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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

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 specify-
          ing 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 under-
       lying 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 sig-
       nal stack which was in effect prior to the call to  sigaltstack().   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 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).

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 estab-
       lished by a call to setrlimit(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 downwards.   sigalt-
       stack() 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() inherits a copy of its parent's alternate signal
       stack settings.

       sigaltstack() supersedes the older sigstack() call.  For  backwards  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 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2008-10-04                    SIGALTSTACK(2)

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