Can Valid Code in Both C and C Behave Differently When Compiled with Language-Specific Compilers?

Can Code Valid in Both C and C Behave Differently with Language-Specific Compilers?

C and C share many similarities, but notable differences exist. This raises the question of whether valid code in both languages might exhibit distinct behaviors when compiled using specific compilers for each language.

Ignoring preprocessor-related hacks and assuming uniformity in implementation-defined aspects, one scenario where such behavior discrepancy arises is the handling of function calls and object declarations.

In this context, the example below demonstrates the disparity between C and C :

#include <stdio.h>

struct f { int x; };

int main() {
    f();
}

int f() {
    return printf("hello");
}

In C , this code triggers the creation of a temporary f object that is immediately destroyed. Thus, it produces no output.

In contrast, C90 allows undeclared functions to be called. As a result, the code will output "hello" when compiled with a C90 compiler.

This difference is rooted in the fact that C90 treats the declaration f() as a function call, while C interprets it as the declaration of an f object. This distinction highlights the subtle nuances that can lead to varying behaviors across different programming languages.

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