Programming Embedded Systems in C and C++

Linking 23

Most object files begin with a header that describes the sections that follow. Each

of these sections contains one or more blocks of code or data that originated

within the original source file. However, these blocks have been regrouped by the

compiler into related sections. For example, all of the code blocks are collected

into a section called text, initialized global variables (and their initial values) into

a section called data, and uninitialized global variables into a section called bss.

There is also usually a symbol table somewhere in the object file that contains the

names and locations of all the variables and functions referenced within the source

file. Parts of this table may be incomplete, however, because not all of the vari-

ables and functions are always defined in the same file. These are the symbols that

refer to variables and functions defined in other source files. And it is up to the

linker to resolve such unresolved references.

Linking

All of the object files resulting from step one must be combined in a special way

before the program can be executed. The object files themselves are individually

incomplete, most notably in that some of the internal variable and function refer-

ences have not yet been resolved. The job of the linker is to combine these object

files and, in the process, to resolve all of the unresolved symbols.

The output of the linker is a new