The sigstack() function allows users to define an alternate stack, called the "signal stack", on which signals are to be processed. When a signal's action indicates its handler should execute on the signal stack (specified with a sigvec(3UCB) call), the system checks to see if the process is currently executing on that stack. If the process is not currently executing on the signal stack, the system arranges a switch to the signal stack for the duration of the signal handler's execution.

A signal stack is specified by a sigstack() structure, which includes the following members:

char *ss_sp; /* signal stack pointer */ int ss_onstack; /* current status */

The ss_sp member is the initial value to be assigned to the stack pointer when the system switches the process to the signal stack. Note that, on machines where the stack grows downwards in memory, this is not the address of the beginning of the signal stack area. The ss_onstack member is zero or non-zero depending on whether the process is currently executing on the signal stack or not.

If nss is not a null pointer, sigstack() sets the signal stack state to the value in the sigstack() structure pointed to by nss. If nss is a null pointer, the signal stack state will be unchanged. If oss is not a null pointer, the current signal stack state is stored in the sigstack() structure pointed to by oss.

Upon successful completion, 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error.

The sigstack() function will fail and the signal stack context will remain unchanged if one of the following occurs.

EFAULT

Either nss or oss points to memory that is not a valid part of the process address space.

sigaltstack(2), sigvec(3UCB), signal(3C)

Signal stacks are not "grown" automatically, as is done for the normal stack. If the stack overflows unpredictable results may occur.

Use of these interfaces should be restricted to only applications written on BSD platforms. Use of these interfaces with any of the system libraries or in multi-threaded applications is unsupported.