Re: [PLUG] ftw.h frustration

["Michael C. Toren" <mct@toren.net>]

Jeff Abrahamson wrote:

> I want to use the function nftw, but it turns out that features.h
> disables this function, even though the files are all GPL.
> 
> (Search for __USE_XOPEN_EXTENDED in /usr/include/features.h to see
> details.)
> 
> Anyone know why glibc would be compiled without this? Worse, the man
> page doesn't reflect this. It all seems like the sort of thing I'd see
> on a proprietary OS ("Sorry, you didn't pay for that option.") rather
> than on linux.

Looking at /usr/include/features.h, it appears that the purpose is not
to restrict what you have access to, but to give you a choice between
which implementation of a common function you'd like to use ("to specify
the desired environment").  According to the FAQ distributed with libc6:

    _GNU_SOURCE: glibc does not make the GNU extensions available
    automatically.  If a program depends on GNU extensions or some
    other non-standard functionality, it is necessary to compile it
    with the C compiler option -D_GNU_SOURCE, or better, to put
    `#define _GNU_SOURCE' at the beginning of your source files,
    before any C library header files are included.

    [...]

    For more information consult the file `NOTES' in the GNU C
    library sources.

And, in the NOTES file:

    Macro: _GNU_SOURCE  If you define this macro, everything is
    included: ISO C89, ISO C99, POSIX.1, POSIX.2, BSD, SVID, X/Open,
    LFS, and GNU extensions.  In the cases where POSIX.1 conflicts
    with BSD, the POSIX definitions take precedence.

So, if you define _GNU_SOURCE, does your program compile as expected?


gabriel rosenkoetter wrote:

> > (Search for __USE_XOPEN_EXTENDED in /usr/include/features.h to see
> > details.)
> 
> Why are you including features.h at all?

It's included by /usr/include/ftw.h, and about 150ish other include
files installed by libc6.

-mct