with the -->
<!-- function call @code{f(y)}, where @code{y} is a floating point -->
<!-- number. This results in the argument being silently converted to an -->
<!-- integer. This is valid C, but in a numerical program it also likely to -->
<!-- be a programming error so we would like to be warned about it. (If we -->
<!-- genuinely wanted to convert @code{y} to an integer then we could use an -->
<!-- explicit cast, @code{(int)y}). -->
<!-- Fortunately GCC provides many additional warnings which can alert you to -->
<!-- problems such as this. You just have to remember to use them. Here is a -->
<!-- set of recommended warning options for numerical programs. -->
<pre class="example"> gcc -ansi -pedantic -Werror -Wall -W
-Wmissing-prototypes -Wstrict-prototypes
-Wtraditional -Wconversion -Wshadow
-Wpointer-arith -Wcast-qual -Wcast-align
-Wwrite-strings -Wnested-externs
-fshort-enums -fno-common -Dinline= -g -O2
</pre>
<p class="noindent">For details of each option consult the manual <cite>Using and Porting
GCC</cite>. The following table gives a brief explanation of what types of
errors these options catch.
<dl>
<dt><code>-ansi -pedantic</code><dd>Use ANSI C, and reject any non-ANSI extensions. These flags help in
writing portable programs that will compile on other systems.
<br><dt><code>-Werror</code><dd>Consider warnings to be errors, so that compilation stops. This prevents
warnings from scrolling off the top of the screen and being lost. You
won't be able to compile the program until it is completely
warning-free.
<br><dt><code>-Wall</code><dd>This turns on a set of warnings for common programming problems. You
need <code>-Wall</code>, but it is not enough on its own.
<br><dt><code>-O2</code><dd>Turn on optimization. The warnings for uninitialized variables in
<code>-Wall</code> rely on the optimizer to analyze the code. If there is no
optimization then these warnings aren't generated.
<br><dt><code>-W</code><dd>This turns on some extra warnings not included in <code>-Wall</code>, such as
missing return values and comparisons between signed and unsigned
integers.
<br><dt><code>-Wmissing-prototypes -Wstrict-prototypes</code><dd>Warn if there are any missing or inconsistent prototypes. Without
prototypes it is harder to detect problems with incorrect arguments.
<br><dt><code>-Wtraditional</code><dd>This warns about certain constructs that behave differently in
traditional and ANSI C. Whether the traditional or ANSI interpretation
is used might be unpredictable on other compilers.
<br><dt><code>-Wconversion</code><dd>The main use of this option is to warn about conversions from signed to
unsigned integers. For example, <code>unsigned int x = -1</code>. If you need
to perform such a conversion you can use an explicit cast.
<br><dt><code>-Wshadow</code><dd>This warns whenever a local variable shadows another local variable. If
two variables have the same name then it is a potential source of
confusion.
<br><dt><code>-Wpointer-arith -Wcast-qual -Wcast-align</code><dd>These options warn if you try to do pointer arithmetic for types which
don't have a size, such as <code>void</code>, if you remove a <code>const</code>
cast from a pointer, or if you cast a pointer to a type which has a
different size, causing an invalid alignment.
<br><dt><code>-Wwrite-strings</code><dd>This option gives string constants a <code>const</code> qualifier so that it
will be a compile-time error to attempt to overwrite them.
<br><dt><code>-fshort-enums</code><dd>This option makes the type of <code>enum</code> as short as possible. Normally
this makes an <code>enum</code> different from an <code>int</code>. Consequently any
attempts to assign a pointer-to-int to a pointer-to-enum will generate a
cast-alignment warning.
<br><dt><code>-fno-common</code><dd>This option prevents global variables being simultaneously defined in
different object files (you get an error at link time). Such a variable
should be defined in one file and referred to in other files with an
<code>extern</code> declaration.
<br><dt><code>-Wnested-externs</code><dd>This warns if an <code>extern</code> declaration is encountered within a
function.
<br><dt><code>-Dinline=</code><dd>The <code>inline</code> keyword is not part of ANSI C. Thus if you want to use
<code>-ansi</code> with a program which uses inline functions you can use this
preprocessor definition to remove the <code>inline</code> keywords.
<br><dt><code>-g</code><dd>It always makes sense to put debugging symbols in the executable so that
you can debug it using <code>gdb</code>. The only effect of debugging symbols
is to increase the size of the file, and you can use the <code>strip</code>
command to remove them later if necessary.
</dl>
<!-- For comparison, this is what happens when the test program above is -->
<!-- compiled with these options. -->
<!-- @example -->
<!-- bash$ gcc -ansi -pedantic -Werror -W -Wall -Wtraditional -->
<!-- Wconversion -Wshadow -Wpointer-arith -Wcast-qual -Wcast-align -->
<!-- Wwrite-strings -Waggregate-return -Wstrict-prototypes -fshort-enums -->
<!-- fno-common -Wmissing-prototypes -Wnested-externs -Dinline= -->
<!-- g -O4 tmp.c -->
<!-- cc1: warnings being treated as errors -->
<!-- tmp.c:7: warning: function declaration isn't a prototype -->
<!-- tmp.c: In function `main': -->
<!-- tmp.c:9: warning: passing arg 1 of `f' as integer rather than floating -->
<!-- due to prototype -->
<!-- tmp.c: In function `f': -->
<!-- tmp.c:14: warning: type of `x' defaults to `int' -->
<!-- tmp.c:15: warning: passing arg 1 of `sqrt' as floating rather than integer -->
<!-- due to prototype -->
<!-- make: *** [tmp] Error 1 -->
<!-- @end example -->
<!-- @noindent -->
<!-- The error in the prototype is flagged, plus the fact that we should have -->
<!-- defined main as @code{int main (void)} in ANSI C. Clearly there is some -->
<!-- work to do before this program is ready to run. -->
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