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     sigaltstack — set and/or get signal stack context


     Standard C Library (libc, -lc)


     #include <signal.h>

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

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


     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.


     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)
     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

     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.


     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.


     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.


     sigaction(2), setjmp(3)


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