CODEWITHSUNDEEP
linuxlinux-kerneloperating-systemlinux-device-driver
Adi
Adi

Reputation: 731

Is file object local to every process or System wide?

As a Linux device driver developer i was in the idea that file object is local structure to every process and its address is available in the fd table for the corresponding fd. But when i came across section 5.6 in Linux Programming interface by Michale Kerrisk which states that

Two different file descriptors that refer to the same open file description share a file offset value. Therefore, if the file offset is changed via one file descriptor (as a consequence of calls to read(), write(), or lseek()), this change is visible through the other file descriptor. This applies both when the two file descrip tors belong to the same process and when they belong to different processes.

I am befuddled...Kindly some one help me improve my understanding.

Upvotes: 4

Views: 472

Answers (1)

user149341
user149341

Reputation:

Each process does have its own file descriptor table, and each time a file is open()ed yields a separate file description. So there is sanity there!

The exception is when a file descriptor is duplicated, either within a process (via dup()) or across processes (by one process fork()ing a copy with all the same FDs, or by passing a file descriptor through a UNIX domain socket). When this happens, the two descriptors end up sharing some properties with each other, including the offset.

This is not necessarily a bad thing. It means, for instance, that two processes that are both writing to a shared file descriptor will not end up overwriting each other's output. It can sometimes have unexpected results, though. But it's not usually something that you'd end up with without knowing about it.

Upvotes: 6

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