How do I use sigaction()? struct sigaction is not defined - c

Question

I am doing simple sigaction example to practice C, but when I try to compile my code, it claims that struct sigaction doesn't exist [1].
When I checked out some old code I had produced I saw that I had added some POSIX string at the very top of the file [2]. But when I read the manual for sigaction (man 2 sigaction) there is nothing about _POSIX_SOURCE in it, the closest being _POSIX_C_SOURCE which doesn't work. How and when do I know which POSIX is the be used? When I try simple code that others have suggested, which is without the _POSIX_SOURCE it doesn't work.
[1]
#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
void sa_handler(int signum)
{
printf("The signal has been replaced with this useless
string!\n");
exit(0);
}
int main(void)
{
struct sigaction sa = {.sa_handler = sa_handler};
int sigret = sigaction(SIGINT, &sa, NULL);
while(1);
return 0;
}
[2]
#define _POSIX_SOURCE
#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
void sa_handler(int signum)
{
printf("The signal has been replaced with this useless
string!\n");
exit(0);
}
int main(void)
{
struct sigaction sa = {.sa_handler = sa_handler};
int sigret = sigaction(SIGINT, &sa, NULL);
while(1);
return 0;
}
When I compile the first example the result are these error messages.
sigaction.c: In function ‘main’:
sigaction.c:13:12: error: variable ‘sa’ has initializer but
incomplete type
struct sigaction sa = {.sa_handler = sa_handler};
^~~~~~~~~
sigaction.c:13:29: error: ‘struct sigaction’ has no member named
‘sa_handler’
struct sigaction sa = {.sa_handler = sa_handler};
^~~~~~~~~~
sigaction.c:13:42: warning: excess elements in struct initializer
struct sigaction sa = {.sa_handler = sa_handler};
^~~~~~~~~~
sigaction.c:13:42: note: (near initialization for ‘sa’)
sigaction.c:13:22: error: storage size of ‘sa’ isn’t known
struct sigaction sa = {.sa_handler = sa_handler};
^~
sigaction.c:14:18: warning: implicit declaration of function
‘sigaction’ [-Wimplicit-function-declaration]
int sigret = sigaction(SIGINT, &sa, NULL);
^~~~~~~~~

Answer 1

when I read the manual for sigaction (man 2 sigaction) there is nothing about _POSIX_SOURCE in it
From man sigaction:L
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
From future_test_macros(7):
_POSIX_SOURCE
Defining this obsolete macro with any value is equivalent to
defining _POSIX_C_SOURCE with the value 1.
Since this macro is obsolete, its usage is generally not doc‐
umented when discussing feature test macro requirements in
the man pages.
So _POSIX_SOURCE is equivalent to _POSIX_C_SOURCE 1 and is obsolete.
How and when do I know which POSIX is the be used?
From man future_test_macros:
Specification of feature test macro requirements in manual pages
When a function requires that a feature test macro is defined, the
manual page SYNOPSIS typically includes a note [....]
So you should check SYNOPSIS section in the manual page of the function/feature you are interested in. For example for man sigaction:
sigaction(): _POSIX_C_SOURCE
siginfo_t: _POSIX_C_SOURCE >= 199309L
So you need to define _POSIX_C_SOURCE for sigaction() and _POSIX_C_SOURCE greater or equal to the value of 199309 for siginfo_t.

Answer 2

You need to define a positive integer for _POSIX_C_SOURCE. For sigaction, it needs to be atleast:
#define _POSIX_C_SOURCE 199309L
Look the the documentation for which POSIX version to use.

Answer 3

You need to add <features.h> before <signals.>, as the features.h contains #define _POST_C_SOURCE with latest value for compatibility purpose.