The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


sigaltstack - set and/or get signal alternate stack context.


#include <signal.h>

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


The sigaltstack() function allows a process to define and examine the state of an alternate stack for signal handlers. Signals that have been explicitly declared to execute on the alternate stack will be delivered on the alternate stack.

If ss is not a null pointer, it points to a stack_t structure that specifies the alternate signal stack that will take effect upon return from sigaltstack(). The ss_flags member specifies the new stack state. If it is set to SS_DISABLE, the stack is disabled and ss_sp and ss_size are ignored. Otherwise the stack will be enabled, and the ss_sp and ss_size members specify the new address and size of the stack.

The range of addresses starting at ss_sp, up to but not including ss_sp+ss_size, is available to the implementation for use as the stack. This interface makes no assumptions regarding which end is the stack base and in which direction the stack grows as items are pushed.

If oss is not a null pointer, on successful completion it will point to a stack_t structure that specifies the alternate signal stack that was in effect prior to the call to sigaltstack(). The ss_sp and ss_size members specify the address and size of that stack. The ss_flags member specifies the stack's state, and may contain one of the following values:

The process is currently executing on the alternate signal stack. Attempts to modify the alternate signal stack while the process is executing on it fails. This flag must not be modified by processes.
The alternate signal stack is currently disabled.

The value SIGSTKSZ is a system default specifying the number of bytes that would be used to cover the usual case when manually allocating an alternate stack area. The value MINSIGSTKSZ is defined to be the minimum stack size for a signal handler. In computing an alternate stack size, a program should add that amount to its stack requirements to allow for the system implementation overhead. The constants SS_ONSTACK, SS_DISABLE, SIGSTKSZ, and MINSIGSTKSZ are defined in <signal.h>.

After a successful call to one of the exec functions, there are no alternate signal stacks in the new process image.

In some implementations, a signal (whether or not indicated to execute on the alternate stack) will always execute on the alternate stack if it is delivered while another signal is being caught using the alternate stack.

Use of this function by library threads that are not bound to kernel-scheduled entities results in undefined behaviour.


Upon successful completion, sigaltstack() returns 0. Otherwise, it returns -1 and sets errno to indicate the error.


The sigaltstack() function will fail if:
The ss argument is not a null pointer, and the ss_flags member pointed to by ss contains flags other than SS_DISABLE.
The size of the alternate stack area is less than MINSIGSTKSZ.
An attempt was made to modify an active stack.




The following code fragment illustrates a method for allocating memory for an alternate stack:

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

On some implementations, stack space is automatically extended as needed. On those implementations, automatic extension is typically not available for an alternate stack. If the stack overflows, the behaviour is undefined.




sigaction(), sigsetjmp(), <signal.h>.

UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
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