выполнение нелокального перехода (performing a nonlocal goto)
Имя (Name)
setjmp, sigsetjmp, longjmp, siglongjmp - performing a nonlocal
goto
Синопсис (Synopsis)
#include <setjmp.h>
int setjmp(jmp_buf env);
int sigsetjmp(sigjmp_buf env, int savesigs);
noreturn void longjmp(jmp_buf env, int val);
noreturn void siglongjmp(sigjmp_buf env, int val);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
setjmp(): see NOTES.
sigsetjmp():
_POSIX_C_SOURCE
Описание (Description)
The functions described on this page are used for performing
"nonlocal gotos": transferring execution from one function to a
predetermined location in another function. The setjmp()
function dynamically establishes the target to which control will
later be transferred, and longjmp() performs the transfer of
execution.
The setjmp() function saves various information about the calling
environment (typically, the stack pointer, the instruction
pointer, possibly the values of other registers and the signal
mask) in the buffer env for later use by longjmp(). In this
case, setjmp() returns 0.
The longjmp() function uses the information saved in env to
transfer control back to the point where setjmp() was called and
to restore ("rewind") the stack to its state at the time of the
setjmp() call. In addition, and depending on the implementation
(see NOTES), the values of some other registers and the process
signal mask may be restored to their state at the time of the
setjmp() call.
Following a successful longjmp(), execution continues as if
setjmp() had returned for a second time. This "fake" return can
be distinguished from a true setjmp() call because the "fake"
return returns the value provided in val. If the programmer
mistakenly passes the value 0 in val, the "fake" return will
instead return 1.
sigsetjmp() and siglongjmp()
sigsetjmp() and siglongjmp() also perform nonlocal gotos, but
provide predictable handling of the process signal mask.
If, and only if, the savesigs argument provided to sigsetjmp() is
nonzero, the process's current signal mask is saved in env and
will be restored if a siglongjmp() is later performed with this
env.
Возвращаемое значение (Return value)
setjmp() and sigsetjmp() return 0 when called directly; on the
"fake" return that occurs after longjmp() or siglongjmp(), the
nonzero value specified in val is returned.
The longjmp() or siglongjmp() functions do not return.
Атрибуты (Attributes)
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│setjmp(), sigsetjmp() │ Thread safety │ MT-Safe │
├──────────────────────────────────────┼───────────────┼─────────┤
│longjmp(), siglongjmp() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
Стандарты (Conforming to)
setjmp(), longjmp(): POSIX.1-2001, POSIX.1-2008, C89, C99.
sigsetjmp(), siglongjmp(): POSIX.1-2001, POSIX.1-2008.
Примечание (Note)
POSIX does not specify whether setjmp() will save the signal mask
(to be later restored during longjmp()). In System V it will
not. In 4.3BSD it will, and there is a function _setjmp() that
will not. The behavior under Linux depends on the glibc version
and the setting of feature test macros. On Linux with glibc
versions before 2.19, setjmp() follows the System V behavior by
default, but the BSD behavior is provided if the _BSD_SOURCE
feature test macro is explicitly defined and none of
_POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE, _GNU_SOURCE, or
_SVID_SOURCE is defined. Since glibc 2.19, <setjmp.h> exposes
only the System V version of setjmp(). Programs that need the
BSD semantics should replace calls to setjmp() with calls to
sigsetjmp() with a nonzero savesigs argument.
setjmp() and longjmp() can be useful for dealing with errors
inside deeply nested function calls or to allow a signal handler
to pass control to a specific point in the program, rather than
returning to the point where the handler interrupted the main
program. In the latter case, if you want to portably save and
restore signal masks, use sigsetjmp() and siglongjmp(). See also
the discussion of program readability below.
The compiler may optimize variables into registers, and longjmp()
may restore the values of other registers in addition to the
stack pointer and program counter. Consequently, the values of
automatic variables are unspecified after a call to longjmp() if
they meet all the following criteria:
• they are local to the function that made the corresponding
setjmp() call;
• their values are changed between the calls to setjmp() and
longjmp(); and
• they are not declared as volatile.
Analogous remarks apply for siglongjmp().
Nonlocal gotos and program readability
While it can be abused, the traditional C "goto" statement at
least has the benefit that lexical cues (the goto statement and
the target label) allow the programmer to easily perceive the
flow of control. Nonlocal gotos provide no such cues: multiple
setjmp() calls might employ the same jmp_buf variable so that the
content of the variable may change over the lifetime of the
application. Consequently, the programmer may be forced to
perform detailed reading of the code to determine the dynamic
target of a particular longjmp() call. (To make the programmer's
life easier, each setjmp() call should employ a unique jmp_buf
variable.)
Adding further difficulty, the setjmp() and longjmp() calls may
not even be in the same source code module.
In summary, nonlocal gotos can make programs harder to understand
and maintain, and an alternative should be used if possible.
Caveats
If the function which called setjmp() returns before longjmp() is
called, the behavior is undefined. Some kind of subtle or
unsubtle chaos is sure to result.
If, in a multithreaded program, a longjmp() call employs an env
buffer that was initialized by a call to setjmp() in a different
thread, the behavior is undefined.
POSIX.1-2008 Technical Corrigendum 2 adds longjmp() and
siglongjmp() to the list of async-signal-safe functions.
However, the standard recommends avoiding the use of these
functions from signal handlers and goes on to point out that if
these functions are called from a signal handler that interrupted
a call to a non-async-signal-safe function (or some equivalent,
such as the steps equivalent to exit(3) that occur upon a return
from the initial call to main()), the behavior is undefined if
the program subsequently makes a call to a non-async-signal-safe
function. The only way of avoiding undefined behavior is to
ensure one of the following:
* After long jumping from the signal handler, the program does
not call any non-async-signal-safe functions and does not
return from the initial call to main().
* Any signal whose handler performs a long jump must be blocked
during every call to a non-async-signal-safe function and no
non-async-signal-safe functions are called after returning
from the initial call to main().
Смотри также (See also)
signal(7), signal-safety(7)
