Symbol Resolution for ELF

Question

I had a question regarding symbol resolution in C linking (in particular with the elf format).

Suppose I have two modules split into separate files module1.c and module2.c:

// module1.c
int main() {
    return 0;
}

==========================

// module2.c
int main = 3;

int p2() {
    return 0;
}

Compiling these two together will give me a linker error because there is a duplicate symbol for main in both modules. My question is, why does the linker not account for the fact that one is a function and one is a variable? This information definitely exists in the symbol tables for the two:

Symbol table '.symtab' contains 9 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS a.c
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    2 
     4: 0000000000000000     0 SECTION LOCAL  DEFAULT    3 
     5: 0000000000000000     0 SECTION LOCAL  DEFAULT    5 
     6: 0000000000000000     0 SECTION LOCAL  DEFAULT    6 
     7: 0000000000000000     0 SECTION LOCAL  DEFAULT    4 
     8: 0000000000000000    11 FUNC    GLOBAL DEFAULT    1 main

No version information found in this file.

===================================================================

Symbol table '.symtab' contains 10 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS b.c
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    2 
     4: 0000000000000000     0 SECTION LOCAL  DEFAULT    3 
     5: 0000000000000000     0 SECTION LOCAL  DEFAULT    5 
     6: 0000000000000000     0 SECTION LOCAL  DEFAULT    6 
     7: 0000000000000000     0 SECTION LOCAL  DEFAULT    4 
     8: 0000000000000000     4 OBJECT  GLOBAL DEFAULT    2 main
     9: 0000000000000000    11 FUNC    GLOBAL DEFAULT    1 p2

You can clearly see one is an object and the other is a function. My question is whether there is any specific reason linker's don't aim to make this distinction between functions and variables? Does this mean in a huge program made of many modules, you can never declare a global variable with the same name as a function?

Answer 1

It's just a rule of the C language: functions and objects don't have distinct namespaces.

So yes - you can't have a function and variable both with external linkage and the same name.

Note that you can give both file-scope variables and functions static linkage using the static keyword, and it's OK to have a function with static linkage in one file and a variable of the same name with static linkage in another file. This reduces the likelihood of conflict in large programs.