This is proc/setjmp_vars.c (Listing 6-6, page 136), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* setjmp_vars.c

   Compiling this program with and without optimization yields different
   results, since the optimizer reorganizes code and variables in a manner
   that does not take account of the dynamic flow of control established by
   a long jump.

   Try looking at the assembler source (.s) for the unoptimized (cc -S)
   and optimized (cc -O -S) versions of this program to see the cause
   of these differences.
#include <stdio.h>
#include <stdlib.h>
#include <setjmp.h>

static jmp_buf env;
static void
doJump(int nvar, int rvar, int vvar)
    printf("Inside doJump(): nvar=%d rvar=%d vvar=%d\n", nvar, rvar, vvar);
    longjmp(env, 1);
main(int argc, char *argv[])
    int nvar;
    register int rvar;          /* Allocated in register if possible */
    volatile int vvar;          /* See text */

    nvar = 111;
    rvar = 222;
    vvar = 333;

    if (setjmp(env) == 0) {     /* Code executed after setjmp() */
        nvar = 777;
        rvar = 888;
        vvar = 999;
        doJump(nvar, rvar, vvar);

    } else {                    /* Code executed after longjmp() */

        printf("After longjmp(): nvar=%d rvar=%d vvar=%d\n", nvar, rvar, vvar);



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