Creating a C program

Overview

• There are four fundamental tasks in the creating of any C program
  • Editing
  • Compiling
  • Linking
  • Executing
• These tasks will become second nature to you because you will be doing it so often
• The processes of editing, compiling, linking, and executing are essentially the same for developing programs in any environment and with any compiled language
• Editing is the process of creating and modifying your C source code
  • source code is inside a file and contains the program instructions you write
  • choose a wise name for your base file name (all source files end in the .c extension)

Compiling

• A compiler converts your source code into machine language and detects and reports errors in your code
  • The input to the compiler is the file you produce during your editing (source file)
• Compilation is a two-stage process
  • the first stage is called the preprocessing phase, during which your code may be modified or added to
  • the second stage is the actual compilation that generates the object code
• the compiler examines each program statement and checks it to ensure that it conforms to the syntax and semantics of the language
  • can also recognize structural errors (dead code)
  • does not find logic errors
  • typical errors reported might be due to an expression that has unbalanced parentheses (syntactic error), or due to the use of a variable that is not “define” (semantic error)

Compiling (cont’d)

• After all errors are fixed, the compiler will then take each statement of the program and translate it into assembly language
• the compiler will then translate the assembly language statement into actual machine instructions
  • the output from the compiler is known as object code and it is stored in files called object files (same name as source file with a .obj or .o extension)
• The standard command to compile your C programs will be cc (or the GNU compiler, which is .gcc)
  • cc -c myprog.c or gcc -c myprog.c
  • if you omit the -c flag, your program will automatically be linked as well

Linking

• After the program has been translated into object code, it is ready to be linked
  • The purpose of the linking phase is to get the program into a final form for execution on the computer
  • linking usually occurs automatically when compiling depending on what system you are on, but, can sometimes be a separate command
• The linker combines the object modules generated by the compiler with additional libraries needed by the program to create the whole executable
  • also detects and reports errors
    • if part of your program is missing or a nonexistent library component is referenced
• Program libraries support and extend the C language by providing routines to carry out operations that are not part of the language
  • input and output libraries, mathematical libraries, string manipulation libraries

Linking (cont’d)

• A failure during the linking phase means that once again you have to go back and edit your source code
• Success will produce an executable file
  • In a Windows environment, the executable file will have an .exe extension
  • in Unix / Linux, there will be no such extension (a.out by default)
  • Many IDEs have a build option, which will compile and link your program in a single operation to produce the executable
• A program of any significant size will consist of several source code files
  • each source code file needs the compiler to generate the object file that need to be linked
• the program is much easier to manage by breaking it up into a number of smaller source files
  • It is cohesive and makes the development and maintenance of the program a lot easier
  • the set of source files that make up the program will usually be integrated under a project name, which is used to refer to the whole program

Executing

• In most IDEs, you’ll find an appropriate menu command that allows you to run or execute your compiled program
  • Otherwise double click the exe file or type a.out on the console in linux manually
• The execution stage is where you run your program
  • each of the statement of the program is sequentially executed in turn
  • If the program requests any data from the user the program temporarily suspends its execution so that the input can be entered
  • Results that are displayed by the program (output) appear in a window called the console
• this stage can also generate a wide variety of error conditions
  • producing the wrong output
• If the program does not perform the intended functionality then it will be necessary to go back and reanalyze the program’s logic
  • known as the debugging phase, correct all the known problems or bugs from the program
  • Will need to make changes to the original source program
    • the entire process of compiling, linking, and executing the program must be repeated until the desired results are obtained