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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.