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

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

SYNOPSIS
     #include <signal.h>

     typedef struct sigaltstack	{
	     void    *ss_sp;
	     size_t  ss_size;
	     int     ss_flags;
     } stack_t;

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

DESCRIPTION
     sigaltstack() allows users	to define an alternate stack on	which signals
     delivered to this thread are to be	processed.  If ss is non-zero and
     SS_DISABLE	is set in ss_flags, the	signal stack will be disabled.	A dis-
     abled stack will cause all	signals	to be taken on the regular user	stack.
     Trying to disable an active stack will cause sigaltstack()	to return -1
     with errno	set to EPERM.

     Otherwise,	ss_sp specifies	a pointer to a space to	be used	as the signal
     stack and ss_size specifies the size of that space.  When a signal's ac-
     tion indicates its	handler	should execute on the signal stack (specified
     with a sigaction(2) call),	the system checks to see if the	thread is cur-
     rently 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 signal	handler's execution.

     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.

     To	permit the space to operate as a stack,	a page-aligned inner region
     will be zeroed and	have MAP_STACK (see mmap(2)) enabled.  Once the
     sigaltstack is disabled, MAP_STACK	remains	on the memory, so it is	best
     to	deallocate the memory via a method that	results	in munmap(2).

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) == -1)
		   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
     Upon successful completion, the value 0 is	returned; otherwise the
     value -1 is returned and the global variable errno	is set to indicate the
     error.

ERRORS
     sigaltstack() will	fail and the signal stack context will remain un-
     changed 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]  The ss_flags member pointed to by the ss	argument contains
	       flags other than	SS_DISABLE.

     [ENOMEM]  Size of alternate stack area is less than or equal to
	       MINSIGSTKSZ.

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

SEE ALSO
     sigaction(2), setjmp(3)

STANDARDS
     The sigaltstack() function	conforms to IEEE Std 1003.1-2008 ("POSIX.1").

HISTORY
     The predecessor to	sigaltstack(), the sigstack() system call, appeared in
     4.2BSD.

FreeBSD	13.0			April 22, 2018			  FreeBSD 13.0

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

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