setjmp(S)                      6 January 1993                      setjmp(S)


 Name

    setjmp, longjmp - non-local goto

 Syntax


    cc  . . .  -lc


    #include  <setjmp.h>

    int setjmp (env)
    jmp_buf env;

    void longjmp (env, val)
    jmp_buf env;
    int val;


 Description

    These functions are useful for dealing with errors and interrupts encoun-
    tered in a low-level subroutine of a program.

    setjmp saves its stack environment in env (whose type, jmpbuf, 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 exe-
    cution continues as if the corresponding call of setjmp 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 returns 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 syntacti-
    cally those automatic variables on whose values they need to rely after
    the second return from setjmp.

 Example


       #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 is:


    +  value of i on 1st return from setjmp: 0

    +  value of i on 2nd return from setjmp: 1


 Warning

    Problems may occur if longjmp is called before env is primed with a call
    to setjmp.

 Note

    If signals are to be saved as part of the environment the sigsetjmp and
    siglongjmp routines should be used instead.

 See also

    signal(S), sigsetjmp(S)

 Standards conformance

    longjmp  setjmp  conform with:
    AT&T SVID Issue 2;
    X/Open Portability Guide, Issue 3, 1989;
    ANSI X3.159-1989 Programming Language -- C;
    IEEE POSIX Std 1003.1-1990 System Application Program Interface (API) [C
    Language] (ISO/IEC 9945-1);
    and NIST FIPS 151-1.