sigaltstack

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 *old_ss);

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

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

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 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:

       ss.ss_flags
              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 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.

       ss.ss_sp
              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.

       ss.ss_size
              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:

       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.

              Alternatively, this value is returned if the process 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().

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

ERRORS
       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 process was already executing on the
              current alternate signal stack).

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

       ┌──────────────┬───────────────┬─────────┐
       │Interface     Attribute     Value   │
       ├──────────────┼───────────────┼─────────┤
       │sigaltstack() │ Thread safety │ MT-Safe │
       └──────────────┴───────────────┴─────────┘
CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, SUSv2, SVr4.

       The SS_AUTODISARM flag is a Linux extension.

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
       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().

   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() (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) {
               perror("malloc");
               exit(EXIT_FAILURE);
           }

           ss.ss_size = SIGSTKSZ;
           ss.ss_flags = 0;
           if (sigaltstack(&ss, NULL) == -1) {
               perror("sigaltstack");
               exit(EXIT_FAILURE);
           }

           sa.sa_flags = SA_ONSTACK;
           sa.sa_handler = handler();      /* Address of a signal handler */
           sigemptyset(&sa.sa_mask);
           if (sigaction(SIGSEGV, &sa, NULL) == -1) {
               perror("sigaction");
               exit(EXIT_FAILURE);
           }

BUGS
       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.

SEE ALSO
       execve(2), setrlimit(2), sigaction(2), siglongjmp(3), sigsetjmp(3),
       signal(7)

COLOPHON
       This page is part of release 5.02 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
       https://www.kernel.org/doc/man-pages/.



Linux                             2017-11-08                    SIGALTSTACK(2)