Checking for success of fwrite in C, perror

Question

With fwrite returning the number of successful elements written to the file, by saying:

if (!(fwrite(...))) {
    fprintf(stderr, "Failure");
    //perror(???)  I sometimes see code that says perror here and I don't know 
    //exactly what this does.
}

Does this check for successful writing to the file? Are there other things to worry about?

Thanks.

Answer 1

In short, not quite. fwrite returns the number of elements successfully written; you need to check this against the number of elements you intended to write i.e. those you passed in argument to fwrite.

What you've done checks that some elements have been written.

Here's a reference for perror.

Interprets the value of the global variable errno into a string and prints that string to stderr (standard error output stream, usually the screen), optionaly preceding it with the custom message specified in str. errno is an integral variable whose value describes the last error produced by a call to a library function. The error strings produced by perror depend on the developing platform and compiler. If the parameter str is not a null pointer, str is printed followed by a colon (:) and a space. Then, whether str was a null pointer or not, the generated error description is printed followed by a newline character ('\n'). perror should be called right after the error was produced, otherwise it can be overwritten in calls to other functions.

Answer 2

You can also use explain_fwrite(), explain_errno_fwrite, ... from libexplain.

The man page explains:

The explain_fwrite function is used to obtain an explanation of an error returned by the fwrite(3) system call. The least the message will contain is the value of strerror(errno), but usually it will do much better, and indicate the underlying cause in more detail.

The errno global variable will be used to obtain the error value to be decoded.

This function is intended to be used in a fashion similar to the following example (the man-page was wrong here, as correctly pointed out by @puchu in the comment below. I corrected the code to address the issue):

if (fwrite(ptr, size, nmemb, fp) < nmemb)
{
    fprintf(stderr, "%s\n", explain_fwrite(ptr, size, nmemb, fp));
    exit(EXIT_FAILURE);
}

Caveat: This method is not thread-safe.

Answer 3

Your code might not check for errors properly. Use

if (fwrite(ptr, size, num, f) != num) {
    // An error occurred, handle it somehow
}

Answer 4

From the Linux man page of fwrite

fread() and fwrite() return the number of items successfully read or written (i.e., not the number of characters). If an error occurs, or the end-of-file is reached, the return value is a short item count (or zero).

so you need to compare with what is expected return value.

In many cases you may need to check for errno equal to EAGAIN or EINTR, in which case you normally want to retry the write request, while in other cases you want to handle short writes gracefully.

For fwrite, on a short write (where less than your entire data was written) you can check feof() and/or ferror() to see if the stream is returning and end-of-file, EOF, such as if a PIPE was closed, or if the stream has its error inducator flag set.

Answer 5

STRERROR(3)            FreeBSD Library Functions Manual            STRERROR(3)

NAME
     perror, strerror, strerror_r, sys_errlist, sys_nerr — system error mes‐
     sages

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdio.h>

     void
     perror(const char *string);

     ...

DESCRIPTION
     ...

     The perror() function finds the error message corresponding to the cur‐
     rent value of the global variable errno (intro(2)) and writes it, fol‐
     lowed by a newline, to the standard error file descriptor.  If the argu‐
     ment string is non‐NULL and does not point to the null character, this
     string is prepended to the message string and separated from it by a
     colon and space (“: ”); otherwise, only the error message string is
     printed.

...

STANDARDS
     The perror() and strerror() functions conform to ISO/IEC 9899:1999
     (“ISO C99”).  ...