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Segmentation[edit]
Most object file formats are structured as separate sections of data, each section containing a certain type of data. These sections are known as "segments" due to the term "memory segment", which was previously a common form of memory management. When a program is loaded into memory by a loader, the loader allocates various regions of memory to the program. Some of these regions correspond to segments of the object file, and thus are usually known by the same names. Others, such as the stack, only exist at run time. In some cases, relocation is done by the loader (or linker) to specify the actual memory addresses. However, for many programs or architectures, relocation is not necessary, due to being handled by the memory management unit or by position-independent code. On some systems the segments of the object file can then be copied (paged) into memory and executed, without needing further processing. On these systems, this may be done lazily, that is, only when the segments are referenced during execution, for example via a memory-mapped file backed by the object file.
Types of data supported by typical object file formats:[6]
- Header (descriptive and control information)
- Code segment ("text segment", executable code)
- Data segment (initialized static variables)
- Read-only data segment (rodata, initialized static constants)
- BSS segment (uninitialized static data, both variables and constants)
- External definitions and references for linking
- Relocation information
- Dynamic linking information
- Debugging information
Segments in different object files may be combined by the linker according to rules specified when the segments are defined. Conventions exist for segments shared between object files; for instance, in DOS there are different memory models that specify the names of special segments and whether or not they may be combined.[7]
Debugging information may either be an integral part of the object file format, as in COFF, or a semi-independent format which may be used with several object formats, such as stabs or DWARF.
The GNU Project's Binary File Descriptor library (BFD library) provides a common API for the manipulation of object files in a variety of formats.
[root@hw ~]# objdump -h /usr/lib64/crti.o
/usr/lib64/crti.o: file format elf64-x86-64
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00000000 0000000000000000 0000000000000000 00000040 2**0
CONTENTS, ALLOC, LOAD, READONLY, CODE
1 .data 00000000 0000000000000000 0000000000000000 00000040 2**0
CONTENTS, ALLOC, LOAD, DATA
2 .bss 00000000 0000000000000000 0000000000000000 00000040 2**0
ALLOC
3 .init 00000015 0000000000000000 0000000000000000 00000040 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
4 .fini 00000004 0000000000000000 0000000000000000 00000058 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
5 .note.GNU-stack 00000000 0000000000000000 0000000000000000 0000005c 2**0
CONTENTS, READONLY
An object file is a computer file containing object code, that is, machine code output of an assembler or compiler. The object code is usually relocatable, and not usually directly executable. There are various formats for object files, and the same machine code can be packaged in different object file formats. An object file may also work like a shared library.
In addition to the object code itself, object files may contain metadata used for linking or debugging, including: information to resolve symbolic cross-references between different modules, relocation information, stack unwinding information, comments, program symbols, debugging or profiling information.