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

       sigaltstack - set and/or get signal stack context

       #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

       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 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 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 au-
              tomatically  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 sig-
              nal 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 sigalt-
       stack().  The old_ss.ss_sp and old_ss.ss_size fields return the  start-
       ing address and size of that stack.  The old_ss.ss_flags may return ei-
       ther 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 chang-
       ing them.

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

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

       |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 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  grow-
       ing  heap,  or  it  reaches  a  limit  established  by  a call to setr-
       limit(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  down-
       ward.   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 unpre-
       dictable 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 par-
       ent's alternate signal stack settings.

       sigaltstack() supersedes the older sigstack() call.  For backward  com-
       patibility,  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.

       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) {

       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  im-
       plementations,  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.

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

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Linux                             2017-11-08                    SIGALTSTACK(2)
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