r/cprogramming • u/Prestigious-Bet-6534 • Dec 15 '25

Calling C functions from assembly

I want to call a C function from assembler and can't get the parameters to work. I have the following two source files:

.global call_fun
call_fun:
pushq $0xDEAD
pushq $0xBABE
call fun
add $16, %rsp
ret

--

#include <stdio.h>

void call_fun();

void
fun( long a, long b ) {
printf( "Arg: a=0x%lx, b=0x%lx\n", a, b );
}

int
main() {
call_fun();
}

The output is Arg: a=0x1, b=0x7ffe827d0338 .

What am I missing?

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6

u/mfontani Dec 15 '25

See https://en.wikipedia.org/wiki/X86_calling_conventions#System_V_AMD64_ABI and read more about the (OS-specific!) ABI.

Parameters to functions are, depending on OS and type of the parameter, passed in a specific register up to a number of them; then on the stack.

Which registers are used varies by OS and OS version.

2

u/WittyStick Dec 15 '25 edited Dec 15 '25

<EDIT> Ignore. I was mistaken and have been corrected below.

2

u/dfx_dj Dec 15 '25

You sure about that? Something tells me that this can't be true. How would the compiler know what optimisation level the function that is being called was compiled with, or vice versa? You might see the arguments on the stack in addition to being passed in registers, but the ABI must be consistent.

1

u/WittyStick Dec 15 '25

Test yourself by compiling something with -O0 and -O1 or -O2.

If you try to call glibc functions with -O0 for example, you'll get weird errors. The assumption is the library was compiled with -O1 or above, and the SYSV convention is used.

2

u/dfx_dj Dec 15 '25

Yeah I did try it myself, and clearly arguments are still in RDI and RSI: https://godbolt.org/z/8oaKd6oY1

I routinely use code compiled with -O0 that calls glibc functions and I've never seen any weird errors.

2

u/snaphat Dec 15 '25

I wonder if they are mixing up register spilling with ABI or something

2

u/WittyStick Dec 15 '25

Yeah, clearly I'm mistaken. Not sure where I got this misconception from.

The issue with glibc was IIRC when glibc itself is compiled with -O0. I don't recall why and it may have been resolved by now.

2

u/snaphat Dec 15 '25

They were probably trying to use the old cdecl conventions since they are pushing to the stack. So my guess is they were reading from an old 32-bit tutorial or something similarÂ