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 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  han-
       dler.

       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 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 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 exe-
              cuting 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  avail-
       able  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 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.  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() 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 3.25 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                                                      2008-10-04                                             SIGALTSTACK(2)