4.2 Linker Sections

ld deals with just four kinds of sections, summarized below.

named sections

text section

data section

These sections hold your program. as and ld treat them as separate but equal sections. Anything you can say of one section is true of another. When the program is running, however, it is customary for the text section to be unalterable. The text section is often shared among processes: it contains instructions, constants and the like. The data section of a running program is usually alterable: for example, C variables would be stored in the data section.

bss section

This section contains zeroed bytes when your program begins running. It is used to hold uninitialized variables or common storage. The length of each partial program's bss section is important, but because it starts out containing zeroed bytes there is no need to store explicit zero bytes in the object file. The bss section was invented to eliminate those explicit zeros from object files.

absolute section

Address 0 of this section is always “relocated” to runtime address 0. This is useful if you want to refer to an address that ld must not change when relocating. In this sense we speak of absolute addresses being “unrelocatable”: they do not change during relocation.

undefined section

This “section” is a catch-all for address references to objects not in the preceding sections.

An idealized example of three relocatable sections follows. The example uses the traditional section names .text and .data. Memory addresses are on the horizontal axis.

                           +-----+----+--+
     partial program # 1:  |ttttt|dddd|00|
                           +-----+----+--+
     
                           text   data bss
                           seg.   seg. seg.
     
                           +---+---+---+
     partial program # 2:  |TTT|DDD|000|
                           +---+---+---+
     
                           +--+---+-----+--+----+---+-----+~~
     linked program:       |  |TTT|ttttt|  |dddd|DDD|00000|
                           +--+---+-----+--+----+---+-----+~~
     
         addresses:        0 ...