setjmp, sigsetjmp, longjmp, siglongjmp - performing a nonlocal goto
int setjmp(jmp_buf env);
int sigsetjmp(sigjmp_buf env, int savesigs);
void longjmp(jmp_buf env, int val);
void siglongjmp(sigjmp_buf env, int val);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)
(): see NOTES.
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
() performs the transfer of execution.
() 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.
() function uses the information saved in env
transfer control back to the point where setjmp
() was called and to
restore ("rewind") the stack to its state at the time of the
() 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
Following a successful longjmp
(), execution continues as if
() 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.
() 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
nonzero, the process's current signal mask is saved in env
and will be
restored if a siglongjmp
() is later performed with this env
() and sigsetjmp
() return 0 when called directly; on the
"fake" return that occurs after longjmp
(), the nonzero value specified in val
() or siglongjmp
() functions do not return.
For an explanation of the terms used in this section, see attributes(7)
|setjmp (), sigsetjmp ()
|longjmp (), siglongjmp ()
(): POSIX.1-2001, POSIX.1-2008, C89, C99.
(): POSIX.1-2001, POSIX.1-2008.
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
feature test macro is explicitly defined and none of
, or _SVID_SOURCE
is defined. Since glibc 2.19,
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
() 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
(). See also the discussion of program readability below.
The compiler may optimize variables into registers, and longjmp
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
- they are local to the function that made the corresponding
- their values are changed between the calls to
setjmp() and longjmp(); and
- they are not declared as volatile.
Analogous remarks apply for siglongjmp
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
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
() call. (To make the programmer's life easier, each
() call should employ a unique jmp_buf
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.
If the function which called setjmp
() returns before longjmp
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
() 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().
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