Linker and Libraries Guide

Defining Absolute Symbols

The following example shows how two absolute symbol definitions can be defined. These definitions are then used to resolve the references from the input file main.c.

$ cat main.c
#include <stdio.h>
extern  int     foo();
extern  int     bar;

void main()
        (void) printf("&foo = 0x%p\n", &foo);
        (void) printf("&bar = 0x%p\n", &bar);
$ cat mapfile
$mapfile_version 2
                foo     { TYPE=FUNCTION; VALUE=0x400 };
                bar     { TYPE=DATA;     VALUE=0x800 };
$ cc -o prog -M mapfile main.c
$ prog
&foo = 0x400
&bar = 0x800
$ elfdump -sN.symtab prog | egrep 'foo$|bar$'
      [45]  0x00000800 0x00000000  OBJT GLOB  D    0 ABS            bar
      [69]  0x00000400 0x00000000  FUNC GLOB  D    0 ABS            foo

When obtained from an input file, symbol definitions for functions or data items are usually associated with elements of data storage. A mapfile definition is insufficient to be able to construct this data storage, so these symbols must remain as absolute values. A simple mapfile definition that is associated with a size, but no value results in the creation of data storage. In this case, the symbol definition is accompanied with a section index. However, a mapfile definition that is accompanied with a value results in the creation of an absolute symbol. If a symbol is defined in a shared object, an absolute definition should be avoided. See Augmenting a Symbol Definition.