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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

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 non-zero 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
       downwards.  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()  inherits  a  copy  of  its
       parent’s alternate signal stack settings.

       sigaltstack()  supersedes  the  older  sigstack()  call.  For backwards
       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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