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What Is Compiling?

Simply put, compiling is the process of translating source code (the human-readable de- scription of a program written by a programmer) into the native language of the com- puter’s processor.

The computer’s processor (or CPU) works at a very elemental level, executing programs in what is called machine language. This is a numeric code that describes very small op- erations, such as “add this byte,” “point to this location in memory,” or “copy this byte.”

Each of these instructions is expressed in binary (ones and zeros). The earliest computer programs were written using this numeric code, which may explain why programmers who wrote it were said to smoke a lot, drink gallons of coffee, and wear thick glasses.

This problem was overcome by the advent of assembly language, which replaced the nu- meric codes with (slightly) easier to use character mnemonics such as CPY (for copy) and MOV (for move). Programs written in assembly language are processed into machine language by a program called an assembler. Assembly language is still used today for certain specialized programming tasks, such as device drivers and embedded systems.

We next come to what are called high-level programming languages. They are called this because they allow the programmer to be less concerned with the details of what the pro- cessor is doing and more with solving the problem at hand. The early ones (developed during the 1950s) included FORTRAN (designed for scientific and technical tasks) and COBOL (designed for business applications). Both are still in limited use today.

While there are many popular programming languages, two predominate. Most programs written for modern systems are written in either C or C++. In the examples to follow, we will be compiling a C program.

Programs written in high-level programming languages are converted into machine lan- guage by processing them with another program, called a compiler. Some compilers translate high-level instructions into assembly language and then use an assembler to per- form the final stage of translation into machine language.

A process often used in conjunction with compiling is called linking. There are many common tasks performed by programs. Take, for instance, opening a file. Many programs perform this task, but it would be wasteful to have each program implement its own rou- tine to open files. It makes more sense to have a single piece of programming that knows how to open files and to allow all programs that need it to share it. Providing support for common tasks is accomplished by what are called libraries. They contain multiple rou- tines, each performing some common task that multiple programs can share. If we look in the /lib and /usr/lib directories, we can see where many of them live. A program called a linker is used to form the connections between the output of the compiler and the libraries that the compiled program requires. The final result of this process is the exe- cutable program file, ready for use.


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