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           #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
                  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).


           SUSv2, SVr4, POSIX.1-2001.


           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.


           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),

    Linux 2010-09-26 SIGALTSTACK(2)


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