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():
	   _XOPEN_SOURCE >= 500
	       || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
	       || /* Glibc versions <= 2.19: */ _BSD_SOURCE

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).

ATTRIBUTES
       For   an	  explanation	of   the  terms	 used  in  this	 section,  see
       attributes(7).

       +--------------+---------------+---------+
       |Interface     | Attribute     | Value	|
       +--------------+---------------+---------+
       |sigaltstack() | Thread safety | MT-Safe |
       +--------------+---------------+---------+

CONFORMING TO
       POSIX.1-2001, POSIX.1-2009, SUSv2, SVr4.

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	 grow-
       ing  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	 down-
       ward.   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 unpre-
       dictable 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 par-
       ent's alternate signal stack settings.

       sigaltstack() supersedes the older sigstack() call.  For backward  com-
       patibility,  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 4.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2016-03-15			SIGALTSTACK(2)