sigaltstack(2) — Linux manual page


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

NAME         top

       sigaltstack - set and/or get signal stack context

SYNOPSIS         top

       #include <signal.h>

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

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

           _XOPEN_SOURCE >= 500
               || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
               || /* Glibc versions <= 2.19: */ _BSD_SOURCE

DESCRIPTION         top

       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 old_ss 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.

       The stack_t type used to type the arguments of this function is
       defined as follows:

           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, the fields of this struc‐
       ture are set as follows:

              This field contains either 0, or the following flag:

              SS_AUTODISARM (since Linux 4.7)
                     Clear the alternate signal stack settings on entry to
                     the signal handler.  When the signal handler returns,
                     the previous alternate signal stack settings are re‐

                     This flag was added in order make it safe to switch
                     away from the signal handler with swapcontext(3).
                     Without this flag, a subsequently handled signal will
                     corrupt the state of the switched-away signal handler.
                     On kernels where this flag is not supported, sigalt‐
                     stack() fails with the error EINVAL when this flag is

              This field specifies the starting address of the stack.  When
              a signal handler is invoked on the alternate stack, the kernel
              automatically aligns the address given in ss.ss_sp to a suit‐
              able address boundary for the underlying hardware architec‐

              This field specifies the 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.

       To disable an existing stack, specify ss.ss_flags as SS_DISABLE.  In
       this case, the kernel ignores any other flags in ss.ss_flags and the
       remaining fields in ss.

       If old_ss 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 old_ss.ss_sp and old_ss.ss_size fields return the
       starting address and size of that stack.  The old_ss.ss_flags may re‐
       turn 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.

              Alternatively, this value is returned if the process is cur‐
              rently executing on an alternate signal stack that was estab‐
              lished using the SS_AUTODISARM flag.  In this case, it is safe
              to switch away from the signal handler with swapcontext(3).
              It is also possible to set up a different alternative signal
              stack using a further call to sigaltstack().

              The alternate signal stack has been marked to be autodisarmed
              as described above.

       By specifying ss as NULL, and old_ss as a non-NULL value, one can ob‐
       tain the current settings for the alternate signal stack without
       changing them.

RETURN VALUE         top

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

ERRORS         top

       EFAULT Either ss or old_ss 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 an invalid

       ENOMEM The specified size of the new alternate signal stack
              ss.ss_size was less than MINSIGSTKSZ.

       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).

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface     Attribute     Value   │
       │sigaltstack() │ Thread safety │ MT-Safe │

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008, SUSv2, SVr4.

       The SS_AUTODISARM flag is a Linux extension.

NOTES         top

       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
       downward.  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(2) inherits a copy of its
       parent's alternate signal stack settings.

       sigaltstack() supersedes the older sigstack() call.  For backward
       compatibility, 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.

BUGS         top

       In Linux 2.2 and earlier, the only flag that could be specified in
       ss.sa_flags was SS_DISABLE.  In the lead up to the release of the
       Linux 2.4 kernel, a change was made to allow sigaltstack() to allow
       ss.ss_flags==SS_ONSTACK with the same meaning as ss.ss_flags==0
       (i.e., the inclusion of SS_ONSTACK in ss.ss_flags is a no-op).  On
       other implementations, and according to POSIX.1, SS_ONSTACK appears
       only as a reported flag in old_ss.ss_flags.  On Linux, there is no
       need ever to specify SS_ONSTACK in ss.ss_flags, and indeed doing so
       should be avoided on portability grounds: various other systems give
       an error if SS_ONSTACK is specified in ss.ss_flags.

EXAMPLES         top

       The following code segment demonstrates the use of sigaltstack() (and
       sigaction(2)) to install an alternate signal stack that is employed
       by a handler for the SIGSEGV signal:

           stack_t ss;

           ss.ss_sp = malloc(SIGSTKSZ);
           if (ss.ss_sp == NULL) {

           ss.ss_size = SIGSTKSZ;
           ss.ss_flags = 0;
           if (sigaltstack(&ss, NULL) == -1) {

           sa.sa_flags = SA_ONSTACK;
           sa.sa_handler = handler();      /* Address of a signal handler */
           if (sigaction(SIGSEGV, &sa, NULL) == -1) {

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.09 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                            2020-06-09                   SIGALTSTACK(2)

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