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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 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  follow-
       ing 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 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 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 sig-
       nal 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 informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-10-04                    SIGALTSTACK(2)