NAME

       sigaltstack — set and/or get signal stack context

LIBRARY

       Standard C Library (libc, -lc)

SYNOPSIS

       #include <signal.h>

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

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

DESCRIPTION

       The sigaltstack() system call allows defining an alternate stack on which signals are to be processed for
       the current thread.  If ss is non-zero, it specifies a pointer to and the size of a signal stack on which
       to  deliver  signals.   When  a  signal's action indicates its handler should execute on the signal stack
       (specified with a sigaction(2) system call), the  system  checks  to  see  if  the  thread  is  currently
       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.

       An active stack cannot be modified.

       If SS_DISABLE is set in ss_flags, ss_sp and ss_size are ignored and the signal stack will be disabled.  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 SS_ONSTACK if the thread is currently on a signal stack and  SS_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_sp = malloc(SIGSTKSZ)) == NULL)
                     /* error return */
             sigstk.ss_size = SIGSTKSZ;
             sigstk.ss_flags = 0;
             if (sigaltstack(&sigstk, NULL) < 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  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

       The sigaltstack() function returns the value 0 if successful; otherwise the value -1 is returned and  the
       global variable errno is set to indicate the error.

ERRORS

       The  sigaltstack() system call 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.

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

       [EINVAL]           The ss_flags field was invalid.

       [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.

Debian                                             May 6, 2010                                    SIGALTSTACK(2)