fread and ferror don't set errno

Question

I'm trying to check when fread() raises an error, so I use ferror().

chunk = fread(buf, 1, 100, file);
if (ferror(file))
  {
    return errno;
  }

But, ferror() man page (man 3 ferror, or just man ferror) says:

ERRORS
These functions should not fail and do not set the external variable errno.

So, how can I know the error type occurred when file has been read, although fread() and ferror() didn't set errno?

Purchase

Did you read fread(3) ? It has a documented behavior on error ! – Equidistance

yes, it doesn't mention anything about setting errno. – Purchase

@BasileStarynkevitch I think his point was exactly that. Sure ferror can tell you an error happened, but since fread is not documented to set errno, how can he tell what the actual error was, if he can at all, since errno won't reflect the specific error condition that lit up the stream error state? – Salade

Related: Does stdio always set errno?. – Bellabelladonna

Answer 1

You can't get there from here.

fread does not set errno (as you have discovered), and as such you cannot really determine much about a specific error state; only that there is one. The exact nature of the error is generally implementation-dependent. There is no C standard-library-based portable way to gather it .

For specific system-level errors, you can slum it to system-calls, possibly suffering with the pitfalls like poor/nonexistent IO buffering along the way. There POSIX can somewhat come to your rescue. Calls like read, do set errno and have a fairly detailed set of possible outcomes. That may be an option for you if the platform you're working with is POSIX compliant and the code is really so critical to be in-the-know.

But from the C standard library, you're not going to find much beyond being told an error has happened. Generally you'll find you don't need more than that anyway.

Salade

POSIX requires fread to set errno on an error. From the POSIX documentation on fread, "Otherwise, if a read error occurs, the error indicator for the stream shall be set, and errno shall be set to indicate the error. " – Crank

@DavidHammen: that snippet is marked as a POSIX extension [CX]. Still, I wonder why GNU does not follow this extension. Isn't it supposed to when _XOPEN_SOURCE / _POSIX_SOURCE are defined with appropriate values? – Juliusjullundur

Answer 2

Those functions don't use errno, so you shouldn't either.

It is worth noting that you can tell if everything went smoothly from the return value of fread(). If the return value of fread() differs from the passed nmemb parameter (100 in your case), then you either reached the end of your file or an error occured reading it (source). So test only in that case:

Just drop the use of errno alltogether:

chunk = fread(buf, 1, 100, file);
if (chunk != 100) { // If fread() returns a number different to the nmemb parameter, either error or EOF occured
    if (ferror(file))
      {
        printf("Error occured while reading file.");
        return -1; // Or what ever return value you use to indicate an error
      }
}

Unrighteous

@BasileStarynkevitch fread has documented behavior on error or end of file. You need to use ferror() to check if it was an error or feof() for end-of-file. – Unrighteous