why am i not able to declare sigset_t with std=c99?

Question

If i compile the below program with std=c99, i get an error, but the program compiles fine without the c99 flag. Why?

#include <signal.h>
void x()
{
    sigset_t dd;
}

int main(void)
{
    x();
    return 0;
}


jim@cola temp]$ gcc -std=c99 blah.c -o blah
blah.c: In function ‘x’:
blah.c:9: error: ‘sigset_t’ undeclared (first use in this function)
blah.c:9: error: (Each undeclared identifier is reported only once
blah.c:9: error: for each function it appears in.)
blah.c:9: error: expected ‘;’ before ‘dd’

it requires `_GNU_SOURCE` to compile this with C99 standard. Try with `gcc -std=c99 -D_GNU_SOURCE blah.c -o blah` instead. — , Nov 29 '12 at 03:22
@user9000 Why bother with `-std=c99` then? Just use `gcc -std=gnu99` and be done. — melpomene, Nov 29 '12 at 03:25
@melpomene what does that has to do with it? `_GNU_SOURCE` makes some GNU extensions available. — , Nov 29 '12 at 03:27

score 2 · Accepted Answer · answered Nov 29 '12 at 03:16

2

Because sigset_t is not part of <signal.h> in standard C and you requested strict standards compatibility with -std=c99. That is, a strictly standard C program can do:

#include <signal.h>

int sigset_t;
int main(void) { return 0; }

and expect it to work.

answered Nov 29 '12 at 03:16

melpomene

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So, what would be the counterpart of sigset_t in c99? – Jimm Nov 29 '12 at 03:20
... nothing? Why do you assume such a thing exists in standard C? – melpomene Nov 29 '12 at 03:20
if you can't find `sigset_t` then define it yourself like in the code below. it is just an `unsigned long` – Aniket Inge Nov 29 '12 at 03:22
3

@Aniket That is terrible advice. – melpomene Nov 29 '12 at 03:24
@melpomene so if i want to use signals specifc functions like sigprocmask and would like c99 features like restrict, what flag should i pass to gcc? – Jimm Nov 29 '12 at 03:32
@user9000 Is that same as #define _GNU_SOURCE ? – Jimm Nov 29 '12 at 03:34

score 2 · Answer 2 · edited Apr 08 '19 at 13:55

sigset_t is not in C99 standard, but it is available in POSIX. You can define _POSIX_SOURCE or _POSIX_C_SOURCE to make sigset_t available.

Here is the definition:

#define _NSIG 64 
#define _NSIG_BPW 32 
#define _NSIG_WORDS (_NSIG / _NSIG_BPW) 

typedef unsigned long old_sigset_t; /* at least 32 bits */ 

typedef struct { 
unsigned long sig[_NSIG_WORDS]; 
} sigset_t;

Also see What does #define _POSIX_SOURCE mean?

why am i not able to declare sigset_t with std=c99?

2 Answers2