Defining Tentative Symbols

A mapfile can also be used to define a COMMON, or tentative, symbol. Unlike other types of symbol definition, tentative symbols do not occupy storage within a file, but define storage that must be allocated at runtime. Therefore, symbol definitions of this kind can contribute to the storage allocation of the output file being generated.

A feature of tentative symbols that differs from other symbol types is that their value attribute indicates their alignment requirement. A mapfile definition can therefore be used to realign tentative definitions that are obtained from the input files of a link-edit.

The following example shows the definition of two tentative symbols. The symbol foo defines a new storage region whereas the symbol bar is actually used to change the alignment of the same tentative definition within the file main.c.

$ cat main.c
#include <stdio.h>
extern  int     foo;
int             bar[0x10];

void main()
{
        (void) printf("&foo = 0x%p\n", &foo);
        (void) printf("&bar = 0x%p\n", &bar);
}
$ cat mapfile
$mapfile_version 2
SYMBOL_SCOPE {
        global:
                foo     { TYPE=COMMON; VALUE=0x4;   SIZE=0x200 };
                bar     { TYPE=COMMON; VALUE=0x102; SIZE=0x40 };
};
$ cc -o prog -M mapfile main.c
ld: warning: symbol 'bar' has differing alignments:
	(file mapfile value=0x102; file main.o value=0x4);
	largest value applied
$ prog
&foo = 0x21264
&bar = 0x21224
$ elfdump -sN.symtab prog | egrep 'foo$|bar$'
      [45]  0x00021224 0x00000040  OBJT GLOB  D    0 .bss           bar
      [69]  0x00021264 0x00000200  OBJT GLOB  D    0 .bss           foo

This symbol resolution diagnostic can be suppressed by using the link-editor's -t option.