sigaltstack(2)
sigaltstack --
set or get signal alternate stack context
Synopsis
#include <signal.h>
int sigaltstack(const stack_t *ss, stack_t *oss);
Description
sigaltstack allows users to define an
alternate stack area on which signals are to be
processed.
If ss is non-zero, it specifies a pointer to,
and the size of a
stack area 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 alternate signal stack [specified with a
sigaction(2)
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 alternate signal
stack for the duration of the signal handler's execution.
The structure sigaltstack includes the following members.
char *ss_sp
int ss_size
int ss_flags
If ss is not NULL, it points to a structure specifying the
alternate signal stack that will take effect upon return from
sigaltstack.
The ss_sp and ss_size fields specify
the new base and size of the stack, which is automatically
adjusted for direction of growth and alignment.
The ss_flags
field specifies the new stack state
and may be set to the following:
SS_DISABLE-
The stack is to be disabled and ss_sp and ss_size are ignored.
If SS_DISABLE is not set, the stack will be enabled.
SS_DISABLE
is the only way users can disable the alternate signal stack.
If oss is not NULL, it points to a structure specifying the
alternate signal stack that was in effect prior to the call to
sigaltstack.
The ss_sp and ss_size fields specify
the base and size of that stack.
The ss_flags field specifies the
stack's state, and may contain 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 will fail.
SS_ONSTACK cannot be modified by
users.
SS_DISABLE-
The alternate signal stack is currently disabled.
Return values
On success, sigaltstack returns 0.
On failure, sigaltstack returns -1 and sets errno to identify the error.
Errors
In the following conditions, sigaltstack fails and sets errno to:
EFAULT-
Either ss or oss points outside the process's allocated address
space.
EINVAL-
ss is non-null and the ss_flags field pointed to by
ss contains invalid flags.
The only flag considered valid is
SS_DISABLE.
EPERM-
An attempt was made to modify an active stack.
ENOMEM-
The size of the alternate stack area is less than MINSIGSTKSZ.
Usage
The value SIGSTKSZ is defined to be the number of bytes that
would be used to cover the usual case when 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
operating system overhead.
The following code fragment is typically used to allocate an
alternate stack.
if ((sigstk.ss_sp = (char *)malloc(SIGSTKSZ)) == NULL)
/* error return */;
sigstk.ss_size = SIGSTKSZ;
sigstk.ss_flags = 0;
if (sigaltstack(&sigstk, (stack_t *)0) < 0)
perror("sigaltstack");
References
getcontext(2),
sigaction(2),
sigsetjmp(3C),
ucontext(5)
Notices
Considerations for threads programming
The Threads Library does not support alternate signal handling stacks
for threads.
See
signal(5)
for further details.
Considerations for lightweight processes
Internally,
an alternative signal stack can be defined per LWP.
© 2004 The SCO Group, Inc. All rights reserved.
UnixWare 7 Release 7.1.4 - 25 April 2004