Getting Started

The only way to learn a new programming language is by writing programs in it. The first program to write is the same for all languages, print the words:

"hello, world"

This is a big hurdle; to leap over it you have to be able to create the program text somewhere, compile it successfully, load it, run it, and find out where your output went. With these mechanical details mastered, everything else is comparatively easy.

In C, the program to print "hello, world" is:

#include <stdio.h> 
int main() { 

    printf("hello, world\n"); 
    return 0;
}

Just how to run this program depends on the system you are using. As a specific example, on the UNIX operating system you must create the program in a file whose name ends in ".c", such as "hello.c", then compile it with the command:

cc hello.c

If you haven't botched anything, such as omitting a character or misspelling something, the compilation will proceed silently, and make an executable file called "a.out". If you run it by typing the command:

a.out 

it will print:

hello, world

Note for modern workflow (MacOS)
running the executable requires a "./" before the name, so that would be: ./a.out

Now, for some explanations about the program itself. A C program, whatever its size, consists of functions and variables. A function contains statements that specify the computing operations to be done, and variables store values used during the computation. C functions are like the subroutines and functions in Fortran or the procedures and functions of Pascal. Our example is a function named main. Normally you are at liberty to give functions whatever names you like, but main is special - your program begins executing at the beginning of main. This means that every program must have a main somewhere.

main will usually call other functions to help perform its job, some that you wrote, and others from libraries that are provided for you. The first line of the program:

#include <stdio.h>

Tells the compiler to include information about the standard input/output library; the line appears at the beginning of many C source files. The standard library is described in Chapter 7 and Appendix B.

One method of communicating data between functions is for the calling function to provide a list of values, called arguments, to the function it calls. The parentheses after the function name surround the argument list. In this example, main is defined to be a function that expects no arguments, which is indicated by the empty list: ().

The first C program

#include <stdio.h>                  *include information about the standard library  
int main() {                        *define a function called main that received no argument values  

    printf("hello, world\n");       *main calls library function "printf" to print this sequence of characters   
    return 0;                       *\n represents the newline character   
}                                   *statements of main are enclosed in braces                                 

Note for modern C
The return statement in main returns a value to us to let us know if the program exited correctly. 0 usually means it ran without errors.
While this statement is technically optional, (many compilers will interpret the lack of return as a 0 return) it is smart and good discipline to add the return 0 explicitly.

The statements of a function are enclosed in braces: {}. The function main contains only one statement:

printf("hello, world\n");

A function is called by naming it, followed by a parenthesized list of arguments, so this calls the function printf with the argument "hello, world\n". printf is a library function that prints output, in this case the string of characters between the quotes.

A sequence of characters in double quotes, like "hello, world\n", is called a character string or string constant. For the moment our only use of character strings will be as arguments for printf and other functions.

The sequence \n in the string is C notation for the newline character, which when printed advances the output to the left margin on the next line. If you leave out the \n (a worthwhile experiment), you will find that there is no line advance after the output is printed. You must use \n to include a newline character in the printf argument; if you try something like:

printf("hello, world 
"); 

the C compiler will produce an error message.
printf never supplies a newline character automatically, so several calls may be used to build up an output line in stages. Our first program could just as well have been written:

#include <stdio.h> 
int main() { 

    printf("hello, "); 
    printf("world"); 
    printf("\n"); 
    return 0;
}

to produce identical output.

Notice that \n represents only a single character. An escape sequence like \n provides a general and extensible mechanism for representing hard-to-type or invisible characters. Among the others that C provides are \t for tab, \b for backspace,\" for the double quote and \\ for the backslash itself. There is a complete list in Section 2.3.

Exercise 1-1. Run the "hello, world" program on your system. Experiment with leaving out parts of the program, to see what error messages you get.

Exercise 1-2. Experiment to find out what happens when prints's argument string contains \c, where c is some character not listed above.