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 *oss);

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

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

       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  oss  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.  Each
       of these arguments is a structure of the following type:

           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, ss.ss_flags is set to  zero,
       and  ss.ss_sp  and  ss.ss_size specify the starting address and 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.

       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.

       To  disable  an  existing stack, specify ss.ss_flags as SS_DISABLE.  In
       this case, the remaining fields in ss are ignored.

       If oss 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 oss.ss_sp and  oss.ss_size  fields  return  the  starting
       address  and size of that stack.  The oss.ss_flags may return either 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.

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

       EFAULT Either  ss  or  oss 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 a  nonzero  value
              other than SS_DISABLE.

       ENOMEM The  specified size of the new alternate signal stack ss.ss_size
              was less than MINSTKSZ.

       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

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

       POSIX.1-2001, POSIX.1-2009, SUSv2, SVr4.

       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():

           stack_t ss;

           ss.ss_sp = malloc(SIGSTKSZ);
           if (ss.ss_sp == NULL)
               /* Handle error */;
           ss.ss_size = SIGSTKSZ;
           ss.ss_flags = 0;
           if (sigaltstack(&ss, NULL) == -1)
               /* Handle error */;

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

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Linux                             2015-07-23                    SIGALTSTACK(2)
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