Why fprintf doesn't write directly into the file unless fflush() is used?

Question

I have written a daemon that writes a value in a file. What I have observed is that when I keep writing on a file, there is nothing visible in the file. in other hand, If I

Answer 1

fprintf is an IO routine provided by the libc, it use caching mechanism by default, before doing a real write into files.

Characters are normally accumulated and transmitted asynchronously to the file in a block, so the cache must exceed the libc(stdio) internal buffer size (BUFSIZE, #defined in stdio.h) or when a fflush() has occurred.

If you want to minimize the caching i suggest you to use O_DIRECT or O_SYNC flags for your open call, but there is some restrictions: you must ensure alignment of your buffers and other stuffs. Read the O_DIRECT section of man 2 open .

you may also read this for further informations on how to control libc buffering.