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.