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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)):

       sigaltstack(): _BSD_SOURCE || _XOPEN_SOURCE >= 500

       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
       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:

	      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 non-zero 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‐
       wards.  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() inherits a copy of its  par‐
       ent's alternate signal stack settings.

       sigaltstack() supersedes the older sigstack() call.  For backwards 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 3.22 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2008-10-04			SIGALTSTACK(2)

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