NAME
sigaltstack - set and/or get signal stack context
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <signal.h>
typedef struct sigaltstack {
char *ss_sp;
size_t ss_size;
int ss_flags;
} stack_t;
int
sigaltstack(const stack_t * restrict ss, stack_t * restrict oss);
DESCRIPTION
The sigaltstack() system call allows users to define an alternate stack
on which signals are to be processed. If ss is non-zero, it specifies a
pointer to and the size of a signal stack on which to deliver signals,
and tells the system if the process is currently executing on that stack.
When a signal’s action indicates its handler should execute on the signal
stack (specified with a sigaction(2) system call), the system checks to
see if the process is currently executing on that stack. If the process
is not currently executing on the signal stack, the system arranges a
switch to the signal stack for the duration of the signal handler’s
execution.
If SS_DISABLE is set in ss_flags, ss_sp and ss_size are ignored and the
signal stack will be disabled. Trying to disable an active stack will
cause sigaltstack() to return -1 with errno set to EINVAL. A disabled
stack will cause all signals to be taken on the regular user stack. If
the stack is later re-enabled then all signals that were specified to be
processed on an alternate stack will resume doing so.
If oss is non-zero, the current signal stack state is returned. The
ss_flags field will contain the value SS_ONSTACK if the process is
currently on a signal stack and SS_DISABLE if the signal stack is
currently disabled.
NOTES
The value SIGSTKSZ is defined to be the number of bytes/chars that would
be used to cover the usual case when allocating an alternate stack area.
The following code fragment is typically used to allocate an alternate
stack.
if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
/* error return */
sigstk.ss_size = SIGSTKSZ;
sigstk.ss_flags = 0;
if (sigaltstack(&sigstk,0) < 0)
perror("sigaltstack");
An alternative approach is provided for programs with signal handlers
that require a specific amount of stack space other than the default
size. The value MINSIGSTKSZ is defined to be the number of bytes/chars
that is required by the operating system to implement the alternate stack
feature. In computing an alternate stack size, programs should add
MINSIGSTKSZ to their stack requirements to allow for the operating system
overhead.
Signal stacks are automatically adjusted for the direction of stack
growth and alignment requirements. Signal stacks may or may not be
protected by the hardware and are not ‘‘grown’’ automatically as is done
for the normal stack. If the stack overflows and this space is not
protected unpredictable results may occur.
RETURN VALUES
The sigaltstack() function returns the value 0 if successful; otherwise
the value -1 is returned and the global variable errno is set to indicate
the error.
ERRORS
The sigaltstack() system call will fail and the signal stack context will
remain unchanged if one of the following occurs.
[EFAULT] Either ss or oss points to memory that is not a valid
part of the process address space.
[EINVAL] An attempt was made to disable an active stack.
[ENOMEM] Size of alternate stack area is less than or equal to
MINSIGSTKSZ.
SEE ALSO
sigaction(2), setjmp(3)
HISTORY
The predecessor to sigaltstack(), the sigstack() system call, appeared in
4.2BSD.