The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


sigaltstack - set and get signal alternate stack context


[XSI] [Option Start] #include <signal.h>

int sigaltstack(const stack_t *restrict
ss, stack_t *restrict oss); [Option End]


The sigaltstack() function allows a process to define and examine the state of an alternate stack for signal handlers for the current thread. Signals that have been explicitly declared to execute on the alternate stack shall 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 shall 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 shall 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 function 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 shall 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 fail. This flag shall 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) shall 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 behavior.


Upon successful completion, sigaltstack() shall return 0; otherwise, it shall return -1 and set errno to indicate the error.


The sigaltstack() function shall 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 sections are informative.


Allocating Memory for an Alternate Stack

The following example illustrates a method for allocating memory for an alternate stack.

#include <signal.h>
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 behavior is undefined.






Signal Concepts, sigaction(), sigsetjmp(), the Base Definitions volume of IEEE Std 1003.1-2001, <signal.h>


First released in Issue 4, Version 2.

Issue 5

Moved from X/OPEN UNIX extension to BASE.

The last sentence of the DESCRIPTION was included as an APPLICATION USAGE note in previous issues.

Issue 6

The DESCRIPTION is updated to avoid use of the term "must" for application requirements.

The restrict keyword is added to the sigaltstack() prototype for alignment with the ISO/IEC 9899:1999 standard.

IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/58 is applied, updating the first sentence to include "for the current thread".

End of informative text.

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