SIGALTSTACK(2)						     Linux Programmer's Manual						    SIGALTSTACK(2)

NAME

       sigaltstack - set and/or get signal stack context

SYNOPSIS

       #include <signal.h>

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

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       sigaltstack():
	   _BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
	   || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L

DESCRIPTION

       sigaltstack()  allows a process to define a new alternate signal stack and/or retrieve the state of an existing alternate signal stack.	An
       alternate signal stack is used during the execution of a signal handler if the establishment of that handler (see  sigaction(2))  requested
       it.

       The normal sequence of events for using an alternate signal stack is the following:

       1. Allocate an area of memory to be used for the alternate signal stack.

       2. Use sigaltstack() to inform the system of the existence and location of the alternate signal stack.

       3. When	establishing  a  signal  handler using sigaction(2), inform the system that the signal handler should be executed on the alternate
	  signal stack by specifying the SA_ONSTACK flag.

       The ss argument is used to specify a new alternate signal stack, while the oss argument is used to retrieve information about the currently
       established  signal  stack.   If  we are interested in performing just one of these tasks then the other argument can be specified as NULL.
       Each of these arguments is a structure of the following type:

	   typedef struct {
	       void  *ss_sp;	 /* Base address of stack */
	       int    ss_flags;  /* Flags */
	       size_t ss_size;	 /* Number of bytes in stack */
	   } stack_t;

       To establish a new alternate signal stack, ss.ss_flags is set to zero, and ss.ss_sp and ss.ss_size specify the starting address and size of
       the stack.  The constant SIGSTKSZ is defined to be large enough to cover the usual size requirements for an alternate signal stack, and the
       constant MINSIGSTKSZ defines the minimum size required to execute a signal handler.

       When a signal handler is invoked on the alternate stack, the kernel automatically aligns the  address  given  in  ss.ss_sp  to  a  suitable
       address boundary for the underlying hardware architecture.

       To disable an existing stack, specify ss.ss_flags as SS_DISABLE.  In this case, the remaining fields in ss are ignored.

       If oss is not NULL, then it is used to return information about the alternate signal stack which was in effect prior to the call to sigalt-
       stack().  The oss.ss_sp and oss.ss_size fields return the starting address and size of that stack.  The oss.ss_flags may return	either	of
       the following values:

       SS_ONSTACK
	      The  process  is	currently  executing  on the alternate signal stack.  (Note that it is not possible to change the alternate signal
	      stack if the process is currently executing on it.)

       SS_DISABLE
	      The alternate signal stack is currently disabled.

RETURN VALUE

       sigaltstack() returns 0 on success, or -1 on failure with errno set to indicate the error.

ERRORS

       EFAULT Either ss or oss is not NULL and points to an area outside of the process's address space.

       EINVAL ss is not NULL and the ss_flags field contains a nonzero value other than SS_DISABLE.

       ENOMEM The specified size of the new alternate signal stack (ss.ss_size) was less than MINSTKSZ.

       EPERM  An attempt was made to change the alternate signal stack while it was active (i.e., the process was already executing on the current
	      alternate signal stack).

CONFORMING TO

       SUSv2, SVr4, POSIX.1-2001.

NOTES

       The  most common usage of an alternate signal stack is to handle the SIGSEGV signal that is generated if the space available for the normal
       process stack is exhausted: in this case, a signal handler for SIGSEGV cannot be invoked on the process stack; if we wish to handle it,	we
       must use an alternate signal stack.

       Establishing an alternate signal stack is useful if a process expects that it may exhaust its standard stack.  This may occur, for example,
       because the stack grows so large that it encounters the upwardly growing heap, or it reaches  a	limit  established  by	a  call  to  setr-
       limit(RLIMIT_STACK, &rlim).  If the standard stack is exhausted, the kernel sends the process a SIGSEGV signal.	In these circumstances the
       only way to catch this signal is on an alternate signal stack.

       On most hardware architectures supported by Linux, stacks grow downward.  sigaltstack() automatically takes account  of	the  direction	of
       stack growth.

       Functions called from a signal handler executing on an alternate signal stack will also use the alternate signal stack.	(This also applies
       to any handlers invoked for other signals while the process is executing on the alternate signal stack.)  Unlike the  standard  stack,  the
       system  does  not automatically extend the alternate signal stack.  Exceeding the allocated size of the alternate signal stack will lead to
       unpredictable results.

       A successful call to execve(2) removes any existing alternate signal stack.  A child process created via fork(2) inherits  a  copy  of  its
       parent's alternate signal stack settings.

       sigaltstack()  supersedes  the  older  sigstack()  call.  For backward compatibility, glibc also provides sigstack().  All new applications
       should be written using sigaltstack().

   History
       4.2BSD had a sigstack() system call.  It used a slightly different struct, and had the major disadvantage that the caller had to  know  the
       direction of stack growth.

EXAMPLE

       The following code segment demonstrates the use of sigaltstack():

	   stack_t ss;

	   ss.ss_sp = malloc(SIGSTKSZ);
	   if (ss.ss_sp == NULL)
	       /* Handle error */;
	   ss.ss_size = SIGSTKSZ;
	   ss.ss_flags = 0;
	   if (sigaltstack(&ss, NULL) == -1)
	       /* Handle error */;

SEE ALSO

       execve(2), setrlimit(2), sigaction(2), siglongjmp(3), sigsetjmp(3), signal(7)

COLOPHON

       This  page is part of release 3.53 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2010-09-26							    SIGALTSTACK(2)