Usage and Definition Tables in the Linker

Here are two files to be linked to create a single executable.

file1.c

/* file1.c */
#include <stdio.h>
int g; /* a global variable */
extern double dg; /* another global var, defined
                    in some other file */
void fctnOne();   /* a function prototype */

int main()
{
  int x;  /* a variable local to main
             (an automatic variable) */
  x = 3;
  dg = 3.14;
  g = 17;
  fctnOne();
  printf("x is %d, g is %d, dg is %f\n",
	 x, g, dg);
  return 0;
}

void fctnTwo()
{
  int x;
  x = 5;
  g = 11;
  dg = dg * 2;
}

file2.c

/* file2.c */
extern int g;
double dg;

void fctnTwo(); /* function prototype */
void fctnOne()
{
  int x = 44;
  g = x;
  dg = dg + 2;
  fctnTwo();
}

The compiler command
gcc file1.c file2.c
would produce two object files, named file1.o and file2.o. Each of these would have unresolved references, that is, references to data and functions whose addresses are not known, because they are in the other file.

Each object file would have two tables attached, a definition table which contains the address of all objects which are global, that is, could be referenced by a function in another file, and the usage table, which contains a list of all unresolved references. Here are the contents of the four tables.

file1.o def	file1.use	file2.def	file2.use
g	dg line 13	dg	g line 10
main	fctnOne line 15	fctnOne	fctnTwo line 12
fctnTwo	printf line 16
	gd line 16
	gd line 17
	gd line 26
	gd line 26