Linker and Libraries Guide

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
extern  int     foo;
int             bar[0x10];

void main()
        (void) printf("&foo = %x\n", &foo);
        (void) printf("&bar = %x\n", &bar);
$ cat mapfile
                foo = COMMON V0x4 S0x200;
                bar = COMMON V0x100 S0x40;
$ cc -o prog -M mapfile main.c
ld: warning: symbol `bar' has differing alignments:
        (file mapfile value=0x100; file main.o value=0x4);
        largest value applied
$ prog
&foo = 20940
&bar = 20900
$ nm -x prog | egrep "foo$|bar$"
[37]    |0x00020900|0x00000040|OBJT |GLOB |0x0  |16     |bar
[42]    |0x00020940|0x00000200|OBJT |GLOB |0x0  |16     |foo

Note –

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