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
extern  int     foo();
extern  int     bar;

void main()
{
        (void) printf("&foo = %x\n", &foo);
        (void) printf("&bar = %x\n", &bar);
}
$ cat mapfile
{
        global:
                foo = FUNCTION V0x400;
                bar = DATA V0x800;
};
$ cc -o prog -M mapfile main.c
$ prog
&foo = 400
&bar = 800
$ nm -x prog | egrep "foo$|bar$"
[37]    |0x00000800|0x00000000|OBJT |GLOB |0x0  |ABS    |bar
[42]    |0x00000400|0x00000000|FUNC |GLOB |0x0  |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.