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Autoconf Macro: ac_compile_check_sizeof
</title>
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<h1>
ac_compile_check_sizeof
</h1>
<h2>
Synopsis
</h2>
<p class="indent" style="white-space:nowrap;">
<code>AC_COMPILE_CHECK_SIZEOF(TYPE [, HEADERS [, EXTRA_SIZES...]])</code>
</p>
<h2>
Description
</h2>
<div class="indent">
<p>
This macro checks for the size of TYPE using compile checks, not run
checks. You can supply extra HEADERS to look into. the check will cycle
through 1 2 4 8 16 and any EXTRA_SIZES the user supplies. If a match is
found, it will #define SIZEOF_`TYPE' to that value. Otherwise it will emit
a configure time error indicating the size of the type could not be
determined.
</p>
<p>
The trick is that C will not allow duplicate case labels. While this is
valid C code:
</p>
<pre>
switch (0) case 0: case 1:;
</pre>
<p>
The following is not:
</p>
<pre>
switch (0) case 0: case 0:;
</pre>
<p>
Thus, the AC_TRY_COMPILE will fail if the currently tried size does not
match.
</p>
<p>
Here is an example skeleton configure.in script, demonstrating the macro's
usage:
</p>
<pre>
AC_PROG_CC
AC_CHECK_HEADERS(stddef.h unistd.h)
AC_TYPE_SIZE_T
AC_CHECK_TYPE(ssize_t, int)
headers='#ifdef HAVE_STDDEF_H
#include <stddef.h>
#endif
#ifdef HAVE_UNISTD_H
#include <unistd.h>
#endif
'
AC_COMPILE_CHECK_SIZEOF(char)
AC_COMPILE_CHECK_SIZEOF(short)
AC_COMPILE_CHECK_SIZEOF(int)
AC_COMPILE_CHECK_SIZEOF(long)
AC_COMPILE_CHECK_SIZEOF(unsigned char *)
AC_COMPILE_CHECK_SIZEOF(void *)
AC_COMPILE_CHECK_SIZEOF(size_t, $headers)
AC_COMPILE_CHECK_SIZEOF(ssize_t, $headers)
AC_COMPILE_CHECK_SIZEOF(ptrdiff_t, $headers)
AC_COMPILE_CHECK_SIZEOF(off_t, $headers)
</pre>
</div>
<h2>
Author
</h2>
<p class="indent">
Kaveh Ghazi <ghazi@caip.rutgers.edu>
</p>
<h2>
Last Modified
</h2>
<p class="indent">
2000-07-19
</p>
<h2>
M4 Source Code
</h2>
<div class="indent">
<pre class="m4source">
AC_DEFUN([AC_COMPILE_CHECK_SIZEOF],
[changequote(<<, >>)dnl
dnl The name to #define.
define(<<AC_TYPE_NAME>>, translit(sizeof_$1, [a-z *], [A-Z_P]))dnl
dnl The cache variable name.
define(<<AC_CV_NAME>>, translit(ac_cv_sizeof_$1, [ *], [_p]))dnl
changequote([, ])dnl
AC_MSG_CHECKING(size of $1)
AC_CACHE_VAL(AC_CV_NAME,
[for ac_size in 4 8 1 2 16 $2 ; do # List sizes in rough order of prevalence.
AC_TRY_COMPILE([#include "confdefs.h"
#include <sys/types.h>
$2
], [switch (0) case 0: case (sizeof ($1) == $ac_size):;], AC_CV_NAME=$ac_size)
if test x$AC_CV_NAME != x ; then break; fi
done
])
if test x$AC_CV_NAME = x ; then
AC_MSG_ERROR([cannot determine a size for $1])
fi
AC_MSG_RESULT($AC_CV_NAME)
AC_DEFINE_UNQUOTED(AC_TYPE_NAME, $AC_CV_NAME, [The number of bytes in type $1])
undefine([AC_TYPE_NAME])dnl
undefine([AC_CV_NAME])dnl
])
</pre>
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