setjmp(3UCB) SunOS/BSD Compatibility Library Functions setjmp(3UCB)NAME
setjmp, longjmp, _setjmp, _longjmp - non-local goto
SYNOPSIS
/usr/ucb/cc [ flag ... ] file ...
#include <setjmp.h>
int setjmp(env)
jmp_buf env;
void longjmp(env, val)
jmp_buf env;
int val;
int _setjmp(env)
jmp_buf env;
void _longjmp(env, val)
jmp_buf env;
int val;
DESCRIPTION
The setjmp() and longjmp() functions are useful for dealing with errors
and interrupts encountered in a low-level subroutine of a program.
The setjmp() function saves its stack environment in env for later use
by longjmp(). A normal call to setjmp() returns zero. setjmp() also
saves the register environment. If a longjmp() call will be made, the
routine which called setjmp() should not return until after the
longjmp() has returned control (see below).
The longjmp() function restores the environment saved by the last call
of setjmp(), and then returns in such a way that execution continues as
if the call of setjmp() had just returned the value val to the function
that invoked setjmp(); however, if val were zero, execution would con‐
tinue as if the call of setjmp() had returned one. This ensures that a
``return'' from setjmp() caused by a call to longjmp() can be distin‐
guished from a regular return from setjmp(). The calling function must
not itself have returned in the interim, otherwise longjmp() will be
returning control to a possibly non-existent environment. All memory-
bound data have values as of the time longjmp() was called. The CPU
and floating-point data registers are restored to the values they had
at the time that setjmp() was called. But, because the register stor‐
age class is only a hint to the C compiler, variables declared as reg‐
ister variables may not necessarily be assigned to machine registers,
so their values are unpredictable after a longjmp(). This is especially
a problem for programmers trying to write machine-independent C rou‐
tines.
The setjmp() and longjmp() functions save and restore the signal mask
while _setjmp() and _longjmp() manipulate only the C stack and regis‐
ters.
None of these functions save or restore any floating-point status or
control registers.
EXAMPLES
Example 1 Examples of setjmp() and longjmp().
The following example uses both setjmp() and longjmp() to return the
flow of control to the appropriate instruction block:
#include <stdio.h>
#include <setjmp.h>
#include <signal.h>
#include <unistd.h>
jmp_buf env; static void signal_handler();
main() {
int returned_from_longjump, processing = 1;
unsigned int time_interval = 4;
if ((returned_from_longjump = setjmp(env)) != 0)
switch (returned_from_longjump) {
case SIGINT:
printf("longjumped from interrupt %d\n",SIGINT);
break;
case SIGALRM:
printf("longjumped from alarm %d\n",SIGALRM);
break;
}
(void) signal(SIGINT, signal_handler);
(void) signal(SIGALRM, signal_handler);
alarm(time_interval);
while (processing) {
printf(" waiting for you to INTERRUPT (cntrl-C) ...\n");
sleep(1);
} /* end while forever loop */
}
static void signal_handler(sig)
int sig; {
switch (sig) {
case SIGINT: ... /* process for interrupt */
longjmp(env,sig);
/* break never reached */
case SIGALRM: ... /* process for alarm */
longjmp(env,sig);
/* break never reached */
default: exit(sig);
}
}
When this example is compiled and executed, and the user sends an
interrupt signal, the output will be:
longjumped from interrupt
Additionally, every 4 seconds the alarm will expire, signalling this
process, and the output will be:
longjumped from alarm
SEE ALSOcc(1B), sigvec(3UCB), setjmp(3C), signal(3C)NOTES
Use of these interfaces should be restricted to only applications writ‐
ten on BSD platforms. Use of these interfaces with any of the system
libraries or in multi-thread applications is unsupported.
BUGS
The setjmp() function does not save the current notion of whether the
process is executing on the signal stack. The result is that a
longjmp() to some place on the signal stack leaves the signal stack
state incorrect.
On some systems setjmp() also saves the register environment. There‐
fore, all data that are bound to registers are restored to the values
they had at the time that setjmp() was called. All memory-bound data
have values as of the time longjmp() was called. However, because the
register storage class is only a hint to the C compiler, variables
declared as register variables may not necessarily be assigned to
machine registers, so their values are unpredictable after a longjmp().
When using compiler options that specify automatic register allocation
(see cc(1B)), the compiler will not attempt to assign variables to reg‐
isters in routines that call setjmp().
The longjmp() function never causes setjmp() to return 0, so program‐
mers should not depend on longjmp() being able to cause setjmp() to
return 0.
SunOS 5.10 7 Apr 1993 setjmp(3UCB)
