sigaltstack(2)							sigaltstack(2)

NAME    [Toc]    [Back]

     sigaltstack - set or get signal alternate stack context

SYNOPSIS    [Toc]    [Back]

     #include <signal.h>

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

DESCRIPTION    [Toc]    [Back]

     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 stack_t 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.  The
     ss_sp field should	be adjusted for	the direction of growth	(see example
     below).  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.

     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.






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sigaltstack(2)							sigaltstack(2)



     sigaltstack fails if any of the following is true:

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

     EINVAL	   If ss is non-null, and the ss_flags field pointed to	by ss
		   contains invalid flags.

     EPERM	   If an attempt was made to modify an active stack.

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

NOTES    [Toc]    [Back]

     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 stack grows downward from high	to lower addresses.  The following
     code fragment is typically	used to	allocate an alternate stack.

	  if ((sigstk.ss_sp = (char *)malloc(SIGSTKSZ))	== NULL)
	       /* error	return */;

	  sigstk.ss_sp += SIGSTKSZ - 1;
	       /* adjust ss_sp to point	to base	of stack */
	  sigstk.ss_size = SIGSTKSZ;
	  sigstk.ss_flags = 0;
	  if (sigaltstack(&sigstk, (stack_t *)0) < 0)
	       perror("sigaltstack");

SEE ALSO    [Toc]    [Back]

     getcontext(2), sigaction(2), sigsetjmp(3C), ucontext(5).

DIAGNOSTICS    [Toc]    [Back]

     On	success, sigaltstack returns zero.  On failure,	it returns -1 and sets
     errno to indicate the error.


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