Question

What really happens when write() system call is executed?

Lets say I have a program which writes certain data into a file using write() function call. Now C library has its own internal buffer and OS too has its own buffer.

What interaction takes place between these buffers ?

Is it like when C library buffer gets filled completely, it writes to OS buffer and when OS buffer gets filled completely, then the actual write is done on the file?

I am looking for some detailed answers, useful links would also help. Consider this question for a UNIX system.

Answer 1

The write() system call (in fact all system calls) are nothing more that a contract between the application program and the OS.

• for “normal” files, the write() only puts the data on a buffer, and marks that buffer as “dirty”
• at some time in the future, these dirty buffers will be collected and actually written to disk. This can be forced by fsync()
• this is done by the .write() “method” in the mounted-filesystem-table
• and this will invoke the hardware’s .write() method. (which could involve another level of buffering, such as DMA)
• modern hard disks have there own buffers, which may or may not have actually been written to the physical disk, even if the OS->controller told them to.

Now, some (abnormal) files don’t have a write() method to support them. Imagine open()ing “/dev/null”, and write()ing a buffer to it. The system could choose not to buffer it, since it will never be written anyway.

Also note that the behaviour of write() does depend on the nature of the file; for network sockets the write(fd,buff,size) can return before size bytes have been sent(write will return the number of characters sent). But it is impossible to find out where they are once they have been sent. They could still be in a network buffer (eg waiting for Nagle …), or a buffer inside the network interface, or a buffer in a router or switch somewhere on the wire.