c - Why is fseek or fflush always required between reading and writing in the update modes?

Question

Q: I'm trying to update a file in place, by using fopen mode "r+", reading a certain string, and writing back a modified string, but it's not working.

A: Be sure to call fseek before you write, both to seek back to the beginning of the string you're trying to overwrite, and because an fseek or fflush is always required between reading and writing in the read/write "+" modes.

My question is why fseek or fflush is always required between reading and writing in the read/write "+" modes? Section 5.2 of Andrew Koenig's C Traps and Pitfalls (1989) mentioned that it is because of a backward compatibility issue. Can anyone explain in detail?

Answer 1

The library buffers input and output operations. Check out setvbuf() and the _IOFBF, _IOLBF parameters to that funktion.

fseek() or fflush() require the library to commit buffered operations.

The standard specifies a seek or flush operation as mandatory to allow the library some shortcuts; otherwise, for every I/O operation, the lib would have to check if the previous operation was also a read op (or a write op), and trigger a flush by itself if the "direction" of the I/O changed. With the specifications as-is, the library may assume the client did the seek / flush before changing I/O direction.