sigaltstack(2) — Linux manual page


sigaltstack(2)             System Calls Manual            sigaltstack(2)

NAME         top

       sigaltstack - set and/or get signal stack context

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <signal.h>

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

   Feature Test Macro Requirements for glibc (see

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

DESCRIPTION         top

       sigaltstack() allows a thread 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
       structure 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 restored.

                     This flag was added in order to 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, sigaltstack() fails
                     with the error EINVAL when this flag is supplied.

              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 suitable address boundary for the underlying
              hardware architecture.

              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 return either of the following values:

              The thread is currently executing on the alternate signal
              stack.  (Note that it is not possible to change the
              alternate signal stack if the thread is currently
              executing on it.)

              The alternate signal stack is currently disabled.

              Alternatively, this value is returned if the thread is
              currently executing on an alternate signal stack that was
              established 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

              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
       obtain 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 thread 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 │

STANDARDS         top


       SS_AUTODISARM is a Linux extension.

HISTORY         top

       POSIX.1-2001, SUSv2, SVr4.

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 standard stack is exhausted: in this case, a signal handler
       for SIGSEGV cannot be invoked on the standard stack; if we wish
       to handle it, we must use an alternate signal stack.

       Establishing an alternate signal stack is useful if a thread
       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 thread 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
       thread 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.  The same
       is also true for a child process created using clone(2), unless
       the clone flags include CLONE_VM and do not include CLONE_VFORK,
       in which case any alternate signal stack that was established in
       the parent is disabled in the child process.

       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), signal(7)

Linux man-pages (unreleased)     (date)                   sigaltstack(2)

Pages that refer to this page: clone(2)execve(2)getrlimit(2)sigaction(2)sigreturn(2)syscalls(2)getcontext(3)makecontext(3)pthread_create(3)sigvec(3)pthreads(7)signal(7)