Here is the generic ``Hello world'' program in C:

#include <stdio.h>
int main (void) {
    printf ("hello world\n");
    return 0;
}

• #include <stdio.h> is a pre-processor directive , i.e., it is interpreted by a program called the `pre-processor', cpp (any line starting with the # character is interpreted by cpp). In this case, #include <stdio.h> simply includes the file /usr/include/stdio.h into the source code (i.e., the line #include <stdio.h> is replaced by the contents of /usr/include/stdio.h), before passing it on to the C compiler. 
• The angle brackets around stdio.h indicate that this is an include-file provided by the C compiler itself (if you wanted to include your own file, then use double quotes instead).
• The reason that we include /usr/include/stdio.h is that it contains a function prototype for the function printf which we call at line 3. Function prototypes are used by ANSI-standard C to check that you are calling the function with the right number and type of arguments. 
• main (void) is a function. Every C program must contain at least one function definition called main. "void" means that the function main doesn't have any arguments. The fact that main (void) is followed by a curly bracket means that this is the definition of main. (To call a function, you give its name, its arguments (in parentheses), and then a semicolon).
• printf ("hello world\n"); is an example of a function being called. "hello world\n" is the argument (in this case a character string).
• A semicolon is used to delimit statements in C. This is one of the most confusing things about C for beginning programmers. We will soon learn some rules for when and where to use semicolons.
• return 0; is an statement that instructs the  main () function to return an integer value of zero to the calling program (in this case, the operating system).

Note also the strange character sequence `\n' in the string constant "hello world\n". `\n' is called a newline character, and is regarded as a single character in C (i.e., it occupies one byte of storage). `\n' is known as an escape sequence, and the back-slash character is called the escape character.