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sigaltstack - set and get signal alternate stack context
int sigaltstack(const stack_t *restrict ss, stack_t *restrict oss);
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
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
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
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
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:
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
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
Signal Concepts , sigaction() , sigsetjmp() , the Base Definitions
volume of IEEE Std 1003.1-2001, <signal.h>
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. 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.opengroup.org/unix/online.html .