2.1 How a Program Becomes a Process

A program is a prepared sequence of instructions to accomplish a defined task. To write a C source program, a programmer creates disk files containing C statements that are organized into functions. An individual C source file may also contain variable and function declarations, type and macro definitions (e.g., typedef ) and preprocessor commands (e.g., #ifdef , #include , #define ). The source program contains exactly one main function.

Traditionally, C source filenames have a .c extension, and header filenames have a .h extension. Header files usually only contain macro and type definitions, defined constants and function declarations. Use the #include preprocessor command to insert the contents of a header file into the source.

The C compiler translates each source file into an object file. The compiler then links the individual object files with the necessary libraries to produce an executable module . When a program is run or executed , the operating system copies the executable module into a program image in main memory.

A process is an instance of a program that is executing. Each instance has its own address space and execution state. When does a program become a process? The operating system reads the program into memory. The allocation of memory for the program image is not enough to make the program a process. The process must have an ID (the process ID ) so that the operating system can distinguish among individual processes. The process state indicates the execution status of an individual process. The operating system keeps track of the process IDs and corresponding process states and uses the information to allocate and manage resources for the system. The operating system also manages the memory occupied by the processes and the memory available for allocation.

When the operating system has added the appropriate information in the kernel data structures and has allocated the necessary resources to run the program code, the program has become a process. A process has an address space (memory it can access) and at least one flow of control called a thread . The variables of a process can either remain in existence for the life of the process (static storage) or be automatically allocated when execution enters a block and deallocated when execution leaves the block (automatic storage). Appendix A.5 discusses C storage classes in detail.

A process starts with a single flow of control that executes a sequence of instructions. The processor program counter keeps track of the next instruction to be executed by that processor (CPU). The CPU increments the program counter after fetching an instruction and may further modify it during the execution of the instruction, for example, when a branch occurs. Multiple processes may reside in memory and execute concurrently, almost independently of each other. For processes to communicate or cooperate, they must explicitly interact through operating system constructs such as the filesystem (Section 5.1), pipes (Section 6.1), shared memory (Section 15.3) or a network (Chapters 18-22).