The general layout of a simple C program includes the following components:

C and C++ are case-sensitive, so PRINTF is not a valid substitute for printf.

/***************************************************************************
 *  Description:
 *      Compute the area of a circle given the radius as input.
 *
 *  History: 
 *  Date        Name        Modification
 *  2013-07-28  Jason Bacon Begin
 ***************************************************************************/

#include <stdio.h>      // Contains prototypes for printf() and scanf()
#include <math.h>       // Defines M_PI
#include <sysexits.h>   // Defines EX_OK

int     main(int argc,char *argv[])

{
    // Variable definitions for main program
    double  radius,
            area;
    
    // Main program statements
    printf("What is the radius of the circle? ");
    scanf("%lf", &radius);
    if ( radius >= 0 )
    {
        area = M_PI * radius * radius;
        printf("The area is %f.\n", area);
    }
    else
        fprintf(stderr,"The radius cannot be negative.\n");

    return EX_OK;
}

#include is an example of a preprocessor directive. #include inserts a header file into the program at the point where it appears.

Header files contain constant definitions, type definitions, and function declarations. Modern C function declarations are called prototypes, and they define the interface to the function completely. A prototype for the printf() function looks like this:

int     printf(const char *format, ...);
    

The example above contains the function definition for main(), the entry point into the program. A definition begins like a declaration/prototype, but also includes the function body (the statements that do the work of the function).

C compilers do their job in one pass through the source file, so forward references (references to objects that are declared or defined later) are not allowed. The compiler only needs to see a prototype for functions that is not defined before it is referenced.

The printf() statement in the example above is a function call. C programs generally contain many function calls, since they are used in lieu of language features that were deliberately left out.

C is a free format language, which means that the compiler treats the end of a line the same as a space or tab. The end of a variable definition or a statement is indicated by a semicolon (;).