3.1. Executable File Formats

The product of compiling a C program is some machine language. But raw machine language isn't enough to allow the OS to run your code. The OS will want to know several pieces of meta-information with regards to your program before loading and running it; such as:

• information necessary to allow dynamic linking

• information about the size of your executable

• how much executable code and how much data space is used by your application

• how to lay out the application in memory

• debugging information

• ...and many many other things.

One way to solve this problem is to use a file format; a file format that contains not only the raw machine language code, but all the required additional information as well. There have been many such file formats devised over the years. Ones that I am familiar with include:

• DOS .exe

  A simple format consisting of a header (which contains all the required meta-information) plus the code itself. Very similar to most bitmap graphics file formats.

• DOS .com

  A "file format" that contains no meta-information whatsoever. This "format" is literally a raw dump of the machine language. All such meta-information is held in assumptions, the OS makes assumptions for any information it needs, and the programmer must follow these assumptions. Therefore, in a sense, it does "contain" meta-information, it's just that this information isn't contained anywhere in the file itself.

• a.out

  A fairly basic file format based on the notion of sections. Unfortunately, it was created before shared libraries became main-stream and couldn't specify the dynamic linking information easily. It also does not allow for an arbitrary number of sections.

• ELF

  A nicer format that expands on the ideas of the a.out format and adds additional features.