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)):

       _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 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:

          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
   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().

   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),


   This  page  is  part of release 4.09 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


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