Path: utzoo!utgpu!watserv1!ria!pruss
From: pruss@ria.ccs.uwo.ca (A.R. Pruss)
Newsgroups: comp.lang.c
Subject: Is jumping inside loops legal?
Summary: Can one legally jump into a loop, and are the results well defined?
Message-ID: <746@ria.ccs.uwo.ca>
Date: 2 Aug 90 20:27:19 GMT
Organization: University of Western Ontario, London
Lines: 47

I am wondering whether it is legal to jump into a loop, either via
a direct goto, or via a switch-case construct.

What I mean is which (if any) of the following two programs are illegal
and will their functioning be consistent on all legal compilers?

--- begin test1.c ---
#include <stdio.h>

int main()
{
 int j=5;
 switch(getchar())
 {
  case 'y':
  case 'z': puts("y or z depressed!");
	    for(j=0; j<5; j++)     /* here is the loop broken into */
  case 'Z':  puts("A 'z' (case unknown) key was pressed.");
	    break;
 }
 return 0;
}
--end test1.c--

--- begin test2.c ---
#include <stdio.h>

int main()
{
 int x=5;
 goto alpha;
 for(x=0; x<7; x++)   /* and here's the other loop being broken into */
alpha:
  putchar('.');
 return 0;
}
--end test2.c--

They both compile under Turbo C 2.0 (on a PC), MIPS C (under Irix),
gcc -pedantic, as well as passing lint with the exception of test2
having a `warning: statement not reached' (which also appeared under
TC 2.0).  Under Borland's new Turbo C++ 1.0 (on a PC, again), test1
refuses to compile (case not in switch), while test2 does indeed compile.

In all cases where the programs compiled, the output was the same.

pruss@ria.ccs.uwo.ca