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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 *oss);

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

       sigaltstack():
           _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
           _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
           || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L

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  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
       sigaltstack().   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:

       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.

RETURN VALUE

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

ERRORS

       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).

CONFORMING TO

       SUSv2, SVr4, POSIX.1-2001.

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():

           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 */;

SEE ALSO

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

COLOPHON

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