Symbol Resolution

Symbol resolution runs the entire spectrum, from simple and intuitive to complex and perplexing. Most resolutions are carried out silently by the link-editor. However, some relocations can be accompanied by warning diagnostics, while others can result in a fatal error condition.

The most common simple resolutions involve binding symbol references from one object to symbol definitions within another object. This binding can occur between two relocatable objects, or between a relocatable object and the first definition found in a shared object dependency. Complex resolutions typically occur between two or more relocatable objects.

The resolution of two symbols depends on their attributes, the type of file that provides the symbol, and the type of file being generated. For a complete description of symbol attributes, see Symbol Table Section. For the following discussions, however, three basic symbol types are identified.

• Undefined – Symbols that have been referenced in a file but have not been assigned a storage address.

• Tentative – Symbols that have been created within a file but have not yet been sized, or allocated in storage. These symbols appear as uninitialized C symbols, or FORTRAN COMMON blocks within the file.

• Defined – Symbols that have been created, and assigned storage addresses and space within the file.

In its simplest form, symbol resolution involves the use of a precedence relationship. This relationship has defined symbols dominate tentative symbols, which in turn dominate undefined symbols.

The following example of C code shows how these symbol types can be generated. Undefined symbols are prefixed with u_. Tentative symbols are prefixed with t_. Defined symbols are prefixed with d_.

$ cat main.c
extern int u_bar;
extern int u_foo();

int t_bar;
int d_bar = 1;

int d_foo()
{
        return (u_foo(u_bar, t_bar, d_bar));
}
$ cc -o main.o -c main.c
$ elfdump -s main.o

Symbol Table Section:  .symtab
    index  value   size  type bind oth ver shndx          name
    ....
      [7]      0      0  FUNC GLOB  D    0 UNDEF          u_foo
      [8]   0x10   0x40  FUNC GLOB  D    0 .text          d_foo
      [9]    0x4    0x4  OBJT GLOB  D    0 COMMON         t_bar
     [10]      0    0x4  NOTY GLOB  D    0 UNDEF          u_bar
     [11]      0    0x4  OBJT GLOB  D    0 .data          d_bar