Can you have multiple "cursors" for the same ifstream ? Would that be thread-safe?

Question

I have multiple threads, and I want each of them to process a part of my file. Can I have a single ifstream object for that and make them read concurrently read different parts ? The parts are non overlapping, so the same line will not be processed by two threads. If yes, how to get multiple cursors ?

Answer 1

A single std::ifstream is associated with exactly one cursor (there's a seekg and tellg method associated with the std::ifstream directly).

If you want the same std::ifstream object to be shared accross multiple threads, you'll have to have some sort of synchronization mechanism between the threads, which might defeat the purpose (in each thread, you'll have to lock, seek, read and unlock each time).

To solve your problem, you can open one std::ifstream to the same file per thread. In each thread, you'd seek to whatever position you want to start reading from. This would only require you to be able to "easily" compute the seek position for each thread though (Note: this is a pretty strong requirement).

Answer 2

C++ file streams are not guaranteed to be thread safe (see e.g. this answer).

The typical solution is anyway to open separate streams on the same file, each instance comes with their own "cursor". However, you need to ensure shared access, and concurrency becomes platform specific.

For ifstream (i.e. only reading from the file), the concurrency issues are usually tame. Even if someone else modifies the file, both streams might see different content, but you do have some kind of eventual consistency.

Reads and writes are usually not atomic, i.e. you might read only part of a write. Writes might not even execute in the order they are issued (see write combining).

Answer 3

Looking at FILE struct it seems like there is a pointer inside FILE, char* curp, pointing to the current active pointer, which may mean that for each FILE object, you'd have one particular part of the file.
This being in C, I don't know how ifstream works and if it uses FILE object/it is built like a FILE object. Might not help you at all, but I thought it would be interesting to share this little information, and that it could may be help someone.