C - Compiling and executing the Programs

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 Steps for Compiling and executing the Programs

A compiler is a software program that analyzes a program developed in a particular computer language and then translates it into a form that is suitable for executionon a particular computer system. Figure below shows the steps that are involved in entering, compiling, and executing a computer program developed in the C programming language and the typical Unix commands that would be entered from the command line.

Step 1: 

The program that is to be compiled is first typed into a file on the computer system. There are various conventions that are used for naming files, typically be any name provided the last two characters are “.c” or file with extension .c. So, the file name prog1.c might be a valid filename for a C program. A text editor is usually used to enter the C program into a file. For example, vi is a popular text editor used on Unix systems. The program that is entered into the file is known as the source program because it represents the original form of the program expressed in the C language. 

Step 2: 

After the source program has been entered into a file, then proceed to have it compiled. The compilation process is initiated by typing a special command on the system. When this command is entered, the name of the file that contains the source program must also be specified. For example, under Unix, the command to initiate program compilation is called cc. If we are using the popular GNU C compiler, the command we use is gcc. 

 Typing the line 

         gcc prog1.c or cc prog1.c 

In the first step of the compilation process, the compiler examines each program statement contained in the source program and checks it to ensure that it conforms to the syntax and semantics of the language. If any mistakes are discovered by the compiler during this phase, they are reported to the user and the compilation process ends right there. The errors then have to be corrected in the source program (with the use of an editor), and the compilation process must be restarted. Typical errors reported during this phase of compilation might be due to an expression that has unbalanced parentheses (syntactic error), or due to the use of a variable that is not “defined” (semantic error).

Step 3: 

When all the syntactic and semantic errors have been removed from the program, the compiler then proceeds to take each statement of the program and translate it into a “lower” form that is equivalent to assembly language program needed to perform the identical task. 

Step 4: 

After the program has been translated the next step in the compilation process is to translate the assembly language statements into actual machine instructions. The assembler takes each assembly language statement and converts it into a binary format known as object code, which is then written into another file on the system. This file has the same name as the source file under Unix, with the last letter an “o” (for object) instead of a “c”. 

Step 5: 

After the program has been translated into object code, it is ready to be linked. This process is once again performed automatically whenever the cc or gcc command is issued under Unix. The purpose of the linking phase is to get the program into a final form for execution on the computer. 

 If the program uses other programs that were previously processed by the compiler, then during this phase the programs are linked together. Programs that are used from the system’s program library are also searched and linked together with the object program during this phase. The process of compiling and linking a program is often called building. 

The final linked file, which is in an executable object code format, is stored in another file on the system, ready to be run or executed. Under Unix, this file is called a.out by default. Under Windows, the executable file usually has the same name as the source file, with the c extension replaced by an exe extension. 

Step 6: 

To subsequently execute the program, the command a.out has the effect of loading the program called a.out into the computer’s memory and initiating its execution. When the program is executed, each of the statements of the program is sequentially executed in turn. If the program requests any data from the user, known as input, the program temporarily suspends its execution so that the input can be entered. Or, the program might simply wait for an event, such as a mouse being clicked, to occur. Results that are displayed by the program, known as output, appear in a window, sometimes called the console. If the program does not produce the desired results, it is necessary to go back and reanalyze the program’s logic. This is known as the debugging phase, during which an attempt is made to remove all the known problems or bugs from the program. To do this, it will most likely be necessary to make changes to original source program.


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