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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.