C: implicit declaration of function 'foo'

[allucid]

I keep getting these warnings and I would like to find out why.

Here's a couple examples of the ones I am getting:

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io.c:98: warning: implicit declaration of function `readdir_r'
markovlet.c:46: warning: implicit declaration of function `random'
random.c:30: warning: implicit declaration of function `srandom'
server.c:169: warning: implicit declaration of function `fdopen'
server.c:194: warning: implicit declaration of function `fileno'

For all the listed functions I checked the manpages and made sure I included all the necessary header files. I don't know where else to look.

[widan]

You're probably compiling with the "-ansi" flag, and those functions aren't defined in ANSI C. They're POSIX functions. Look in the man pages, at the very bottom there is a section "CONFORMING TO":

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CONFORMING TO
       The fopen and freopen functions conform  to  ANSI  X3.159-1989  (``ANSI
       C'').    The   fdopen   function   conforms   to   IEEE  Std1003.1-1988
       (``POSIX.1'').

When you compile in "-ansi" mode, gcc does not define some constants (__USE_GNU, __USE_POSIX, __USE_BSD, maybe others), that cause some function prototypes to be skipped in system headers (for example in stdio.h):

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#ifdef	__USE_POSIX
/* Create a new stream that refers to an existing system file descriptor.  */
extern FILE *fdopen (int __fd, __const char *__modes) __THROW;
#endif

[allucid]

I am not using -ansi but I am using -std=c99. Changing that to -std=gnu99 seemed fixed those warnings (it also caused another error but I'm working on fixing that). The other person who is working on the project with me is using gcc with the same flags but is not getting all the warnings that I am which added to my confusion. Thanks for the help!

[TrueDFX]

You can use -std=c99, as long as you also add -D_POSIX_C_SOURCE=200112L (or better, #define it before #including any of the headers). POSIX requires it anyway.