Structure of C Program.

Sample C program

#include <stdio.h>
    #define LAST 10
      
    int main()
    {
        int i, sum = 0;
       
        for ( i = 1; i <= LAST; i++ ) {
          sum += i;
        } /*-for-*/
        printf("sum = %d\n", sum);

        return 0;
    }

The main parts are:

• preprocessor directives (notable by the # character and the lack of semicolons)
• global variable declarations
• function declarations
• int main()
• function definitions

Here's a brief description of those parts:

Comments:

These start with /* and continue, ignoring newlines, until the next */. Notice that this means there is no nesting of comments. We adhere to ANSI-C standard (until C99 becomes more ubiquitous), so you cannot assume that // comments until the end of a line will work. It may turn out that some compilers accept these, but you cannot assume that all compilers (particularly the one used by the marking TA) will.

#include directives:

includes declarations of functions from header files for standard libraries. For example, #include <stdio.h> includes declarations of functions for the standard library that are useful for input/output. Note the distinct absence of semicolon

#define directives:

Performs textual replacement, which is useful to define constants. For example, this is legal C:

#include <stdio.h>
    #define BEGIN {
    #define END }
    int main()
    BEGIN
        printf("Hello World!\n");

        return 0;
    END

int main():

An executable program must have a main method, which is what's called when the program is executed. int indicates that this function returns an integer value to the shell where it is executed. In this course we will normally return 0.