Address of externed array differs between compilation units

Question

I have an array foo declared in a C file, which is then declared as extern in the corresponding header. I then access foo from another file, but the address of the first element in the array is an invalid address, and differs from that in the compilation unit where foo is defined. This has been built from the Pebble SDK, using ARM CodeSourcery compilers.

Example code:

test1.h

extern int foo[];
void testRoutine();

test1.c

#include <pebble.h>
#include "test1.h"

void testRoutine()
{
    APP_LOG(APP_LOG_LEVEL_INFO, "Hello World! foo: 0x%x", (unsigned int)&foo[0]);
}

int foo[] = {1,2,3,4,5,6};

main.c

#include <pebble.h>
#include "test1.h"

int main()
{
    APP_LOG(APP_LOG_LEVEL_INFO, "Start of test");
    testRoutine(); // Print the address of foo from the perspective of test1.c
    APP_LOG(APP_LOG_LEVEL_INFO, "foo: 0x%x", (unsigned int)&foo[0]);

    return 0;
}

Expected output:

Start of test
Hello World! foo: 0x12345678
foo: 0x12345678

Actual output:

Start of test
Hello World! foo: 0x20081234
foo: 0x21941234 (placeholders, I can't get the real addresses right now)

This code works as expected in standard x86 GCC, but does not work on the ARM compiler. Have I messed up the code, or is there a compiler bug here?

Answer 1

I tried to reproduce your problem without success on Debian 64 bits:

Main.c:

#include <stdio.h>
#include "test1.h"

int main()
{
    testRoutine(); // Print the address of foo from the perspective of test1.c
    printf("foo: 0x%x\n", (unsigned int)&foo[0]);
    return 0;
}

test1.h:

extern int foo[];
void testRoutine();

test1.c:

#include <stdio.h> 
#include "test1.h"                                                    

void testRoutine()                                                    
{                                                                     
    printf("Hello World! foo: 0x%x\n", (unsigned int)&foo[0]);        
}                                                                     

int foo[] = {1,2,3,4,5,6};

The result is:

$ ./program 
Hello World! foo: 0x600970
foo: 0x600970

However gcc warnings are explicit:

test1.c:7:38: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]

You should try with:

APP_LOG(APP_LOG_LEVEL_INFO, "foo: 0x%p", &foo[0]);

Edit: Since you said you are running it on a 32 bits platform where sizeof(unsigned int)==sizeof(int *), the cast is not the problem. There is some other problem in your program unrelated to this code (stack corruption?).