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/usr/ucb/cc [flag . . . ] file . . .#include <setjmp.h>
int setjmp(jmp_buf env);
longjmp(jmp_buf env, int val);
int _setjmp(jmp_buf env);
_longjmp(jmp_buf env, int val);
int sigsetjmp(sigjmp_buf env, int savemask);
siglongjmp(sigjmp_buf env, int val);
setjmp saves its stack environment in env for later use by longjmp. A normal call to setjmp returns 0. 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).
longjmp 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 0, execution would continue as if the call of setjmp had returned 1. This ensures that a ``return'' from setjmp caused by a call to longjmp can be distinguished 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 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. This is especially a problem for programmers trying to write machine-independent C routines.
setjmp and longjmp save and restore the signal mask (see sigsetmask(3bsd)), while _setjmp and _longjmp manipulate only the C stack and registers. If the savemask flag to sigsetjmp is non-zero, the signal mask is saved, and a subsequent siglongjmp using the same env will restore the signal mask. If the savemask flag is 0, the signal mask is not saved, and a subsequent siglongjmp using the same env will not restore the signal mask. In all other ways, _setjmp and sigsetjmp function in the same way that setjmp does, and _longjmp and siglongjmp function in the same way that longjmp does.
None of these functions save or restore any floating-point status or control registers.
On some systems setjmp also saves the register environment. Therefore, 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(1)), the compiler will not attempt to assign variables to registers in routines that call setjmp.
longjmp never causes setjmp to return 0, so programmers should not depend on longjmp being able to cause setjmp to return 0.
The following code fragment indicates the flow of control of the setjmp and longjmp combination:
function declaration
...
jmp_buf my_environment;
...
if (setjmp(my_environment)) {
/* register variables have unpredictable values */
code after the return from longjmp
...
} else {
/* do not modify register vars in this leg of code */
this is the return from setjmp
...
}