c++ - What does an object file contain?

Question

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?

Answer 1

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.