This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
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 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
If oss is not a null pointer, upon 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:
SS_ONSTACK 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.
SS_DISABLE 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.
The sigaltstack() function shall fail if:
EINVAL The ss argument is not a null pointer, and the ss_flags member
pointed to by ss contains flags other than SS_DISABLE.
ENOMEM The size of the alternate stack area is less than MINSIGSTKSZ.
EPERM 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)perror("sigaltstack");
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.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 SIGALTSTACK(3P)