Thread: Contradictory outputs

Quoting the standard:

Originally Posted by c99 draft, fopen()

When a file is opened with update mode (’+’ as the second or third character in the
above list of mode argument values), both input and output may be performed on the
associated stream. However, output shall not be directly followed by input without an
intervening call to the fflush function or to a file positioning function (fseek,
fsetpos, or rewind), and input shall not be directly followed by output without an
intervening call to a file positioning function, unless the input operation encounters end-
of-file. Opening (or creating) a text file with update mode may instead open (or create) a
binary stream in some implementations.

Because some systems may choose to implement an input buffer and and output buffer. This works great, if all you're doing is input OR output.
But if you're doing input AND output, then some kind of synchronisation event (say seek or flush) is needed to restore consistency between the two.