How to place a variable at a given absolute address in memory (with GCC)

Question

The RealView ARM C Compiler supports placing a variable at a given memory address using the variable attribute at(address):

int var __attribute_         


        

Answer 1

Minimal runnable linker script example

The technique was mentioned at: https://stackoverflow.com/a/4081574/895245 but now I will now provide a concrete example.

main.c

#include 

int myvar __attribute__((section(".mySection"))) = 0x9ABCDEF0;

int main(void) {
    printf("adr %p\n", (void*)&myvar);
    printf("val 0x%x\n", myvar);
    myvar = 0;
    printf("val 0x%x\n", myvar);
    return 0;
}

link.ld

SECTIONS
{
  .mySegment 0x12345678 : {KEEP(*(.mySection))}
}

GitHub upstream.

Compile and run:

gcc -fno-pie -no-pie -o main.out -std=c99 -Wall -Wextra -pedantic link.ld main.c
./main.out

Output:

adr 0x12345678
val 0x9abcdef0
val 0x0

So we see that it was put at the desired address.

I cannot find where this is documented in the GCC manual, but the following syntax:

gcc link.ld main.c

seems to append the given linker script to the default one that would be used.

-fno-pie -no-pie is required, because the Ubuntu toolchain is now configured to generate PIE executables by default, which leads the Linux kernel to place the executable on a different address every time, which messes with our experiment. See also: What is the -fPIE option for position-independent executables in gcc and ld?

TODO: compilation produces a warning:

/usr/bin/x86_64-linux-gnu-ld: warning: link.ld contains output sections; did you forget -T?

Am I doing something wrong? How to get rid of it? See also: How to remove warning: link.res contains output sections; did you forget -T?

Tested on Ubuntu 18.10, GCC 8.2.0.