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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.