Manual Pages  — SIGALTSTACK

sigaltstack – set and/or get signal stack context

LIBRARY SYNOPSIS DESCRIPTION NOTES RETURN VALUES ERRORS SEE ALSO HISTORY

Standard C Library (libc, -lc)

#include <signal.h>

typedef struct {
        char    *ss_sp;
        size_t  ss_size;
        int     ss_flags;
} stack_t;

int
sigaltstack(const stack_t * restrict ss, stack_t * restrict oss);

The value SIGSTKSZ is defined to be the number of bytes/chars that would be used to cover the usual case when allocating an alternate stack area. The following code fragment is typically used to allocate an alternate stack.

if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
        /* error return */
sigstk.ss_size = SIGSTKSZ;
sigstk.ss_flags = 0;
if (sigaltstack(&sigstk, NULL) < 0)
        perror("sigaltstack");

An alternative approach is provided for programs with signal handlers that require a specific amount of stack space other than the default size. The value MINSIGSTKSZ is defined to be the number of bytes/chars that is required by the operating system to implement the alternate stack feature. In computing an alternate stack size, programs should add MINSIGSTKSZ to their stack requirements to allow for the operating system overhead.

Signal stacks are automatically adjusted for the direction of stack growth and alignment requirements. Signal stacks may or may not be protected by the hardware and are not ``grown'' automatically as is done for the normal stack. If the stack overflows and this space is not protected unpredictable results may occur.

The sigaltstack function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

The sigaltstack() system call will fail and the signal stack context will remain unchanged if one of the following occurs.

[EFAULT]
	Either ss or oss points to memory that is not a valid part of the process address space.
[EPERM]
	An attempt was made to modify an active stack.
[EINVAL]
	The ss_flags field was invalid.
[ENOMEM]
	Size of alternate stack area is less than or equal to MINSIGSTKSZ.

The predecessor to sigaltstack(), the sigstack() system call, appeared in BSD 4.2 .