Provided by: freebsd-manpages_8.0-1_all bug


     sigaltstack - set and/or get signal stack context


     Standard C Library (libc, -lc)


     #include <signal.h>

     typedef struct sigaltstack {
             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 users to define an alternate stack
     on which signals are to be processed.  If ss is non-zero, it specifies a
     pointer to and the size of a signal stack on which to deliver signals,
     and tells the system if the process is currently executing on that stack.
     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 process is currently executing on that stack.  If the process
     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

     If SS_DISABLE is set in ss_flags, ss_sp and ss_size are ignored and the
     signal stack will be disabled.  Trying to disable an active stack will
     cause sigaltstack() to return -1 with errno set to EINVAL.  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 process 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

           if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
                   /* error return */
           sigstk.ss_size = SIGSTKSZ;
           sigstk.ss_flags = 0;
           if (sigaltstack(&sigstk,0) < 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.

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

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


     sigaction(2), setjmp(3)


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