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    <p>On an x86 architecture (using gcc 3.4.6,) the return value of int functions is stored in the eax register. Floating-point numbers are returned via an XMM register or in ST0. [Edit: Thanks Bastien Leonard!]</p> <p>Void functions simply don't put anything into eax and return.</p> <pre><code>#include &lt;stdio.h&gt; int f1(void) { printf("f1\n"); return 9.0f; } void f2(void) { printf("f2\n"); } double f3(void) { return 10.0; } int main(void) { int x = f1(); f2(); double y = f3(); return 0; } </code></pre> <p>builds with gcc -S to:</p> <pre><code>main: pushl %ebp movl %esp, %ebp subl $24, %esp andl $-16, %esp movl $0, %eax addl $15, %eax addl $15, %eax shrl $4, %eax sall $4, %eax subl %eax, %esp call f1 movl %eax, -4(%ebp) call f2 call f3 fstpl -16(%ebp) movl $0, %eax leave ret .size main, .-main .section .note.GNU-stack,"",@progbits .ident "GCC: (GNU) 3.4.6" </code></pre> <p>with f2 defined:</p> <pre><code>f2: pushl %ebp movl %esp, %ebp subl $8, %esp subl $12, %esp pushl $.LC1 call printf addl $16, %esp leave ret .size f2, .-f2 </code></pre> <p>and f1 defined:</p> <pre><code>f1: pushl %ebp movl %esp, %ebp subl $8, %esp subl $12, %esp pushl $.LC0 call printf addl $16, %esp movl $9, %eax leave ret </code></pre> <p>If other people get different code from different versions of gcc, that would be interesting to see XD (Maybe in other situations int functions return on the stack...)</p> <p>I would imagine that in all implementations, though, a void function simply returns nothing. It either leaves the return registers alone and/or drops nothing on (or into) the stack.</p>
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    1. POWhat data type does memory see when I use void?
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    1. COOn x86 and x86_64 platforms on linux, int functions will never use the stack to return. Try returning a long long on x86, or a large struct (or class) on any platform if you want to see returns on the stack.
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    2. CO@ParoXoN: float and double are returned in ST0 or an XMM register (in your code f3's returned value is retrieved with FSTP, which "pops" ST0).
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    3. COAh that makes sense, thanks :) My mistake!
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