CODEWITHSUNDEEP
clinuxunixsignals
oliver
oliver

Reputation: 6791

what does it mean if sa_sigaction is set to NULL when calling sigaction()?

What does the following code do?

struct sigaction saStruct;
saStruct.sa_flags = SA_NOCLDWAIT|SA_NODEFER|SA_SIGINFO;
saStruct.sa_sigaction = NULL;
sigemptyset(&saStruct.sa_mask);
sigaction(SIGCHLD, &saStruct, NULL);

In particular, what does it mean that sa_sigaction is set to NULL? Is this a complicated way to express something much more simple? I couldn't find any reference to this usage in the sigaction man page.

I'm primarily interested in answers for Linux (>= 2.6).

Upvotes: 3

Views: 1392

Answers (1)

ecatmur
ecatmur

Reputation: 157484

NULL happens to be equivalent to SIG_DFL; don't depend on this.

#define SIG_ERR  ((__sighandler_t) -1)  /* Error return.  */
#define SIG_DFL  ((__sighandler_t)  0)  /* Default action.  */
#define SIG_IGN  ((__sighandler_t)  1)  /* Ignore signal.  */

SIG_DFL etc. are intended to be assigned to the sa_handler member, but (and again, don't depend on this) sa_handler is on Linux in a union with sa_sigaction so assigning NULL to sa_sigaction is equivalent to assigning SIG_DFL to sa_handler.

    union
      {
        /* Used if SA_SIGINFO is not set.  */
        __sighandler_t sa_handler;
        /* Used if SA_SIGINFO is set.  */
        void (*sa_sigaction) (int, siginfo_t *, void *);
      }
    __sigaction_handler;
# define sa_handler __sigaction_handler.sa_handler
# define sa_sigaction   __sigaction_handler.sa_sigaction

Upvotes: 3

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