Компилятор не дает написать main без int. Ошибка в книге, или в С так можно?

exits and the void return from main require a bit of explanation. The traditional way of representing errors and status returns in C is with numbers: a return from main or the argument to exit represents a status return from a program, and errno stores information about error returns from functions. The traditional behavior is to have zero mean no error and any other value mean error; ANSI C defines EXIT_SUCCESS and EXIT_FAILURE for status returns from programs.

This gets restricted very quickly. ANSI C only defines three standard values for errno (domain error, range error, and illegal multibyte sequence) and two values for status return. And sometimes an integer won’t tell you enough. For example, let’s take the UNIX lseek system call, which manipulates the file read/write position:

long lseek(int fd, long offset, int from);

If any argument is invalid (for example, from not 0, 1, or 2), lseek returns with errno set to EINVAL (specific to UNIX). But this doesn’t tell you which argument was invalid, or how many; it only says that something was not right. We can add the appropriate errno values to resolve this problem. But what about a library that defines over 1,000 values for errno? On machines with small int sizes, this chokes your program and defeats the purposes of both sides.

A better idea is to give the programmer the ability to handle any error that comes in without worry of losing standards compliance or clarity, and to generate any error without falling into a surfeit of possibility. So the designers of Plan 9 decided to use strings instead of numbers. Each program has an error string which is set by routines when an error occurs. And each program returns a string to the host environment with the exits system call. The value given to exits can be accessed from rc through the environment variable $status.