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

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

SYNOPSIS
     #include <&lt;signal.h>&gt;

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

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

DESCRIPTION
     sigaltstack() allows users to define an alternate stack on which signals
     delivered to this thread are to be processed.  If ss is non-zero and
     SS_DISABLE is set in ss_flags, the signal stack will be disabled.  A dis-
     abled stack will cause all signals to be taken on the regular user stack.
     Trying to disable an active stack will cause sigaltstack to return -1
     with errno set to EPERM.

     Otherwise, ss_sp specifies a pointer to a space to be used as the signal
     stack and ss_size specifies the size of that space.  When a signal's
     action indicates its handler should execute on the signal stack (speci-
     fied with a sigaction(2) call), the system checks to see if the thread is
     currently executing on that stack.  If the thread is not currently exe-
     cuting on the signal stack, the system arranges a switch to the signal
     stack for the duration of the signal handler's execution.

     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 cur-
     rently 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, 0) == -1)
                   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 pro-
     tected 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, the value 0 is returned; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

ERRORS
     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]  The ss_flags member pointed to by the ss argument contains
               flags other than SS_DISABLE.

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

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

SEE ALSO
     sigaction(2), setjmp(3)

STANDARDS
     The sigaltstack function conforms to IEEE Std 1003.1-2008 (``POSIX.1'').

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

BSD                            November 15, 2014                           BSD