上节-Relocatable Object Files

Symbols and Symbol Tables

Each relocatable object module, m, has a symbol table that contains information about the symbols that are defined and referenced by m. In the context of a linker, there are three different kinds of symbols:

• Global symbols that are defined by module m and that can be referenced by other modules. Global linker symbols correspond to nonstatic C functions and global variables that are defined without the C static attribute.
• . Global symbols that are referenced by module m but defined by some other module. Such symbols are called externals and correspond to C functions and variables that are defined in other modules.
• Local symbols that are defined and referenced exclusively by module m. Some local linker symbols correspond to C functions and global variables that are defined with the static attribute. These symbols are visible anywhere within module m, but cannot be referenced by other modules. The sections in an object file and the name of the source file that corresponds to module m also get local symbols

It is important to realize that local linker symbols are not the same as local program variables. The symbol table in .symtab does not contain any symbols that correspond to local nonstatic program variables. These are managed at run time on the stack and are not of interest to the linker

Interestingly, local procedure variables that are defined with the C static
attribute are not managed on the stack. Instead, the compiler allocates space in
.data or .bss for each definition and creates a local linker symbol in the symbol
table with a unique name. For example, suppose a pair of functions in the same
module define a static local variable x:

/695