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24.3.5 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 Interrupted Primitives, 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_ONSTACK

If this flag is set for a particular signal number, the system uses the signal stack when delivering that kind of signal. See Signal Stack. If a signal with this flag arrives and you have not set a signal stack, the system terminates the program with SIGILL.

— 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 Interrupted Primitives.