If the argument "act" is not a null pointer, it points to a structure specifying the action to be associated with the specified signal. If the argument "oact" is not a null pointer, the action previously associated with the signal is stored in the location pointed to by the argument "oact". If the argument "act" is a null pointer, signal handling is unchanged. The SIGKILL and SIGSTOP signals shall not be added to the signal mask using sigaction(), but will not result in sigaction reporting a failure. If the SA_SIGINFO flag is cleared in the sa_flags field of the sigaction structure, the sa_handler field indentifies the action to be associated with the specified signal. If the SA_SIGINFO flag is set in the sa_flags field, and the implementation supports the Realtime Signals Extension option or the XSI Extension option, the sa_sigaction field specifies a signal catching function. If the SA_SIGINFO bit is cleared and the sa_handler field specifies a signal catching function, or if the SA_SIGINFO bit is set, the sa_mask field indentifies a set of signals that shall be added to the signal mask of the thread before the signal-catching function is invoked. If the sa_handler field specifies a signal-catching function, the sa_mask field indentifies a set of signals that shall be added to the process' signal mask before the signal-catching function is invoked. If the SA_NOCLDSTOP bit is set in the sa_flags field when calling sigaction on SIGCHLD, then the SIGCHLD signal will not be generated when children stop or stopped children continue. If sig is SIGCHLD and the SA_NOCLDSTOP flag is not set in sa_flags, and the implementation supports the SIGCHLD signal, then a SIGCHLD signal shall be generated for the calling process whenever any of its child processes stop. Not setting the SA_NOCLDSTOP flag on the SIGCHLD signal will result in a SIGCHLD signal being generated for the parent process whenever any of it's stopped children are continued. If the SA_ONSTACK flag is set in sa_flags, and an alternate signal stack has been declared with sigaltstack(), the signal shall be delivered to the calling process on the stack. Otherwise, the signal shall be delivered on the current stack. If the SA_ONSTACK flag is not set in sa_flags, the signal shall be delivered on the current stack. If the SA_RESETHAND flag is set in sa_flags, then the disposition of the signal shall be reset to SIG_DFL and the SA_SIGINFO flag shall be cleared on entry to the signal handler. The routine behaves as if SA_NODEFER was also set. If the SA_RESETHAND flag is not set in sa_flags, the disposition of the signal shall not be modified on entry to the signal handler. Interruptable functions (functions specified to fail with errno set to [EINTR] when interrupted by a signal) shall restart and shall not fail with [EINTR] if the SA_RESTART flag is set in sa_flags. If the SA_RESTART flag is not set in sa_flags, then interruptable functions will fail with with errno set to [EINTR]. If the SA_SIGINFO flag is not set for a signal and the signal is caught then the signal-catching function shall be entered as "void func(int signo);". If the SA_SIGINFO flag is set for a signal and the signal is caught then the signal -catching function shall be entered as "void func(int signo, siginfo_t *info, void *context);" If the SA_NOCLDWAIT flag is set and sig equals SIGCHLD, then child processes of the calling process shall not be trasformed into zombie processes when they terminate. If a process has set the SA_NOCLDWAIT flag on SIGCHLD and subseqeunetly waits for a its children, and the process has not unwaited-for children that were trasformed into zombie processes, it shall block until all of its children terminate, and wait(), waitid(), and waitpid() shall fail and set errno to [ECHILD]. If the SA_NODEFER flag is set for a signal and the signal is caught, the signal shall not be added to the process' signal mask on entry to the signal handler unless it is included in sa_mask. If the SA_NODEFER flag is not set for a signal and the signal is caught, the signal shall be added to the process' signal mask on entry to the signal handler. When a signal-catching function installed by sigaction() is entered, a new signal mask is calculated and installed for the duration of the signal-catching function (or until a call to either sigprocmask() or sigsuspend() is made.) The new signal mask installed on entry to a signal-catching function installed by sigaction shall be calculated as the union of the current signal mask and the value of sa_mask for the signal being delivered (unless SA_NODEFER or SA_RESETHAND is set) and then including the signal being delivered. If and when the user's signal handler returns normally, the original signal mask is restored. If sigaction fails, no new signal handler is installed. If the signal action was set with the signal() function, getting it into oact then reinstalling it with act must be valid. If SA_SIGINFO is set and realtime signals extension is supported, queueable signals generated by sigqueue or some other functions are delivered in FIFO order. sigaction returns -1 and errno is set to EINVAL if signal number is invalid or an attempt to do an operation which is not allowed is made.