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.

Something is very broken on your system if you have to create an ifdef to use unlink. — Fred the Magic Wonder Dog, Nov 15 '13 at 17:40

score 2 · Answer 1 · answered Nov 15 '13 at 18:04

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");

answered Nov 15 '13 at 18:04

Arun Taylor

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Hmmmm, that compiled like a charm. You are probably correct in that there are some problems with the code, I did not write it my-self but am stuck with trying to clean up the mess – cpaitor Nov 15 '13 at 18:27

alk · Accepted Answer · 2013-11-15T20:59:31.237

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.

edited Nov 15 '13 at 20:59

answered Nov 15 '13 at 17:16

alk

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Hmmm, no good, that just gave me the warning about implicit declaration of swab back – cpaitor Nov 15 '13 at 17:30
@cpaitor: Which gcc version and platform are you using? Also please see my updated answer. – alk Nov 15 '13 at 17:32
gcc 4.6.2, openuSuSe 12.1, I figured that `__` was not to be used, Oh I also updated my question as the unlink warning was not really there, sorry for that – cpaitor Nov 15 '13 at 17:46
1

@cpaitor: (and alk) You should put feature-test macro definitions (`#define _XOPEN_SOURCE`) before *any* #include, not just before `#include `. Otherwise it's possible that some other header includes the internal header which defines `swab`, and it will not be included again with the new feature-test setting. – rici Nov 15 '13 at 18:08
@rici: that did the trick. A follow up, do I need (or rather should I) to set the value of _XOPEN_SOURCE to some specific value? – cpaitor Nov 15 '13 at 18:24
1

@cpaitor: For swab, no. For other interfaces, possibly; the `man` page for the function call should tell you. (`500` enables Unix98, `600` enables Posix2001, and `700` enables Posix2008, assuming your standard library implements those interfaces.) – rici Nov 15 '13 at 18:46
@rici: Thanks for clarifing what I initially did not mention. I updated my answer accordingly. – alk Nov 15 '13 at 20:53

implicit declaration of function swab

2 Answers2