How to compile C files with gcc, step by step.

C Program

Before talking of compiling and running C program in Linux let’s see why C is so popular. He was Dennis Ritchie who developed C language in 1969 to 1973. C was developed from the beginning as the system programming language for UNIX. Most of the UNIX kernel, and all of its supporting tools and libraries, were written in C. Initially, C was designed to implement the UNIX operating system. Later other folks found it useful for their programs without any hindrance, and they began using it. Even today, C is the first choice for system-level programming. This tutorial explains compilation and execution of C program is in detail.

GCC

The original GNU C Compiler (GCC) is developed by Richard Stallman, the founder of the GNU Project. Richard Stallman founded the GNU project in 1984 to create a complete Unix-like operating system as free software, to promote freedom and cooperation among computer users and programmers.

The following command (provided that gcc is installed on your Linux box) compiles C program helloworld.c and creates an executable file called helloworld. Don't forget to set appropriate permissions to helloworld.c, so that you won't get execute permission errors.

[root@host ~]# gcc helloworld.c -o helloworld

While compiling helloworld.c the gcc compiler reads the source file helloworld.c and translates it into an executable helloworld. The compilation is performed in four sequential phases by the compilation system (a collection of four programs - preprocessor, compiler, assembler, and linker).

Let’s see all four steps to compile and run C program one by one with gcc.

1. Preprocessing

This is the first stage of compilation process where preprocessor directives (macros and header files are most common) are expanded. To perform this step gcc executes the following command internally.

[root@host ~]# cpp helloworld.c > helloworld.i

The result is a file helloworld.i that contains the source code with all macros expanded. If you execute the above command in isolation then the file helloworld.i will be saved to disk and you can see its content by vi or any other editor you have on your Linux box.

2. Compilation

In this phase compilation proper takes place. The compiler (ccl) translates helloworld.i into helloworld.s. File helloworld.s contains assembly code. You can explicitly tell gcc to translate helloworld.i to helloworld.s by executing the following command.

[root@host ~]# gcc -S helloworld.i

The command line option -S tells the compiler to convert the preprocessed code to assembly language without creating an object file. After having created helloworld.s you can see the content of this file. While looking at assembly code you may note that the assembly code contains a call to the external function printf.

3. Assembly

Here, the assembler (as) translates helloworld.s into machine language instructions, and generates an object file helloworld.o. You can invoke the assembler at your own by executing the following command.

[root@host ~]# as helloworld.s -o helloworld.o

The above command will generate helloworld.o as it is specified with -o option. And, the resulting file contains the machine instructions for the classic "Hello World!" program, with an undefined reference to printf.

4. Linking

This is the final stage in compilation of “Hello World!” program. This phase links object files to produce final executable file. An executable file requires many external resources (system functions, C run-time libraries etc.). Regarding our “Hello World!” program you have noticed that it calls the printf function to print the 'Hello World!' message on console. This function is contained in a separate pre compiled object file printf.o, which must somehow be merged with our helloworld.o file. The linker (ld) performs this task for you. Eventually, the resulting file helloworld is produced, which is an executable. This is now ready to be loaded into memory and executed by the system.

Output

During the whole compilation process there are other files also in role along with the source code file. If you see the very first statement of helloworld.c it is #include <stdio.h> (includes header file). Likewise, while compiling a C program you have to work with following types of files.

If you want more information about this check this video. I hope this blog helps you to understand better the compilation process.

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