Is '\n' necessary to be given to printf() in C?

Question

I am reading the book "The C Programming Language" by Brian Kernighan and Dennis Ritchie(2nd edition, published by PHI). In the first article 1.1 Getting started of the first chapter A Tutorial Introduction, page number 7, they say that one must use \n in the printf() argument, otherwise the C compile will produce an error message. But when I compiled the program without \n in printf(), it went fine. I did not see any error message. I am using Dev-C portable with "MinGW GCC 4.6.2 32-bit" compiler.

Why I do not get the error message?

Answer 1

Here is the passage in question, from page 7 of the second edition of K&R:

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.

This means that you can't embed a literal newline in a quoted string.

Either one of the lines below, however, are fine:

printf("hello, world");   /* does not print a newline */
printf("hello, world\n"); /* prints a newline */

All the text above is saying is that you can't have a quoted string that spans multiple lines in the source code.

You can also escape a newline with a backslash. The C preprocessor will remove the backslash and newline, so the following two statements are equivalent:

printf("hello, world\
");
printf("hello, world");

And if you have a lot of text, you can put multiple quoted strings next to each other, or separated by whitespace, and the compiler will join them for you:

printf("hello, world\n"
       "this is a second line of text\n"
       "but you still need to include backslash-n to break each line\n");

Answer 2

You don't get a compile-time error message because there is no error.

In the first article they say that one must use \n in the printf() argument, otherwise the C compiler will produce an error message.

Can you cite (by section and/or page number) where that statement appears? I seriously do not believe that K&R (you're using the second edition, right?) says that. If it did say that, it would be an error in the book.

Update: What the book says, quite correctly, is that a newline in a string literal is represented by the two-character sequence \n, not by an actual newline character. A string literal must be on a single logical source line; something like

printf("hello
world");

is a syntax error. This applies to all string literals, whether they're printf format strings or not.

An actual newline in a string literal is an error. A \n sequence that represents a newline is optional; its lack is not an error, but a printf format string should usually end with a \n.

There is no requirement for a printf call to include the \n character, and I've never seen a compiler complain about a printf that lacks a \n.

There is an issue here, but it's not a compile-time error.

Some examples:

printf("No newline");

This is a perfectly legal call. It prints the specified string on standard output without a newline character.

printf("hello%c", '\n');

There's no \n in the format string, but it prints hello followed by a newline. Again, this is perfectly legal.

The actual issue is that you should (almost) always print a newline at the very end of your output. This complete program:

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

is legal, but its behavior may be undefined in some implementations. The relevant rule is in the standard, section 7.21.2 paragraph 2 (the quote is from the N1570 draft):

A text stream is an ordered sequence of characters composed into lines, each line consisting of zero or more characters plus a terminating new-line character. Whether the last line requires a terminating new-line character is implementation-defined.

Whether that terminating newline character is required or not, it's (almost always) a very good idea to end your output with a newline. If I run it on my system, I get the string hello immediately followed by my shell prompt on the same line. It's not illegal, but it's inconvenient and ugly.

But that applies only at the very end of the program's output. This program is perfectly valid and has well defined behavior:

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

Still, the easiest and most reliable way to produce clean output is for each printf call to print exactly one line, which ends with exactly one '\n' character. This isn't a universal rule; sometimes it's convenient to print a line a piece at a time, or to print two or more lines in a single printf.

Answer 3

Very often, if you don't end your printf format string with a \n, some of the output stays in the stdout buffer, and you need to call fflush to get all the output shown.

^{This means that if you don't get all the expected output you should add fflush at appropriate places (e.g. before calls to fork).}

But you won't get a compiler message in such case, because it is not an error (it may be a mistake many beginners are doing). If you really wanted, you could customize your compiler (e.g. with MELT if using a recent GCC compiler) to get the warning. I believe it is not worth the effort (because there are legitimate calls to printf without any \n....)

An example of legitimate printf calls without newlines would be if you coded a (recursive) function to output an expression from its AST; you certainly should not emit a newline after each token.

See documentation of printf(3), fflush(3), stdio(3), setvbuf(3) etc...