Return value of fgets()

Question

I have just recently started working with I/O in C. Here is my question -
I have a file, from which I read my input. Then I use fgets() to get strings in a buffer which I utilise in some way. Now, what happens if the input is too short for the buffer i.e. if the first read by fgets() reaches EOF. Should fgets() return NULL(as I have read in fgets() documentation)? It seems that it doesn't and I get my input properly. Besides even my feof(input) does not say that we have reached EOF.
Here is my code snippet.

char    buf[BUFSIZ];
FILE    *input,
        *output;

input   = fopen(argv[--argc], "r");
output  = fopen(argv[--argc], "w");

/**
 *  If either of the input or output were unable to be opened
 *          we exit
 */
if (input == NULL) {
    fprintf(stdout, "Failed to open file - %s.\n", argv[argc + 1]);
    exit(EXIT_FAILURE);
}

if (output == NULL) {
    fprintf(stdout, "Failed to open file - %s.\n", argv[argc + 0]);
    exit(EXIT_FAILURE);
}

if (fgets(buf, sizeof(buf), input) != NULL) {
    ....
}

/**
 *  After the fgets() condition exits it is because, either -
 *      1) The EOF was reached.
 *      2) There is a read error.
 */
if (feof(input)) {
    fprintf(stdout, "Reached EOF.\n");
}
else if (ferror(input)) {
    fprintf(stdout, "Error while reading the file.\n");
}

Credendum

Note that feof(input) will only return true after attempting to read past EOF. It doesn't tell you that you are at EOF. – Oodles

Can you explain that in the context of the example above. Suppose, fgets() reads only 100 bytes (BUFSIZE is 512), should fgets() return NULL? – Credendum

Ok, now I realise how dumb my comment was; anyways I get what you mean. How do I check if EOF was reached? – Credendum

Answer 1

The documentation for fgets() does not say what you think it does:

From my manpage

fgets() reads in at most one less than size characters from stream and stores them into the buffer pointed to by s. Reading stops after an EOF or a newline. If a newline is read, it is stored into the buffer. A terminating null byte ('\0') is stored after the last character in the buffer.

And later

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

I don't read that as saying an EOF will be treated as an error condition and return NULL. Indeed it says a NULL would only occur where EOF occurs when no characters have been read.

The POSIX standard (which defers to the less accessible C standard) is here: http://pubs.opengroup.org/onlinepubs/009695399/functions/fgets.html and states:

Upon successful completion, fgets() shall return s. If the stream is at end-of-file, the end-of-file indicator for the stream shall be set and fgets() shall return a null pointer. If a read error occurs, the error indicator for the stream shall be set, fgets() shall return a null pointer, and shall set errno to indicate the error.

This clearly indicates it's only going to return a NULL if it's actually at EOF when called, i.e. if any bytes are read, it won't return NULL.

Marienbad

Can you explain feof() part, why doesn't it print anything. EOF has been reached right? – Credendum

feof() tests the end of file indicator, which is only set if you've already had an error where you have attempted to read past the end of the file. It does not (confusingly) indicate you are at the end of the file. – Marienbad

@yadav_vishal I had already explained feof behavior in my prior comment to your original post. – Oodles

@mbratch Yeah, I read it again carefully and understood what you meant, but how do I check if I reached EOF. If not by feof() then how? – Credendum

You do the fgets() again, you will get EOF. That's the normal way to test. – Marienbad

@yadav_vishal what abligh said. You continue using fgets until you see it return NULL to end the loop. At that point, feof should be TRUE since the NULL means fgets attempted to read past EOF. – Oodles

Got it! Thanks! I used an if(), should've used a while(). – Credendum

@mbratch At that point, feof will only be nonzero if no error occured. For example, if fgets encountered an i/o error during the read, fgets will return NULL and feof will return zero. – Archduke

@Archduke yes, that is true. There still needs to be a check for error. – Oodles

"it's only going to return a NULL if it's actually at EOF when called" Small clarification: If a rare input error occurs, NULL is also returned - so some characters may have been read prior to or as part of the error. – Penza