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... one earlier. All commands to the preprocessor begin with # marks and do NOT end with semicolons. This tells the preprocessor "search for the file called 'stdio. h's ome where where the compiler stashed it and put its entire contents right here as if they had been typed here all along." stdio. h is the Standard Input/Output header file.
This is because the C language itself has no idea what a screen or a display is, and is incapable of doing anything interesting to us like printing stuff on the screen. If we include stdio. h, we get to use a bunch of functions already written for us, that take care of nasty details like sending stuff to the screen and formatting it properly. More about header files later. The next line in our program is
int main (void);This is a "function prototype", and it ends with a semicolon.
This tells the compiler "we have a function called 'main'. It takes no arguments and returns a value of type 'int'. ('int' values are integers, but 'int' has a special meaning that will be covered later. ) Now that you know about it, I might go and use this function somewhere else in this file! The actual definition of what this function really does is below, but you can't complain that I haven't told you what this function really does because I've declared its existence to you right here." And then we go and immediately define what the function 'main' does. Enclosed in
int main (void) { / Stuff / } Is everything that main does. The braces enclose a "compound statement", in which many "statements" can appear. Statements are terminated by typing a semicolon. Compound statements end with the closing brace - don't put a semicolon after the closing brace. If you are curious, in certain circumstances you can get away with not using any braces at all. (If you " re ahead of me: this is when you only intend to put one statement there, and not a compound statement. ) But, "to be safe", always use the braces. Always format them as I have done: Put the opening brace on the first line, with a space between it and whatever proceeds it, then indent any code that comes below it four spaces, and then finish with a closing brace on a line of its own, at the original indentation level. This is essentially the "One True Brace Style", and it is a Good Thing. So, our compiler knows about the main () function, and it will call it upon program execution and clean up stuff once it's over.
But what does main actually do?
printf ("Hello, world! n"); return 0; As we can see, main calls the function printf and passes one argument to it. Main then returns 0. When we return 0 from main, that means everything went okay. What is the printf function? We neither gave a function prototype for it, nor did we define what printf does! Actually, we have. Including the header file stdio. h put the correct function prototype for printf at the beginning of our program, just as if we had typed it ourselves.
This is good, because the function prototype for printf is pretty nasty. We also have the definition of what printf does already put in there for us by the compiler. (It's expected that any sane program will use standard I/O functions and the like, so the relevant code is automagically "linked" in by the compiler. ) So, that's good! What does 'printf' mean? It means "Print Formatted." This is good, because formatting is a generally ugly process and we'd much rather have the compiler writer figure it out for us than using our own brainpower. What, precisely, is passed to printf? This will actually require a short digression.
C by itself is a very simple language. It has no concept of what a "string" is. You might recognize that term from other programming languages: a string is a sequence of characters (letters). Strings are customarily enclosed in double quotes: "I am a string. " When we give something enclosed in double quotes to printf, some introductory books will say we are passing a string to printf. This is not quite true. When we write "Hello, world!
n" there, we create an array. Never mind exactly what an array is now - you " ll be able to play with your own arrays later. The C compiler, upon seeing "Hello, world! n", stores the individual characters of that "string literal" somewhere in memory. This little region of memory is automatically "allocated" for you, and is called an array. This array of 15 characters (their data type is actually 'char'), is the filled - again, all this is done for you - with numbers corresponding somehow to "Hello, world!
n." This unnamed array (you didn't give it a name, after all - it was automatically made for you because you typed "Hello, world! n") is then passed to printf, which happily takes it, looks at the numeric values stored in that array, and puts up "Hello, world!" on the screen. You are almost certainly wondering what that 'n' I kept typing is, and why you had to type it into the Hello, world! program. That is an "escaped newline." When typing a text file, we use "newlines" to create a new line for us to type with. But what if we want to store a "newline" character somewhere, to tell a printing function that we actually want to print out a new line on the screen?
We certainly can't type something like
"This is one line. And this is another"That just won't work. Rather, if we type 'n' the compiler can recognize this, and knows to stick in whatever numeric value corresponds to a "newline" into the unnamed array mentioned above. This keeps it separate from the newlines we have to use ourselves to actually type the code. If you omit the newline 'n' in the Hello, world! program, nothing particularly bad will happen, but whatever prompt you use might be printed immediately after, like:
C: >hello.This is not exactly what you want. By the way, 'n' occupies a single space in our array, since it is a single newline character. You may also be wondering why I said "Hello, world! n" creates an array of 15 characters. The Hello, world!
exe Hello, world! C: >
itself takes up thirteen characters (5 for Hello, one for the comma, one for the space, five for world, one for the exclamation mark). A fourteenth is used by the newline n. The fifteenth element of the array is a "null" character. It is numerically stored as zero, and we can represent the character by
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