GCC Warning: implicit declaration of function 'puts' is invalid in C99 - macos

Question

I am beginning Zed Shaw's Learn C The Hard Way. I have downloaded XCode and the Command Line Tools. But when I compile the very first program:
int main(int argc, char *argv[]) {
puts("Hello world.");
return 0;
}
I get this warning:
ex1.c:2:1: warning: implicit declaration of function 'puts' is invalid
in C99
[-Wimplicit-function-declaration]
The program does compile and execute correctly.
I'm using OSX 10.8.3. Entering 'gcc -v' gives:
Using built-in specs. Target: i686-apple-darwin11 Configured with:
/private/var/tmp/llvmgcc42/llvmgcc42-2336.11~182/src/configure
--disable-checking --enable-werror --prefix=/Applications/Xcode.app/Contents/Developer/usr/llvm-gcc-4.2 --mandir=/share/man --enable-languages=c,objc,c++,obj-c++ --program-prefix=llvm- --program-transform-name=/^[cg][^.-]*$/s/$/-4.2/ --with-slibdir=/usr/lib --build=i686-apple-darwin11 --enable-llvm=/private/var/tmp/llvmgcc42/llvmgcc42-2336.11~182/dst-llvmCore/Developer/usr/local
--program-prefix=i686-apple-darwin11- --host=x86_64-apple-darwin11 --target=i686-apple-darwin11 --with-gxx-include-dir=/usr/include/c++/4.2.1 Thread model: posix gcc version 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)
Please help.

Answer 1

You need to include stdio.h, i.e.
#include <stdio.h>
at the start to import the function definition.

Answer 2

This "book" should be renamed to Learn To Hate C By Following Meaningless Examples That Are Blatantly Wrong.
The correct code in modern C would be plain and easy
#include <stdio.h> // include the correct header
int main(void) { // no need to repeat the argument mantra as they're not used
puts("Hello world.");
} // omit the return in main as it defaults to 0 anyway
While the original example
int main(int argc, char *argv[]) {
puts("Hello world.");
return 0;
}
would have been just plain bad in 1989, in 1999 (that is 18 years prior to the writing of this answer, and almost as many years before the "book" was written) the C standard was revised. In the C99 revision, this kind of implicit function declaration was made illegal - and naturally it remains illegal in the current revision of the standard (C11). Thus using puts without #includeing the relevant header, i.e. prepending #include <stdio.h> (or declaring the puts function with int puts(const char*);) is a constraint error.
A constraint error is an error that must cause the compiler output a diagnostics message. Additionally such a program is considered an invalid program. However the peculiar thing about the C standard is that it allows a C compiler to also successfully compile an invalid program, though a compiler may as well reject it. Therefore, such an example is hardly a good starting point in a book that is supposed to teach C to beginners.