AX_COMPILE_CHECK_SIZEOF(TYPE [, HEADERS [, EXTRA_SIZES...]])
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_COMPILE_IFELSE 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 '
AX_COMPILE_CHECK_SIZEOF(char) AX_COMPILE_CHECK_SIZEOF(short) AX_COMPILE_CHECK_SIZEOF(int) AX_COMPILE_CHECK_SIZEOF(long) AX_COMPILE_CHECK_SIZEOF(unsigned char *) AX_COMPILE_CHECK_SIZEOF(void *) AX_COMPILE_CHECK_SIZEOF(size_t, $headers) AX_COMPILE_CHECK_SIZEOF(ssize_t, $headers) AX_COMPILE_CHECK_SIZEOF(ptrdiff_t, $headers) AX_COMPILE_CHECK_SIZEOF(off_t, $headers)
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