Need help understanding this for loop code in C

Question

Consider the following code in C: ``` void main() { int a=0; for(printf("
A"); a; printf("
B")); printf("
C"); printf("
D"); } ``` When I compile it using Turb C++ version 3.0 and gcc-4.3.4, I get the following as the output in BOTH the cases : ``` A C D ``` However, if I compile the following code: ``` void main() { for(printf("
A"); 0; printf("
B")); printf("
C"); printf("
D"); } ``` The output by gcc-4.3.4 is the same as in the previous case but turbo c++ 3.0 produces the following output : ``` A B C D ``` First of all, I have no idea what's happening here! Plus, how come the output by the gcc compiler is the same for both the codes but in the case of turboc++ 3.0 compiler, the output is different? Can someone please shed some light? EDIT : Actually someone was asked this question in an interview for an IT company and when he failed to give the answer, the interviewer gave this explanation. But I find this to be stupid. How can you ask someone to use a "bug" as if it's a "f…

Accepted Answer

The TCC output for the 2nd example is wrong.

From the C99 standard:

The statement

for ( clause-1 ; expression-2 ; expression-3 ) statement

behaves as follows: The expression expression-2 is the controlling expression that is evaluated before each execution of the loop body. The expression expression-3 is evaluated as a void expression after each execution of the loop body. [...]

Obviously, there are no iterations here, so expression-3 should never be executed.

Similarly, in the C90 standard (or at least in a draft that I found), it says:

Except for the behavior of a continue statement in the loop body, the statement

```
     for (  expression-1 ;  expression-2 ;  expression-3 )  statement
```

and the sequence of statements

```
      expression-1 ;
     while ( expression-2) {
               statement
              expression-3 ;
     }
```

are equivalent.

Need help understanding this for loop code in C

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