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



NAME
     sigaltstack - set and/or get signal stack context

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

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

     _i_n_t
     sigaltstack(ss, oss)
     _s_t_r_u_c_t _s_i_g_a_l_t_s_t_a_c_k *_s_s;
     _s_t_r_u_c_t _s_i_g_a_l_t_s_t_a_c_k *_o_s_s;

DESCRIPTION
     Sigaltstack allows users to define an alternate stack on
     which signals are to be processed.  If _s_s is non-zero, it
     specifies a pointer to and the size of a _s_i_g_n_a_l _s_t_a_c_k 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 _s_s__f_l_a_g_s, _s_s__b_a_s_e and _s_s__s_i_z_e are
     ignored and the signal stack will be disabled.  Trying to
     disable an active stack will cause sigaltstack to return -1
     with _e_r_r_n_o 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 _o_s_s is non-zero, the current signal stack state is
     returned.	The _s_s__f_l_a_g_s 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 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)
	  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 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.

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

ERRORS
     Sigaltstack will fail and the signal stack context will
     remain unchanged if one of the following occurs.

     EFAULT		 Either _s_s or _o_s_s 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.









Printed 11/26/99	September 3, 1997			2