sigstack

Purpose

     Sets and gets signal stack context.

Syntax

     #include <signal.h>

     int sigstack (instack, outstack)
     struct sigstack *instack, *outstack;

Description

     The sigstack  system call  defines an alternate  stack on
     which signals are to be processed.

     If the value of the instack parameter is nonzero, then it
     points to  a sigstack structure, which  has the following
     members:

          caddr_t  ss_sp;
          int      ss_onstack;

     The value of instack"->"ss_sp specifies the stack pointer
     of the new  signal stack.  Since stacks  grow from numer-
     ically greater addresses to lower ones, the stack pointer
     passed to  the sigstack system  call should point  to the
     numerically  high  end of  the  stack  area to  be  used.
     instack"->"ss_onstack should  be set to 1  if the process
     is  currently  executing  on that  stack;  otherwise,  it
     should be 0.

     If the value  of the outstack parameter  is nonzero, then
     it points to a sigstack structure into which the sigstack
     system call stores the current signal stack state.

     If the  value of the instack  parameter is 0 (that  is, a
     NULL pointer),  then the signal  stack state is  not set.
     If the  value of  the outstack parameter  is 0,  then the
     previous signal stack state is not reported.

     When  a signal  occurs whose  handler  is to  run on  the
     signal stack, the system checks  to see if the process is
     already executing  on that  stack.  If  so, then  it con-
     tinues to do so even  after the handler returns.  If not,
     then the signal handler runs on the signal stack, and the
     original stack is restored when the handler returns.

     Use the  sigvec system call  to specify whether or  not a
     given signal's  handler routine is  to run on  the signal
     stack.

     Warning:  A signal stack  does not automatically increase
     in size as a normal  stack does.  If the stack overflows,
     unpredictable results may occur.

Return Value

     Upon successful completion, a value of 0 is returned.  If
     the sigstack system call fails, a value of -1 is returned
     and errno is set to indicate the error.

Diagnostics

     The  sigstack  system call  fails  and  the signal  stack
     context remains unchanged if the following is true:

     EFAULT     The instack or outstack parameter &pointsout..

Related Information

     In this book:  "signal," "sigvec," and "setjmp, longjmp."