I am reading The C Programming Language (2nd Edition). On page 157 and 158 the author gives a code snippet of fopen in the Unix system. At the end of the snippet the author added:

In particular, our fopen does not recognize the "b" that signals binary access, since that is meaningless on UNIX systems, nor the "+" that permits both reading and writing.

Why does the author say it's meaningless? (The "b" and "+" mentioned here are file access modes)


Some non-Unix systems treat binary and text files in different ways. For example under DOS, Windows and OS/2 (which wouldn’t have been relevant when fopen was designed, but serve as useful examples), opening a file in text mode and writing to it will convert line endings from “C” convention (\n) to whatever the platform requires. On other systems, opening a file in binary mode will cause it to be processed in records. This is what fopen’s “b” flag controls: files opened without it are opened in text mode, files opened with it are opened in binary mode. Since Unix-style systems don’t have this distinction, “b” is ignored (and doesn’t cause an error).

My copy of the book doesn’t mention “+”, but I’m guessing fopen didn’t support it then (it does now).


According to the POSIX specification of fopen() (which did not exist when that book was written):

The character b shall have no effect, but is allowed for ISO C standard conformance.

The b mode must be accepted, but it has no effect on a POSIX system.

Furthermore, the + mode:

When a file is opened with update mode (+ as the second or third character in the mode argument), both input and output may be performed on the associated stream.

So that definitely has an effect even on a POSIX system.

Note that the C programming language has changed a bit since the K&R book was written.

  • 1
    POSIX wasn’t a thing when K&R wrote their book... – Stephen Kitt Mar 13 '18 at 14:16
  • 1
    @StephenKitt Ah, yes, but the question wonders why they (the b and + modes) are meaningless. – Kusalananda Mar 13 '18 at 14:17

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