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SIGALTSTACK(2)		    BSD	System Calls Manual		SIGALTSTACK(2)

     sigaltstack -- set	and/or get signal stack	context

     #include <signal.h>

     struct sigaltstack	{
	     char    *ss_sp;
	     size_t  ss_size;
	     int     ss_flags;

     sigaltstack(const struct sigaltstack *ss, struct sigaltstack *oss);

     Sigaltstack() 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 sig-
     nal's action indicates its	handler	should execute on the 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 cur-
     rently 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	cur-
     rently on a signal	stack and SS_DISABLE if	the signal stack is currently

     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

	   if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
		   /* error return */
	   sigstk.ss_size = SIGSTKSZ;
	   sigstk.ss_flags = 0;
	   if (sigaltstack(&sigstk,0) <	0)
     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

     Signal stacks are automatically adjusted for the direction	of stack
     growth and	alignment requirements.	 Signal	stacks may or may not be pro-
     tected 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.

     Upon successful completion, a value of 0 is returned.  Otherwise, a value
     of	-1 is returned and errno is set	to indicate the	error.

     Sigstack()	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

     sigaction(2), setjmp(3)

     The predecessor to	sigaltstack(), the sigstack() system call, appeared in

4.2 Berkeley Distribution	  May 1, 1995	     4.2 Berkeley Distribution


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