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SIGALTSTACK(2)		  FreeBSD System Calls Manual		SIGALTSTACK(2)

NAME
     sigaltstack -- set	and/or get signal stack	context

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <signal.h>

     typedef struct {
	     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 defining an alternate	stack on which
     signals are to be processed for the current thread.  If ss	is non-zero,
     it	specifies a pointer to and the size of a signal	stack on which to
     deliver signals.  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 thread is currently executing on that
     stack.  If	the thread is not currently executing on the signal stack, the
     system arranges a switch to the signal stack for the duration of the sig-
     nal handler's execution.

     An	active stack cannot be modified.

     If	SS_DISABLE is set in ss_flags, ss_sp and ss_size are ignored and the
     signal stack will be disabled.  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 thread is cur-
     rently 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, NULL) < 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 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.

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.

     [EPERM]		An attempt was made to modify an active	stack.

     [EINVAL]		The ss_flags field was invalid.

     [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.

FreeBSD	9.3			  May 6, 2010			   FreeBSD 9.3

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | NOTES | RETURN VALUES | ERRORS | SEE ALSO | HISTORY

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