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.

Answer 2

According to fputs own documentation, yes, EOF does set errno. The man pages infer it indirectly as opposed to stating it outright, which hopefully will be amended. The function fputs returns an integer that will either be positive on success or EOF on failure. So the key to error handling fputs is to setup a code block that checks the return value of fputs as it is being called. The following is a snippet of how I've been taught to handle fputs errors.

if (fputs(buffer, stdout) == EOF)
{
  fprintf(stderr, "fputs returned EOF: %s\n", strerror(errno));
  // .. and now do whatever cleanup you need to do.
  // or be lazy and exit(-1)
}

Here I am writing the contents of buffer to standard output and checking to see if fputs returns EOF. EOF indicates an error code was set, so as long as you follow the documentation on the man pages for fputs, you should be able to create a bunch of if statements to check the various error codes errno can be set to.

(1) What is buffer? Some character array I declared elsewhere.

(2) What does fprintf do? It prints output to a passed in file descriptor, which is in this case standard error (stderr... it prints to console like stdout, but for errors).

(3) What is strerror? It is a function defined in the string.h header that prints error information for the passed in error code. It has information for every single error code that errno can be set to. The header string.h should NOT be confused with strings.h, which is a BSD linux header file that does not contain strerror(3).

Edit: Ok, I messed up. You were looking for an answer on fgets, not fputs.

To check for an error on fgets, do the following

if (fgets(buffer, BUF_SIZE, myFile) == NULL)
{ 
  // print out error as a string to stderr
  fprintf(stderr, "fgets error occurred: %s\n", strerror(errno));
  // do cleanup
}
// special: you also need to check errno AFTER the if statement...

The thing is, the only way you are getting an error on this is if the stream becomes unreadable, which is either due to permissions or trying to read something that is in write mode. In the case of a network, it may be possible for something to cut off your connection in the middle of reading, in which case you need to check the error code after the fgets if statement as well. But it will set the error code if something went wrong.

At least that is if the man pages are correct. See the linux man pages for more details. Only error code that can be set is the "I can't read this thing" code, which is errno == EBADF