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

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

SYNOPSIS
     #include <sys/types.h>
     #include <signal.h>

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

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

DESCRIPTION
     Sigaltstack() allows users to define an alternate stack on which signals are to be pro-
     cessed.  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 signal'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 currently executing on the signal stack, the
     system arranges a switch to the signal stack for the duration of the signal handler's execu-
     tion.

     If SA_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 pro-
     cessed 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 SA_ONSTACK if the process is currently on a signal stack and SA_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 typ-
     ically 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,0) < 0)
		   perror("sigaltstack");
     An alternative approach is provided for programs with signal handlers that require a spe-
     cific 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 protected 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, a value of 0 is returned.  Otherwise, a value of -1 is returned
     and errno is set to indicate the error.

ERRORS
     Sigstack() will fail and the signal stack context will remain unchanged if one of the fol-
     lowing 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 MINSIGSTKSZ.

SEE ALSO
     sigaction(2), setjmp(3)

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

4.2 Berkeley Distribution		   June 4, 1993 		4.2 Berkeley Distribution
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