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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: Effect of CPP tags
Date: Sun, 31 Dec 2023 16:07:35 -0800
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Janis Papanagnou <janis_papanagnou+ng@hotmail.com> writes:

> This is a CPP question that arose last month.  It's not about an
> actual issue with the software, just out of curiosity and to be sure
> it works reliable (it seemingly does).
>
> In a C99 program on Linux (Ubuntu) I intended to use usleep() and
> then also strnlen().
>
> When I added usleep() and its include file I got an error and was
> asked to define the CPP tag '_BSD_SOURCE'.  I did so, and because I
> wanted side effects of that tag kept as small as possible I
> prepended it just before the respective #include and put it at the
> end of my #include list
>
> ...other #includes...
> #define _BSD_SOURCE
> #include <unistd.h>
>
> But as got obvious *that* way there had been side-effects and I
> had to put the tag at the beginning of all include files (which
> astonished me)
>
> #define _BSD_SOURCE
> #include <unistd.h>
> ...other #includes here...
>
> For the strnlen() function I needed another CPP tag, '_GNU_SOURCE'.
> So now I have both CPP tag definitions before the includes

I second the recommendations of Lowell Gilbert and others not to
define _BSD_SOURCE or _GNU_SOURCE (especially not _GNU_SOURCE)
but instead seek alternatives, which are readily available for
the two functionalities being sought in this case.

> #define _GNU_SOURCE /* necessary for strnlen() in string.h */
> #define _BSD_SOURCE /* necessary for usleep() in unistd.h */
> ...all #includes here...

For strnlen(), put an inline definition in a header file:

    #ifndef HAVE_strnlen_dot_h_header
    #define HAVE_strnlen_dot_h_header

    #include <stddef.h>

    static inline size_t
    strnlen( const char *s, size_t n ){
        extern void *memchr( const void *, int, size_t );
        const  char *p = memchr( s, 0, n );
        return  p  ? (size_t){ p-s }  : n;
    }

    #include <string.h>

    #endif

Disclaimer:  this code has been compiled but not tested.

(If you want you could call this header file "string.h" and do
a #include "string.h".  I'm not advocating doing that, just
pointing it out as an alternative.  I expect some people like the
idea, and others dislike it, and I don't want to get involved in
a style war.)

For usleep(), define an alternate function usnooze(), to be used
in place of usleep().  In header file usnooze.h:

    extern  int usnooze( unsigned long long );

In source file usnooze.c:

    #ifndef _POSIX_C_SOURCE
    #define _POSIX_C_SOURCE 199309L
    #endif

    #include "usnooze.h"

    #include <errno.h>
    #include <time.h>

    int
    usnooze( unsigned long long snooze_time_in_microseconds ){
        typedef unsigned long long ULL;
        typedef struct timespec TS;

        ULL seconds     = snooze_time_in_microseconds / 1000000;
        ULL nanoseconds = snooze_time_in_microseconds % 1000000 * 1000;
        TS need = { .tv_sec = seconds, .tv_nsec = nanoseconds };
        TS more;
        int rc;
        unsigned most = 1000;

        while(  errno = 0,  rc = nanosleep( & need, & more ),  rc != 0  ){
            if(  errno != EINTR  ||  most-- == 0  )  return  -1;
            need = more;
        }

        return  0;
    }

The point of putting the definition of usnooze() in a separate file
is so the needed #define _POSIX_C_SOURCE doesn't contaminate any
more program source than it has to.