Threads and file descriptors

Question

Am sorry for not doing my own research and asking it here. I am slightly short of time.

Is it possible to have two file descriptor table for two or more threads spwaned from a single thread. The two or more child threads are concurrently accessing same file, so is the offset for two different opens for same file from different threads, thread specific?

Thanks

Answer 1

The file descriptors are shared between the threads. If you want "thread specific" offsets, why not have each thread use a different file descriptor (open(2) multiple times) ?

Answer 2

No, there is only one file descriptor table per process, and it's shared among all the threads.

From your problem description, you might want to look into the pread() and pwrite() functions.

Answer 3

In Linux, you can unshare() the file descriptor table via the CLONE_FILES flag, but I would advise against it.

Answer 4

Try pread()/pwrite().

You can still share the same filedescriptor among multiple threads,i.e, parallel reads/writes to the same file is guaranteed to be atomic using pread()/pwrite() as you will need to specify offset and number of bytes to read/write respectively.