SIGALTSTACK(2-SVR4) RISC/os Reference Manual  SIGALTSTACK(2-SVR4)



NAME
     sigaltstack - set or get signal alternate stack context

SYNOPSIS
     #include <signal.h>

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

DESCRIPTION
     sigaltstack allows users to define an alternate stack area
     on which signals are to be processed.  If ss is non-zero, it
     specifies a pointer to, and the size of a stack area on
     which to deliver signals, and tells the system if the pro-
     cess is currently executing on that stack. When a signal's
     action indicates its handler should execute on the alternate
     signal stack [specified with a sigaction(2) call], the sys-
     tem 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 alter-
     nate signal stack for the duration of the signal handler's
     execution.

     The structure sigaltstack includes the following members.

          int  *ss_sp
          long ss_size
          int  ss_flags

     If ss is not NULL, it points to a structure specifying the
     alternate signal stack that will take effect upon return
     from sigaltstack.  The ss_sp and ss_size fields specify the
     new base and size of the stack, which is automatically
     adjusted for direction of growth and alignment.  The
     ss_flags field specifies the new stack state and may be set
     to the following:

     SS_DISABLE    The stack is to be disabled and ss_sp and
                   ss_size are ignored.  If SS_DISABLE is not
                   set, the stack will be enabled.

     If oss is not NULL, it points to a structure specifying the
     alternate signal stack that was in effect prior to the call
     to sigaltstack.  The ss_sp and ss_size fields specify the
     base and size of that stack.  The ss_flags field specifies
     the stack's state, and may contain the following values:

     SS_ONSTACK    The process is currently executing on the
                   alternate signal stack.  Attempts to modify
                   the alternate signal stack while the process
                   is executing on it will fail.

     SS_DISABLE    The alternate signal stack is currently



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SIGALTSTACK(2-SVR4) RISC/os Reference Manual  SIGALTSTACK(2-SVR4)



                   disabled.

     sigaltstack fails if any of the following is true:

     EFAULT        Either ss or oss points outside the process's
                   allocated address space.

     EINVAL        An attempt was made to disable an active stack
                   or the ss_flags field specifies invalid flags.

     ENOMEM        The size of the alternate stack area is less
                   than MINSIGSTKSZ.

NOTES
     The value SIGSTKSZ is defined to be the number of bytes that
     would be used to cover the usual case when allocating an
     alternate stack area.  The value MINSIGSTKSZ is defined to
     be the minimum stack size for a signal handler. In computing
     an alternate stack size, a program should add that amount to
     its stack requirements to allow for the operating system
     overhead.

     The following code fragment is typically used to allocate an
     alternate stack.

          if ((sigstk.ss_sp = (char *)malloc(SIGSTKSZ)) == NULL)
               /* error return */;

          sigstk.ss_size = SIGSTKSZ;
          sigstk.ss_flags = 0;
          if (sigaltstack(&sigstk, (stack_t *)0) < 0)
               perror("sigaltstack");

SEE ALSO
     getcontext(2), sigaction(2), sigsetjmp(3C), ucontext(5).

DIAGNOSTICS
     On success, sigaltstack returns zero.  On failure, it
     returns -1 and sets errno to indicate the error.
















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