SIGALTSTACK(2)	    UNIX Programmer's Manual	   SIGALTSTACK(2)

     sigaltstack - set and/or get signal stack context

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

     struct sigaltstack {
	  caddr_t ss_base;
	  int  ss_size;
	  int	  ss_flags;

     sigaltstack(ss, oss)
     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 pro-
     cess 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 execution.

     If SA_DISABLE is set in ss_flags, ss_base 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
     SA_ONSTACK if the process is currently on a signal stack and
     SA_DISABLE if the signal stack is currently disabled.

     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_base = malloc(SIGSTKSZ)) == NULL)
	  /* error return */

Printed 11/26/99	September 3, 1997			1

SIGALTSTACK(2)	    UNIX Programmer's Manual	   SIGALTSTACK(2)

     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 operat-
     ing system to implement the alternate stack feature.  In
     computing an alternate stack size, programs should add MIN-
     SIGSTKSZ 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.

     Upon successful completion, a value of 0 is returned.  Oth-
     erwise, a value of -1 is returned and errno is set to indi-
     cate the error.

     Sigaltstack 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

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

     sigaction(2), setjmp(3)

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

Printed 11/26/99	September 3, 1997			2

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