What does an object file contain?

During the various stages of compilation in C or C++, I know that an object file gets generated (i.e., any_name.o file). What does this .o file contain? I can't open it since it's a binary file.

Could anybody please help me? Are the contents of the object file mainly dependent on the compiler which we use on Unix?

Object files can contain a bunch of stuff: Basically it's some or all of the list below:

• Symbol Names
• Compiled code
• Constant data, eg. strings
• Imports - which symbols the compiled code references (gets fixed up by linker)
• Exports - which symbols the object file makes available to OTHER object files.

The linker turns a bunch of object files into an executable, by matching up all the imports and exports, and modifying the compiled code so the correct functions get called.

There are several standardized formats (COFF, ELF on Unix), basically they are variants of the same formats that those used for executables but missing some informations. These missing informations will be completed when linking.

Objects files formats basically contains the same informations:

• binary code resulting of compilation (for a target processor)
• static data used by that part of the program (like constant strings, etc). You can make a finer distinction between BSS (exported data) and Text (data that won't be modified by the program). But that is mostly important for compiler and linker. Note that like binary code, data are also dependant on target (big-endian, little-endian, 32bits, 64bits).
• tables of symbols exported by this part of the program (mostly functions entry points)
• tables of external symbols used by this part of the program

When objects will be linked together the parts of the code that refers to external symbols will be replaced by actual values (well, that is still oversimplified, there is a last part that will be done at loading time when running the program, but that's the idea).

The object file may also contain more symbols information that strictly necessary for resolving imports and export (useful for debug). That information can be removed using the strip command.

First read the wiki page. You can use objdump to examine such a file :)

Use the file command for things like this. It's an ELF object file on a modern Linux system. E.g. if compiled for 32-bit x86.

ELF 32-bit LSB relocatable, Intel 80386, version 1 (SYSV), not stripped

In contrast, a dynamically linked executable might look like:

ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.15, not stripped

To see headers, including section names, you can use:

objdump -x any_name.o

To disassemble:

objdump -d any_name.o

First, binary files can be opened! Don't be scared of it, you need just the right tools! Being binary data, a text editor is not the right tool of course; a right tool could be a hex editor, or an advanced editor like emacs, or a tool that instead of simply "outputting" bytes in their "hex" representation and letting you alone with your interpretation of the data, knows that particular format and "interprets" the data properly, at some level (e.g. GIMP interprets a PNG file as an image and shows it, a PNG analyser will "decompose" the data inside PNG sections showing telling you the flags in certain bytes, ...etc).

In your case, the general answer is that the object file contains your compiled code (and data), plus all extra informations needed by the linker, and eventually more.

How these informantions are "organized" and in some case in what the "eventually more" consists, it depends on the specific object format. Some wikipedia links listing some of the possibilities are this, this, this, this ...

Each of these may have its tools to analyse the content; e.g. readelf for ELF, objdump for several formats (try objdump -i) depending on how it was compiled.

The object file is the compiled source.

This means that it's machine code, which is dependent on the target platform (you can compile for Unix on Windows if you really want to) and the compiler used. Different compilers will produce different machine code from the same source file.

The file contains binary data which must be run through a linker to generate an executable. It is essentially a bunch of machine code instructions with named sections (corresponding to your functions). From wikipedia's 'Object File' article:

> In computer science, an object file is > an organized collection of separate, > named sequences of machine > code[citation needed]. Each sequence, > or object, typically contains > instructions for the host machine to > accomplish some task, possibly > accompanied by related data and > metadata (e.g. relocation > information, stack unwinding > information, comments, program > symbols, debugging or profiling > information). A linker is typically > used to generate an executable or > library by combining parts of object > files.

In the GNU compilation environment you can look with objdump both in the executable and in the object file.

As you can see the object contains only the code of functions declared/referenced within the compiled file (the file contains only the main function with a scanf call and a printf call).

$ objdump -t scanf_sample.o

scanf_sample.o:     file format pe-i386

SYMBOL TABLE:
[  0](sec -2)(fl 0x00)(ty   0)(scl 103) (nx 1) 0x00000000 scanf_sample.c
File
[  2](sec  1)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _main
[  3](sec  1)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .text
AUX scnlen 0x91 nreloc 9 nlnno 0
[  5](sec  2)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .data
AUX scnlen 0x0 nreloc 0 nlnno 0
[  7](sec  3)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .bss
AUX scnlen 0x0 nreloc 0 nlnno 0
[  9](sec  4)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .rdata
AUX scnlen 0x54 nreloc 0 nlnno 0
[ 11](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 1) 0x00000000 ___main
AUX tagndx 0 ttlsiz 0x0 lnnos 0 next 0
[ 13](sec  0)(fl 0x00)(ty   0)(scl   2) (nx 0) 0x00000000 __alloca
[ 14](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _memset
[ 15](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _scanf
[ 16](sec  0)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _printf

If you use objdump on an executable you can see a lot more functions (besides those found inside the object). This proves that the object file contains only the functions defined in the source file with references to other functions. Those references will be resolved at linking phase.

Read more about linking, compilation and objects.