Macro: ac_compile_check_sizeof

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# ac_compile_check_sizeof

## Synopsis

`AC_COMPILE_CHECK_SIZEOF(TYPE [, HEADERS [, EXTRA_SIZES...]])`

## Version

1.1.1.1 (2001/07/26) Miscellaneous @ ac-archive-0.5.32

## Author

Kaveh Ghazi <ghazi@caip.rutgers.edu>

## Description

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.

The trick is that C will not allow duplicate case labels. While this is valid C code:

`switch (0) case 0: case 1:;`

The following is not:

`switch (0) case 0: case 0:;`

Thus, the AC_TRY_COMPILE will fail if the currently tried size does not match.

Here is an example skeleton configure.in script, demonstrating the macro's usage:

`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)`

## M4 Source Code

    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
    ])