c - Using fseek to backtrack

Question

Is using fseek to backtrack character fscanf operations reliable?

Like for example if I have just fscanf-ed 10 characters but I would like to backtrack the 10 chars can I just fseek(infile, -10, SEEK_CUR) ?

For most situations it works but I seem to have problems with the character ^M. Apparently fseek registers it as a char but fscanf doesn't register it, thus in my previous example a 10 char block containing a ^M would require fseek(infile, -11, SEEK_CUR) instead. fseek(infile, -10, SEEK_CUR) would make bring it short by 1 character.

Why is this so?

Edit: I was using fopen in text mode

Answer 1

You're seeing the difference between a "text" and a "binary" file. When a file is opened in text mode (no 'b' in the fopen second argument), the stdio library may (indeed, must) interpret the contents of the file according to the operating system's conventions for text files. For example, in Windows, a line ends with , and this gets translated to a single by stdio, since that is the C convention. When writing to a text file, a single gets output as .

This makes it easier to write portable C programs that handle text files. Some details become complicated, however, and fseeking is one of them. Because of this, the C standard only defines fseek in text files in a few cases: to the very beginning, to the very end, to the current position, and to a previous position that has been retrieved with ftell. In other words, you can't compute a location to seek to for text files. Or you can, but you have to take care of the all the platform-specific details yourself.

Alternatively, you can use binary files and do the line-ending transformations yourself. Again, portability suffers.

In your case, if you just want to go back to where you last did fscancf, the easiest would be to use ftell just before you fscanf.