Main function in c programming

Question

I searched the net regarding my question.  I continue to play with C off and on over the years, but now I aim to really work my way through the K&R book.
In the book one sees:
main()
hello world stuff stuff
This used to be no issue at all. It would compile and make a.out with no warning, nor complaint. Now I am working through the K&R book because I want to. I also have the first edition of Mark's Learn C on the Mac, but given how old it, and the second edition is, with regard to Xcode, I'll just stay with the terminal, BBEdit and K&R. By the way, I also saw numerous 1 star ratings on the Learn C book on amazon. Perhaps folks here could tell me how they feel about that book if they have used it and in particular the negative.
Now I get this warning, and I wonder why main has to have a type; what changed? Some new standard, the mac compiler. Now although I get the warning, default to int and all that, the compiler still creates a.out and it runs fine with the old ./a.out line command.
kr1.c:3:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
main()
^~~~
1 warning generated.

Answer 1

Why not just use a properly defined main function? This avoids having the compiler guess about about the intended return type and the number and types of the inputs. The C99 version of Standard C permits the two versions below:
int main(void) {...}
int main(int argc, char *arg[]) {...}
See http://en.wikipedia.org/wiki/Entry_point#C_and_C.2B.2B for more details, and OS specific main variants.
The last (2:nd) edition of K&R (http://en.wikipedia.org/wiki/The_C_Programming_Language) was released in 1988 preceding standard (ANSI) C and its C89, C95, C99 and C11 variants (http://en.wikipedia.org/wiki/ANSI_C) so K&R is at the very least somewhat out of date with modern C.