SETJMP(3C) (C Programming Language Utilities) SETJMP(3C)
NAME
setjmp, longjmp - non-local goto
SYNOPSIS
#include <setjmp.h>
int setjmp (env)
jmpbuf env;
void longjmp (env, val)
jmpbuf env;
int val;
DESCRIPTION
These functions are useful for dealing with errors and
interrupts encountered in a low-level subroutine of a
program.
setjmp saves its stack environment in env (whose type,
jmp_buf, is defined in the <setjmp.h> header file) for later
use by longjmp. It returns the value 0.
longjmp restores the environment saved by the last call of
setjmp with the corresponding env argument. After longjmp
is completed, program execution continues as if the
corresponding call of setjmp (which must not itself have
returned in the interim) had just returned the value val.
longjmp cannot cause setjmp to return the value 0. If
longjmp is invoked with a second argument of 0, setjmp will
return 1. At the time of the second return from setjmp, all
external and static variables have values as of the time
longjmp is called (see example). The values of register and
automatic variables are undefined.
In a future release, C language users will be able to
identify syntactically those automatic variables on whose
values they need to rely after the second return from
setjmp.
EXAMPLE
Page 1 May 1989
SETJMP(3C) (C Programming Language Utilities) SETJMP(3C)
#include <setjmp.h>
jmp_buf env;
int i = 0;
main ()
{
void exit();
if(setjmp(env) != 0) {
(void) printf("value of i on 2nd return from setjmp: %d\n", i);
exit(0);
}
(void) printf("value of i on 1st return from setjmp: %d\n", i);
i = 1;
g();
/*NOTREACHED*/
}
g()
{
longjmp(env, 1);
/*NOTREACHED*/
}
If the a.out resulting from this C language code is run, the
output will be:
value of i on 1st return from setjmp:0
value of i on 2nd return from setjmp:1
SEE ALSO
signal(2).
WARNING
If longjmp is called even though env was never primed by a
call to setjmp, or when the last such call was in a function
that has since returned, absolute chaos is guaranteed.
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SETJMP(3C) (C Programming Language Utilities) SETJMP(3C)
Page 3 May 1989