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

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

SYNOPSIS
     #include <signal.h>

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

DESCRIPTION
     sigaltstack() 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 sig-
     nal's action indicates its handler should execute on the signal stack
     (specified with a sigaction(2) call), the system checks to see if the
     process is currently executing on that stack.  If the process is not cur-
     rently executing on the signal stack, the system arranges a switch to the
     signal stack for the duration of the signal handler's execution.

     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 SA_ONSTACK if the process 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) < 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 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, a value of 0 is returned.  Otherwise, a value
     of -1 is returned and 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]  An attempt is made to disable an active stack.

     [EINVAL]  The ss argument is not a null pointer, and the ss_flags member
               pointed to by ss contains flags other than SS_DISABLE.

     [ENOMEM]  The size of the alternate stack area is less than or equal to
               MINSIGSTKSZ.

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

LEGACY SYNOPSIS
     #include <sys/types.h>
     #include <signal.h>

     The include file <sys/types.h> is necessary.

     struct sigaltstack {
             char   *ss_sp;
             int     ss_size;
             int     ss_flags;
     };

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

     The variable types have changed.  Specifically, the sigaltstack struct is
     no longer used.

COMPATIBILITY
     Use of the (obsolete) sigaltstack struct will cause compiler diagnostics.
     Use stack_t, defined in <signal.h>.

SEE ALSO
     sigaction(2), setjmp(3), compat(5)

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

4.2 Berkeley Distribution        June 4, 1993        4.2 Berkeley Distribution