Understanding the Linux Kernel

Chapter 19. Program Execution

The concept of a "process," described in Chapter 3, was used in Unix from the beginning to represent the behavior of groups of running programs that compete for system resources. This final chapter focuses on the relationship between program and process. We'll specifically describe how the kernel sets up the execution context for a process according to the contents of the program file. While it may not seem like a big problem to load a bunch of instructions in memory and point the CPU to them, the kernel has to deal with flexibility in several areas:

Different executable formats

Linux is distinguished by its ability to run binaries that were compiled for other operating systems.

Shared libraries

Many executable files don't contain all the code required to run the program but expect the kernel to load in functions from a library at runtime.

Other information in the execution context

This includes the command-line arguments and environment variables familiar to programmers.

A program is stored on disk as an executable file , which includes both the object code of the functions to be executed and the data on which such functions will act. Many functions of the program are service routines available to all programmers; their object code is included in special files called " libraries." Actually, the code of a library function may either be statically copied in the executable file (static libraries), or be linked to the process at run time (shared libraries, ...