End of FILE* pointer is not equal to size of written data.

Question

Very simply put, I have the following code snippet:

FILE* test = fopen("C:\\core.u", "w");
printf("Filepointer at: %d\n", ftell(test));
fwrite(data, size, 1, test);
printf("Written: %d bytes.\n", size);
fseek(test, 0, SEEK_END);
printf("Filepointer is now at %d.\n", ftell(test));
fclose(test);

and it outputs:

Filepointer at: 0
Written: 73105 bytes.
Filepointer is now at 74160.

Why is that? Why does the number of bytes written not match the file pointer?

Answer 1

Since you're opening the file in text mode, it will convert end-of-line markers, such as LF, into CR/LF.

This is likely if you're running on Windows (and you probably are, given that your file name starts with "c:\").

If you open the file in "wb" mode, I suspect you'll find the numbers are identical:

FILE* test = fopen("C:\\core.u", "wb");

The C99 standard has this to say in 7.19.5.3 The fopen function:

The argument mode points to a string. If the string is one of the following, the file is open in the indicated mode. Otherwise, the behaviour is undefined.

r open text file for reading
w truncate to zero length or create text file for writing
a append; open or create text file for writing at end-of-file
rb open binary file for reading
wb truncate to zero length or create binary file for writing
ab append; open or create binary file for writing at end-of-file
r+ open text file for update (reading and writing)
w+ truncate to zero length or create text file for update
a+ append; open or create text file for update, writing at end-of-file
r+b or rb+ open binary file for update (reading and writing)
w+b or wb+ truncate to zero length or create binary file for update
a+b or ab+ append; open or create binary file for update, writing at end-of-file

You can see they distinguish between w and wb. I don't believe an implementation is required to treat the two differently but it's usually safer to use binary mode for binary data.

Answer 2

what does fwrite return? normally the return value should be the number of bytes written. Also, what does the ftell() answer with right before the fseek?

It might help to know what operating system, C compiler version and C library.

Answer 3

A filepointer is a cookie. It has no value. The only thing you can use it for is to seek to the same place in a file. I'm not even sure if ISO C guarantees that ftell returns increasing values. If you don't believe this, please look at the different seek() modes. They exist precisely because the position is not a simple byte offset.

Answer 4

windows doesn't actually write all data out to the file without a flush and possibly an fsync. Maybe that's why