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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).

       ┌──────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├──────────────┼───────────────┼─────────┤
       │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/.