Question

When I do a fopen and then a fread, when is the file actually/partially loaded in the memory
during fopen or fread?

Or is it partially loaded at fopen based on size of file and then fully loaded at time
of fread?

Similarly what happens internally at the OS level when fwrite is called?
Is the file loaded into memory at that time, or a page swap happens retriving just that part
of file in memory?

What happens at the OS level at each of these calls with respect to file loading in memory?

Answer 1

• fopen() only creates a handle to the file.
• fread() actually reads the file into a memory buffer (OS-level buffering may occur transparently to the client.)
• fwrite() writes data into the file, though its committing to the storage may get delayed (e.g. with journalled filesystem.)