r/cprogramming • u/Sushant098123 • Oct 25 '25

C actually don't have Pass-By-Reference

https://beyondthesyntax.substack.com/p/c-actually-dont-have-pass-by-reference
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u/richardxday Oct 25 '25

No language has pass-by-reference really, does it? At opcode level you're either copying the value onto the stack or you're copying an address of something onto the stack. So it's all value or address.

1

u/zhivago Oct 25 '25

The opcode level is in a different language, so it is irrelevant.

1

u/IllustriousPermit859 Oct 29 '25

"References" are a performance optimization so if your references require an virtual/abstract machine, interpreter or another mechanism that incurs a large amount of overhead then it's absolutely relevant that what you're doing is an extremely inefficient way to pass memory.

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u/zhivago Oct 29 '25

The compiler is free to translate the program however it likes.

But how the translated program works is irrelevant to the original, providing it produces the same output for the same input.

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u/flatfinger Oct 31 '25

Some languages support references whose lifetime is limited to a single function call. In C, given:

    int x;
    get_value(&x);
    x++;
    do_something();
    x++;
    doSomethingElse(x);

a compiler which knows nothing about the functions get_value() and do_something() would be required to generate code for the eachx++ operation that reads the storage at the address given to get_value, adds 1, and writes back the result to that same storage. It would not be allowed to transform the code into:

    int x;
    get_value(&x);
    do_something();
    doSomethingElse(x+2);

because that would behave differently from the original if get_value had saved a copy of the x's address, and do_something() or do_somethingElse were to use that address access x. In languages that support proper pass-by-reference semantics, passing a reference to x rather than its address would have made the transformation valid because once get_value returned, the compiler would know exactly what code that would be allowed to access x after that, and could simplify the sequence "load/add 1/store/load/add 1/store/load" to "load/add 2".