Ubuntu Manpage: sigaltstack - set and/or get signal stack context

NAME

       sigaltstack - set and/or get signal stack context

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <signal.h>

       int sigaltstack(const stack_t *_Nullable restrict ss,
                       stack_t *_Nullable restrict old_ss);

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

       sigaltstack():
           _XOPEN_SOURCE >= 500
               || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
               || /* glibc <= 2.19: */ _BSD_SOURCE

DESCRIPTION

       sigaltstack()  allows  a  thread  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 old_ss  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.

       The stack_t type used to type the arguments of this function is defined as follows:

           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, the fields of this structure are set as follows:

       ss.ss_flags
              This field contains either 0, or the following flag:

              SS_AUTODISARM (since Linux 4.7)
                     Clear the alternate signal stack settings on entry to the signal handler.  When the  signal
                     handler returns, the previous alternate signal stack settings are restored.

                     This  flag  was  added in order to make it safe to switch away from the signal handler with
                     swapcontext(3).  Without this flag, a subsequently handled signal will corrupt the state of
                     the   switched-away  signal  handler.   On  kernels  where  this  flag  is  not  supported,
                     sigaltstack() fails with the error EINVAL when this flag is supplied.

       ss.ss_sp
              This field specifies the starting address of the stack.  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.

       ss.ss_size
              This field specifies the 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.

       To disable an existing stack, specify ss.ss_flags as SS_DISABLE.  In this case, the  kernel  ignores  any
       other flags in ss.ss_flags and the remaining fields in ss.

       If  old_ss  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 old_ss.ss_sp  and  old_ss.ss_size  fields  return  the
       starting address and size of that stack.  The old_ss.ss_flags may return either of the following values:

       SS_ONSTACK
              The thread is currently executing on the alternate signal stack.  (Note that it is not possible to
              change the alternate signal stack if the thread is currently executing on it.)

       SS_DISABLE
              The alternate signal stack is currently disabled.

              Alternatively, this value is returned if the thread is currently executing on an alternate  signal
              stack  that was established using the SS_AUTODISARM flag.  In this case, it is safe to switch away
              from the signal handler  with  swapcontext(3).   It  is  also  possible  to  set  up  a  different
              alternative signal stack using a further call to sigaltstack().

       SS_AUTODISARM
              The alternate signal stack has been marked to be autodisarmed as described above.

       By  specifying  ss  as  NULL, and old_ss as a non-NULL value, one can obtain the current settings for the
       alternate signal stack without changing them.

RETURN VALUE

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

ERRORS

       EFAULT Either ss or old_ss 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 an invalid flag.

       ENOMEM The specified size of the new alternate signal stack ss.ss_size was less than MINSIGSTKSZ.

       EPERM  An attempt was made to change the alternate signal stack while it was active (i.e., the thread was
              already executing on the current alternate signal stack).

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │Interface                                                                     │ Attribute     │ Value   │
       ├──────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │sigaltstack()                                                                 │ Thread safety │ MT-Safe │
       └──────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       POSIX.1-2008.

       SS_AUTODISARM is a Linux extension.

HISTORY

       POSIX.1-2001, SUSv2, SVr4.

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 standard stack is exhausted: in this case, a signal handler for SIGSEGV
cannot be invoked on the standard stack; if we wish to handle it, we must use an alternate signal stack.

Establishing an alternate signal stack is useful if a thread 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 thread 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 thread 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. The same is also true for a
child process created using clone(2), unless the clone flags include CLONE_VM and do not include
CLONE_VFORK, in which case any alternate signal stack that was established in the parent is disabled in
the child process.

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.

BUGS

       In Linux 2.2 and earlier, the only flag that could be specified in ss.sa_flags was  SS_DISABLE.   In  the
       lead  up  to  the  release  of  the  Linux  2.4 kernel, a change was made to allow sigaltstack() to allow
       ss.ss_flags==SS_ONSTACK with the same meaning as ss.ss_flags==0 (i.e., the  inclusion  of  SS_ONSTACK  in
       ss.ss_flags  is a no-op).  On other implementations, and according to POSIX.1, SS_ONSTACK appears only as
       a reported flag in  old_ss.ss_flags.   On  Linux,  there  is  no  need  ever  to  specify  SS_ONSTACK  in
       ss.ss_flags,  and indeed doing so should be avoided on portability grounds: various other systems give an
       error if SS_ONSTACK is specified in ss.ss_flags.

EXAMPLES

       The following code segment demonstrates the  use  of  sigaltstack()  (and  sigaction(2))  to  install  an
       alternate signal stack that is employed by a handler for the SIGSEGV signal:

           stack_t ss;

           ss.ss_sp = malloc(SIGSTKSZ);
           if (ss.ss_sp == NULL) {
               perror("malloc");
               exit(EXIT_FAILURE);
           }

           ss.ss_size = SIGSTKSZ;
           ss.ss_flags = 0;
           if (sigaltstack(&ss, NULL) == -1) {
               perror("sigaltstack");
               exit(EXIT_FAILURE);
           }

           sa.sa_flags = SA_ONSTACK;
           sa.sa_handler = handler();      /* Address of a signal handler */
           sigemptyset(&sa.sa_mask);
           if (sigaction(SIGSEGV, &sa, NULL) == -1) {
               perror("sigaction");
               exit(EXIT_FAILURE);
           }