Red Hat ENTERPRISE LINUX 3 - DEVELOPER TOOLS GUIDE Instrukcja Użytkownika Pobierz pdf (Strona 11)

Chapter 2. Basic Principles of the Tools 7

Special programs exist to manipulate object ﬁles. For example, objdump can disassemble an object

ﬁle back into assembler source code and ar can group together multiple object ﬁles into an archive or

library ﬁle.

2.6.2. Assembler Directives

Assembler directives are commands inside the assembler source ﬁles that control how the object ﬁle

is generated. They are also known as pseudo-ops, or pseudo-operations, because they can look like

commands in the assembler programming language.

Assembler directives always start with a period (.). The rest of their name is letters, usually in lower

case. They have a wide range of different uses, such as specifying alignments, inserting constants into

the output, and selecting in which sections of the output ﬁle the assembled machine instructions are

placed.

2.7. ld, the GNU Linker

The GNU linker, ld, combines multiple object ﬁles together and creates an executable program from

them. It does this by resolving references between the different object ﬁles, grouping together similar

sections in the object ﬁles into one place, arranging for these sections to be loaded at the correct

addresses in memory, and generating the necessary header information at the start of a ﬁle that allows

it to be run.

The linker moves blocks of bytes of your program to their load-time addresses. These blocks slide

to their addresses as rigid units; their length does not change and neither does the order of the bytes

within them. Such a rigid unit is called a section. Assigning runtime addresses to sections is called

relocation. It includes the task of adjusting mentions of object-ﬁle addresses so they refer to the proper

runtime addresses.

Each section in an object ﬁle has a name and a size. Most sections also have an associated block

of data, known as the section contents. A section may be marked as allocatable, meaning that space

should be reserved for it in memory when the executable starts running. A section may also be marked

as loadable, meaning that its contents should be loaded into memory when the executable starts. A

section which is allocatable but not loadable will have a zero-ﬁlled area of memory created for it.

A section, which is neither loadable nor allocatable, typically contains some sort of debugging infor-

mation. Every loadable or allocatable output section has two addresses associated with it. The ﬁrst is

the virtual memory address (VMA), the address the section will have when the executable is running.

The second is the load memory address (LMA), which is the address in memory where the section will

loaded. In most cases the two addresses will be the same. An example of when they might be different

is when a data section is loaded into ROM, and then copied into RAM when the program starts. This

technique is often used to initialize global variables in a ROM-based system. In this case, the ROM

address would be the LMA, and the RAM address would be the VMA. To review the sections in an

object ﬁle, use the objdump binary utility with the -h option.

Every object ﬁle also has a list of symbols, known as the symbol table. A symbol may be deﬁned or

undeﬁned. Each symbol has a name, and each deﬁned symbol has an address. If you compile a C or

C++ program into an object ﬁle, you get a deﬁned symbol for every deﬁned function and global or

static variable. Every undeﬁned function or global variable, which is referenced in the input ﬁle, will

become an undeﬁned symbol. You can refer to the symbols in an object ﬁle by using the nm binary

utility, or by using the objdump binary utility with the -t option.

The linker is controlled by a linker script which is a text ﬁle containing commands in a simple lan-

guage. The main purpose of the script is to describe how sections in the input ﬁles should be mapped

into sections in the output ﬁle and to control memory layout of the output ﬁle. When necessary, the

linker script also directs the linker to perform other operations.

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