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25.3.5 Flags for sigaction ¶

The sa_flags member of the sigaction structure is a catch-all for special features. Most of the time, SA_RESTART is a good value to use for this field.

The value of sa_flags is interpreted as a bit mask. Thus, you should choose the flags you want to set, OR those flags together, and store the result in the sa_flags member of your sigaction structure.

Each signal number has its own set of flags. Each call to sigaction affects one particular signal number, and the flags that you specify apply only to that particular signal.

In the GNU C Library, establishing a handler with signal sets all the flags to zero except for SA_RESTART, whose value depends on the settings you have made with siginterrupt. See Primitives Interrupted by Signals, to see what this is about.

These macros are defined in the header file signal.h.

Macro: int SA_NOCLDSTOP ¶

This flag is meaningful only for the SIGCHLD signal. When the flag is set, the system delivers the signal for a terminated child process but not for one that is stopped. By default, SIGCHLD is delivered for both terminated children and stopped children.

Setting this flag for a signal other than SIGCHLD has no effect.

Macro: int SA_NOCLDWAIT ¶

This flag is meaningful only for the SIGCHLD signal. When the flag is set, the terminated child will not wait for the parent to reap it, or become a zombie if not reaped. The child will instead be reaped by the kernel immediately on termination, similar to setting SIGCHLD to SIG_IGN.

Setting this flag for a signal other than SIGCHLD has no effect.

Macro: int SA_NODEFER ¶

Normally a signal is added to the signal mask while running its own handler; this negates that, so that the same signal can be received while it’s handler is running. Note that if the signal is included in sa_mask, it is masked regardless of this flag. Only useful when assigning a function as a signal handler.

Macro: int SA_ONSTACK ¶

If this flag is set for a particular signal number, the system uses the signal stack when delivering that kind of signal. See Using a Separate Signal Stack. If a signal with this flag arrives and you have not set a signal stack, the normal user stack is used instead, as if the flag had not been set.

Macro: int SA_RESETHAND ¶

Resets the handler for a signal to SIG_DFL, at the moment specified handler function begins. I.e. the handler is called once, then the action resets.

Macro: int SA_RESTART ¶

This flag controls what happens when a signal is delivered during certain primitives (such as open, read or write), and the signal handler returns normally. There are two alternatives: the library function can resume, or it can return failure with error code EINTR.

The choice is controlled by the SA_RESTART flag for the particular kind of signal that was delivered. If the flag is set, returning from a handler resumes the library function. If the flag is clear, returning from a handler makes the function fail. See Primitives Interrupted by Signals.

Macro: int SA_SIGINFO ¶

Indicates that the sa_sigaction three-argument form of the handler should be used in setting up a handler instead of the one-argument sa_handler form.