sigaltstack(2)							   System Calls 						    sigaltstack(2)

NAME

       sigaltstack - set or get signal alternate stack context

SYNOPSIS

       #include <signal.h>

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

DESCRIPTION

       The sigaltstack() function allows a thread to define and examine the state of an alternate stack area on which signals are processed. If ss
       is non-zero, it specifies a pointer to and the size of a stack area on which to deliver signals, and informs the system whether the  thread
       is  currently  executing on that stack.	When a signal's action indicates its handler should  execute on the alternate signal stack (speci-
       fied with a sigaction(2) call), the system checks whether the thread chosen to execute the signal handler is currently  executing  on  that
       stack.  If  the	thread	is not currently executing on the signal stack, the system arranges a switch to the alternate signal stack for the
       duration of the signal handler's execution.

       The  stack_t structure includes the following members:

	 int   *ss_sp
	 long  ss_size
	 int   ss_flags

       If ss is not NULL, it points to a structure specifying the alternate signal stack that will take effect upon successful return from sigalt-
       stack().  The ss_sp and ss_size members specify the new base and size of the stack, which is automatically adjusted for direction of growth
       and alignment.  The ss_flags member specifies the new stack state  and may be set to the following:

       SS_DISABLE    The stack is to be disabled and ss_sp and ss_size are ignored. If SS_DISABLE is not set, the stack will be enabled.

       If oss is not NULL, it points to a structure specifying the alternate signal stack that was in effect prior to the call	to  sigaltstack().
       The  ss_sp  and	ss_size members specify the base and size of that stack.  The ss_flags member specifies the stack's state, and may contain
       the following values:

       SS_ONSTACK    The thread is currently executing on the alternate signal stack. Attempts to modify the  alternate  signal  stack	while  the
		     thread is executing on it will fail.

       SS_DISABLE    The alternate signal stack is currently disabled.

RETURN VALUES

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

ERRORS

       The sigaltstack() function will fail if:

       EFAULT	 The ss or oss argument points to an illegal address.

       EINVAL	 The ss argument is not a null pointer, and the ss_flags member pointed to by ss contains flags other than SS_DISABLE.

       ENOMEM	 The size of the alternate stack area is less than MINSIGSTKSZ.

       EPERM	 An attempt was made to modify an active stack.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Standard			   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |Async-Signal-Safe 	   |
       +-----------------------------+-----------------------------+

SEE ALSO

       getcontext(2), mmap(2), sigaction(2), ucontext.h(3HEAD), attributes(5), standards(5)

NOTES

       The  value  SIGSTKSZ  is  defined  to be the number of bytes that would be used to cover the usual case when allocating an alternate  stack
       area.  The value MINSIGSTKSZ is defined to be the minimum stack size for a signal handler.  In computing an alternate stack size, a program
       should add that amount to its stack requirements to allow for the operating system overhead.

       The  following code fragment is typically used to allocate an alternate stack with an adjacent red zone (an unmapped page) to guard against
       stack overflow, as with default stacks:

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

	 stack_t sigstk;
	 sigstk.ss_sp = mmap(NULL, SIGSTKSZ, PROT_READ | PROT_WRITE,
		 MAP_PRIVATE | MAP_ANON, -1, 0);
	 if (sigstk.ss_sp == MAP_FAILED)
		 /* error return */;
	 sigstk.ss_size = SIGSTKSZ;
	 sigstk.ss_flags = 0;
	 if (sigaltstack(&sigstk, NULL) < 0)
		 perror("sigaltstack");

SunOS 5.11							    1 Nov 2003							    sigaltstack(2)