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

       sigaltstack - set and/or	get signal stack context

       #include	<signal.h>

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

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

	   _BSD_SOURCE || _XOPEN_SOURCE	>= 500 ||
	   || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L

       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

       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 au-
       tomatically 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 al-
       ternate	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  ad-
       dress  and  size	 of that stack.	 The oss.ss_flags may return either of
       the following values:

	      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.)

	      The alternate signal stack is currently disabled.

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

       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).

       SUSv2, SVr4, POSIX.1-2001.

       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().

       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.

       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 */;

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

       This  page  is  part of release 3.74 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

Linux				  2010-09-26			SIGALTSTACK(2)


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