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sigaltstack(2)
NAME
  sigaltstack - set or get signal alternate stack context

SYNOPSIS
  #include <signal.h>

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

  The following function declaration does not conform to current standards
  and is supported only for backward compatibility:

  #include <signal.h>

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

STANDARDS
  Interfaces documented on this reference page conform to industry standards
  as follows:

  sigaltstack():  XPG4-UNIX

  Refer to the standards(5) reference page for more information about
  industry standards and associated tags.

PARAMETERS
  ss  If valid, specifies a pointer to a structure that indicates what will
      be in effect upon return from the call to the sigaltstack function.

  oss If not NULL, specifies the pointer to a structure that contains the
      alternate signal stack that was in effect prior to the call to the
      sigaltstack function.

DESCRIPTION
  This function enables another stack area to be defined where signals can be
  examined for their execution status and processed. If a signal's action,
  specified by the sigaction(2) function, indicates that a signal should
  execute on an alternate stack, that signal is examined for its processing
  status.  A process that is not currently executing on the signal stack is
  switched to an alternate stack for the duration of the handler's execution.

  The sigaltstack structure is set up as follows:

       void	  *ss_sp     /* SVID3 uses caddr_t ss_sp
       int	  ss_flags
       size_t	  ss_size

  The values for the fields are:

  ss_sp
      Signal stack pointer.

  ss_flags
      Specifies the new stack state and may be set to either SS_DISABLE or
      SS_ONSTACK as follows:

	·  If ss is not NULL, the new state may be set to SS_DISABLE, which
	   specifies that 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, the stack state may be either SS_ONSTACK or
	   SS_DISABLE.	The value SS_ONSTACK indicates that the process is
	   currently executing on the alternate stack and that any attempt to
	   modify it during execution will fail.  The value SS_DISABLE
	   indicates that the current signal stack is disabled.

  ss_size
      Specifies the size of the stack.

  The value SIGSTKSZ defines the average number of bytes used when allocating
  an alternate stack area.  The value MINSIGSTKSZ defines the minimum stack
  size for a signal handler.  When processing an alternate stack size, your
  program should include these values in the stack requirement to plan for
  the overhead of the operating system.

RETURN VALUES
  Upon successful completion, zero (0) is returned.  On error, the value -1
  is returned and errno is set to indicate the error.

ERRORS
  The sigaltstack() function sets errno to the specified values for the
  following conditions:

  [EINVAL]
      The ss parameter 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 MINSIGSTKSZ.

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

SEE ALSO
  Functions:  getcontext(2), sigaction(2)

  Routines:  sigsetjmp(3)

  Files:  ucontext(5)

  Standards:  standards(5)

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