setjmp, sigsetjmp, longjmp, siglongjmp - performing a nonlocal goto
#include <setjmp.h>
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)):
setjmp(): see NOTES.
sigsetjmp(): _POSIX_C_SOURCE
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() 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.
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.
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 |
setjmp(),
longjmp(): POSIX.1-2001, POSIX.1-2008, C89, C99.
sigsetjmp(),
siglongjmp(): 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
_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().
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.
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().
signal(7),
signal-safety(7)