POSIX Threading and Resource Sharing
In this lesson, you will learn how multiple threads sharing a single process's file descriptor table can lead to unexpected behaviors when interacting with the filesystem.
When you create a new thread using the POSIX threads library (pthreads), that thread exists within the same process as the parent. Because threads are lightweight, they do not get their own independent file descriptor tables. Instead, all threads in a process share a single, global table of file descriptors.
The Memory and State Implications
This shared table means that if one thread opens a file, every other thread in that process can access it using the resulting file descriptor. However, this convenience introduces a significant challenge: the shared file offset.
Every entry in the file descriptor table points to an "Open File Object" in the kernel. This object maintains the current file offset—the "cursor" that tracks where the next read or write operation will occur. Because all threads share the same file descriptor, they also share that cursor.
Consider two threads writing to the same file simultaneously:
// Thread A and Thread B both share a file descriptor 'fd'
// Thread A: write(fd, "AAAA", 4);
// Thread B: write(fd, "BBBB", 4);
If Thread A begins writing, it moves the offset pointer. If Thread B performs a write operation before Thread A finishes, the kernel's shared file offset will be modified by Thread B's operation. This results in "interleaved" data or data being overwritten because the threads are racing to control the same shared pointer.
Unlike memory (where you might use mutexes to protect variables), the shared nature of the file descriptor table means the filesystem state itself is global to the process. If one thread calls lseek() to jump to the end of a file to append data, it changes the position for every other thread in that process, which may cause those threads to read from or write to the wrong location in the file. To prevent this, developers must ensure that concurrent threads do not share file descriptors if they require independent file positions, or they must implement strict application-level synchronization.
Key Takeaway: Because threads share the same file descriptor table and associated file offsets, concurrent I/O operations from different threads can inadvertently interfere with each other’s file positions.