this post was submitted on 11 Feb 2025
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Programmer Humor

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unsafeCode (lemmy.ml)
submitted 3 days ago by cm0002@lemmy.world to c/programmer_humor@programming.dev
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[โ€“] xmunk@sh.itjust.works 21 points 3 days ago (1 children)

Just to clarify. It is defined behavior - there's plenty of undefined behavior in C but that ain't one of them.

[โ€“] FuckBigTech347@lemmygrad.ml 2 points 2 days ago

Interesting feature, I had no idea. I just verified this with gcc and indeed the return register is always set to 0 before returning unless otherwise specified.

spoiler

int main(void)
{
    int foo = 10;
}

produces:

push   %rbp
mov    %rsp,%rbp
movl   $0xa,-0x4(%rbp) # Move 10 to stack variable
mov    $0x0,%eax       # Return 0
pop    %rbp
ret

int main(void)
{
    int foo = 10;
    return foo;
}

produces:

push   %rbp
mov    %rsp,%rbp
movl   $0xa,-0x4(%rbp) # Move 10 to stack variable
mov    -0x4(%rbp),%eax # Return foo
pop    %rbp
ret