SIGALTSTACK(2)            Linux Programmer's Manual           SIGALTSTACK(2)

NAME         top

       sigaltstack - set and/or get signal stack context

SYNOPSIS         top

       #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

DESCRIPTION         top

       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.

RETURN VALUE         top

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

ERRORS         top

       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).

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

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

CONFORMING TO         top

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

NOTES         top

       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.

EXAMPLE         top

       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         top

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

COLOPHON         top

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Linux                            2016-03-15                   SIGALTSTACK(2)