Setting a constant in rodata - c

Question

I am trying to understand how to set the value of a string in the rodata segment as loading it using an environment variable gives me issues.
I want to externally set a constant string in the rodata section. This function should be independent of the code executed. So, when I do
"objdump -c foo"
the rodata section must enlist this string without the file foo.c having to do it.
How do I set a constant in the .rodata section ?
Edit: Linux OS and using GCC
I cannot use an environment var as that would mean that the c code is modified, I want the c code untouched and add the constant, say "Goo" to the rodata segment.

Answer 1

Then you need to write a program that lets you modify the binary file.
Read the ELF file specifications.
Then write a program that modifies the ELF program and section headers and adds the data to the .rodata section.

Answer 2

I've managed to write a small bash script that does more or less what I think you want.
First let's consider this sample program:
test.c
#include <stdio.h>
const char message[1024] = "world";
int main()
{
printf("hello %s\n", message);
}
The target variable will be message. Note that I will not change the size of the variable, that would be a mess, you be careful to reserve as much memory as you will ever need.
Now the script:
patchsym
#!/bin/bash
# usage: patchsym PROGRAM SYMBOL < NEWCONTENT
EXE="$1"
SYMBOL="$2"
OFFS=$((0x$(objdump -t "$EXE" | grep " $SYMBOL$" | cut -d ' ' -f 1)))
OFFS=2176
dd of="$EXE" bs=1 seek=$OFFS conv=notrunc
The new message content will be:
newmsg
universe^#
(where ^# is actually a NUL character).
Now just do:
$ gcc test.c -o test
$ ./test
hello world
$ ./patchsym test message < newmsg
$ ./test
hello universe