sigaltstack -- set or get signal alternate stack context


   #include <signal.h>

int sigaltstack(const stack_t *ss, stack_t *oss);


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(S) 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:

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:

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.

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.


In the following conditions, sigaltstack fails and sets errno to:

Either ss or oss points outside the process's allocated address space.

ss is non-null and the ss_flags field pointed to by ss contains invalid flags. The only flag considered valid is SS_DISABLE.

An attempt was made to modify an active stack.

The size of the alternate stack area is less than MINSIGSTKSZ.


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");


getcontext(S), sigaction(S), sigsetjmp(S), ucontext(M)


Considerations for threads programming

The Threads Library does not support alternate signal handling stacks for threads. See signal(M) for further details.

Considerations for lightweight processes

Internally, an alternative signal stack can be defined per LWP.
© 2005 The SCO Group, Inc. All rights reserved.
SCO OpenServer Release 6.0.0 - 01 June 2005