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It is important to understand that while some computer languages e. Scheme or Basic are normally used with an interactive interpreter where you type in commands that are immediately executed , C doesn't work that way. C source code files are always compiled into binary code by a program called a "compiler" and then executed. This is actually a multi-step process which we describe in some detail here.
Regular source code files. These files contain function definitions, and have names which end in ". These files contain function declarations also known as function prototypes and various preprocessor statements see below. They are used to allow source code files to access externally-defined functions.
Header files end in ". These files are produced as the output of the compiler. They consist of function definitions in binary form, but they are not executable by themselves. Object files end in ". These are produced as the output of a program called a "linker". The linker links together a number of object files to produce a binary file which can be directly executed. Binary executables have no special suffix on Unix operating systems, although they generally end in ". There are other kinds of files as well, notably libraries ".
Before the C compiler starts compiling a source code file, the file is processed by a preprocessor. This is in reality a separate program normally called " cpp ", for "C preprocessor" , but it is invoked automatically by the compiler before compilation proper begins.
What the preprocessor does is convert the source code file you write into another source code file you can think of it as a "modified" or "expanded" source code file. That modified file may exist as a real file in the file system, or it may only be stored in memory for a short time before being sent to the compiler. Either way, you don't have to worry about it, but you do have to know what the preprocessor commands do. Preprocessor commands start with the pound sign " ".
There are several preprocessor commands; two of the most important are:. For instance, the statement:. This is important in case you need to change the constant value later on; it's much less bug-prone to change it once, in the define , than to have to change it in multiple places scattered all over the code. This is used to access function definitions defined outside of a source code file. In this case, we use include in order to be able to use functions such as printf and scanf , whose declarations are located in the file stdio.
C compilers do not allow you to use a function unless it has previously been declared or defined in that file; include statements are thus the way to re-use previously-written code in your C programs. There are a number of other preprocessor commands as well, but we will deal with them as we need them. After the C preprocessor has included all the header files and expanded out all the define and include statements as well as any other preprocessor commands that may be in the original file , the compiler can compile the program.
It does this by turning the C source code into an object code file, which is a file ending in ". Object code is not directly executable, though. In order to make an executable, you also have to add code for all of the library functions that were include d into the file this is not the same as including the declarations, which is what include does. This is the job of the linker see the next section. This tells the compiler to run the preprocessor on the file foo. The -c option means to compile the source code file into an object file but not to invoke the linker.
If your entire program is in one source code file, you can instead do this:. This tells the compiler to run the preprocessor on foo.
The -o option states that the next word on the line is the name of the binary executable file program. If you don't specify the -o , i.
Note also that the name of the compiler we are using is gcc , which stands for "GNU C compiler" or "GNU compiler collection" depending on who you listen to. Other C compilers exist; many of them have the name cc , for "C compiler". On Linux systems cc is an alias for gcc. The job of the linker is to link together a bunch of object files. This includes both the object files that the compiler created from your source code files as well as object files that have been pre-compiled for you and collected into library files.
These files have names which end in. Like the preprocessor, the linker is a separate program called ld. Also like the preprocessor, the linker is invoked automatically for you when you use the compiler. The normal way of using the linker is as follows:. This line tells the compiler to link together three object files foo.
This is all you need to know to begin compiling your own C programs. Generally, we also recommend that you use the -Wall command-line option:. The -Wall option causes the compiler to warn you about legal but dubious code constructs, and will help you catch a lot of bugs very early.
If you want to be even more anal and who doesn't? The -Wstrict-prototypes option means that the compiler will warn you if you haven't written correct prototypes for all your functions.
The -ansi and -pedantic options cause the compiler to warn about any non-portable construct e. The GNU Info documentation on gcc. This is far more information than most people could possibly absorb in the average millenium.