sigaltstack - set and/or get signal alternate stack context.
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
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:
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 fails.
This flag must not be modified by
SS_DISABLE The alternate signal stack is currently
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
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After a successful call to one of the exec functions, there are no
alternate signal stacks in the new process image.
Upon successful completion, sigaltstack() returns 0. Otherwise, it
returns -1 and sets errno to indicate the error.
The sigaltstack() function will 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
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)
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
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
Each thread may define an alternate signal handling stack.
LWP (Light Weight Processes) Considerations
Each LWP may define an alternate signal handling stack.
sigaction(2), setjmp(3C), <signal.h>.
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First released in Issue 4, Version 2.
Hewlett-Packard Company - 3 - HP-UX Release 11i: November 2000