implicit declaration of function swab

Question

EDIT: the warning in the first paragraph was due to me reading the old log from the compiler (before including unistd.h, sorry for the missconfusion, the second problem still resides) I have a code that makes use of the c function unlink (declared in unistd.h). Upon compilation with gcc -Wall I get the warning: implicit declaration of function ‘unlink’ Now I now that this is just a warning but it kinda annoys me and I would like what would be the proper way to resolve this. My current solution is to simply add the line extern int unlink(const char *path); to the code but this seems a bit silly as I am also including unistd.h

I used to got a warning about the use of the function swab (also declared in unistd.h) but managed, after inspection of unistd.h, to resolve this by adding the lines

#ifndef __USE_XOPEN
#define __USE_XOPEN 
#endif

Not sure that this is the proper thing to do so any insights to this would also be appreciated.

Answer 1

Referring your second issue ("implicit declaration of function sawb"):

Replace

#ifndef __USE_XOPEN
#define __USE_XOPEN 
#endif

with

#define _XOPEN_SOURCE

... placing it before all other #include statements #include <unistd.h> should make the prototyping for swab() be available.

---

#defines starting with a double underscore __ are reserved for internal use and shall not be set by a program directly.

Answer 2

You most likely have a typo or something simple wrong.....try this.

#include <unistd.h>

int
main()
{
    int r;  

    r = unlink("foo");

    return(r);
}

Compile it

gcc -Wall foo.c

Above should be a clean compile. You can pre-process and do a grep to verify that unlink declaration is pulled in.

sys> gcc -E foo.c | grep unlink
int unlink(const char *);
 r = unlink("foo");