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

       sigaltstack - set and/or get signal stack context

       #include <signal.h>

       int sigaltstack(const stack_t *restrict ss, stack_t *restrict 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 <= 2.19: */ _BSD_SOURCE

       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

       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 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 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 obtain
       the current settings for the alternate signal stack without changing

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

       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 flag.

       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).

       For an explanation of the terms used in this section, see attributes(7).

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

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

       The SS_AUTODISARM flag is a Linux extension.

       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

       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

       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

       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.

       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.

       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) {

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

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Linux                              2021-03-22                     SIGALTSTACK(2)