Override a function at link time by weakening it with objcopy

build.sh

#!/bin/sh -x
rm -f *.o weak

CC=clang-9
LD=$CC

CFLAGS="-fno-inline-functions -O0"

$CC -c weak.c -o weak.o $CFLAGS
$CC -c weak_b.c -o weak_b.o $CFLAGS
$CC -c weak_c.c -o weak_c.o $CFLAGS

# weaken the symbol moveme in weak_c.o
llvm-objcopy-9 --weaken-symbol=moveme weak_c.o weak_c.o

$LD weak.o weak_b.o weak_c.o -o weak && \
    ./weak

output.txt

$ ./weak
success!

weak.c

void moveme(void);

int main(void) {

    // whichever one was linked gets called
    moveme();
    
    return 0;
}

weak_b.c

#include<stdio.h>

void moveme(void) {
    puts("success!");
}

weak_c.c

#include <stdio.h>

void moveme(void) {
    puts("this function gets weakened before linking and this should not be called");
}