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

NAME
     ssiiggaallttssttaacckk - set and/or get signal stack context

SYNOPSIS
     ##iinncclluuddee <>

     int
     ssiiggaallttssttaacckk(const stackt *restrict ss, stackt *restrict oss);

DESCRIPTION
     SSiiggaallttssttaacckk() 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 SADISABLE is set in ssflags, sssp and sssize are ignored and the
     signal stack will be disabled.  Trying to disable an active stack will
     cause ssiiggaallttssttaacckk 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
     ssflags field will contain the value SAONSTACK if the process is cur-
     rently on a signal stack and SADISABLE if the signal stack is currently
     disabled.

NNOOTTEESS
     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.sssp = malloc(SIGSTKSZ)) == NULL)
                   /* error return */
           sigstk.sssize = SIGSTKSZ;
           sigstk.ssflags = 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.

EERRRROORRSS
     SSiiggaallttssttaacckk() 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 ssflags member
               pointed to by ss contains flags other than SSDISABLE.

     [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
     ##iinncclluuddee <>
     ##iinncclluuddee <>

     The include file  is necessary.

     struct sigaltstack {
             char   *sssp;
             int     sssize;
             int     ssflags;
     };

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

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

CCOOMMPPAATTIIBBIILLIITTYY
     Use of the (obsolete) sigaltstack struct will cause compiler diagnostics.
     Use stackt, defined in .

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

HISTORY     The predecessor to ssiiggaallttssttaacckk, the ssiiggssttaacckk() system call, appeared in
     4.2BSD.

4.2 Berkeley Distribution        June 4, 1993        4.2 Berkeley Distribution