Does EOF set errno?

Question

I always struggle with return values of system calls - they are just so inconsistent! Normally I check if they are NULL or -1 and then call perror. However, for fgets, the man page says:

gets() and fgets() return s on success, and NULL on error or when end of file occurs while no characters have been read.

which means the return value NULL is not necessarily an error - it can also be EOF. Is errno set when the end of file is reached? Can I still call perror in this case?

If not, what is the common way to tell if the call returned an error versus EOF. I want to use perror with NULL string for errors and a custom string for EOF.

Answer 1

Use ferror and feof to distinguish between error and EOF. There's no general way to find out exactly what the error was, if there was an error, but you can tell that there was one.

Standard C (f)gets (and (f)getc) are not required to set errno, although a conforming library implementation can set errno to a non-zero value pretty much at will.

Posix does requires that (f)get{c,s} set errno on read errors, but that only helps you after you have determined that there was a read error (by calling ferror). It's also important to remember that library functions never set errno to 0, but may set errno to a non-zero value even if no error occurs. So you cannot test errno as a replacement for checking the error return of any library function, including fgets. And since end-of-file is not an error, errno will (probably) not be modified on EOF, so its value in that case is meaningless.