Redefine a symbol in a compiled elf object file

build.sh

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

CC=clang-9
LD=$CC

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

$CC -c redefine.c -o redefine.o $CFLAGS
$CC -c redefine_b.c -o redefine_b.o $CFLAGS
$CC -c redefine_c.c -o redefine_c.o $CFLAGS

# rename the moveme function(symbol) to setaside
llvm-objcopy-9 --redefine-sym moveme=setaside redefine_c.o redefine_c.o

$LD redefine.o redefine_b.o redefine_c.o -o redefine && \
    ./redefine

output.txt

$ ./redefine
success!
this function is called setaside() by the time linking happens

redefine.c

void moveme(void);
void setaside(void);

int main(void) {

    moveme();

    setaside();
    
    return 0;
}

redefine_b.c

#include<stdio.h>

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

redefine_c.c

#include <stdio.h>

void moveme(void) {
    puts("this function is called setaside() by the time linking happens");
}