overview

this repo is a custom threading library built in c, designed to mimic core pthread functionality from scratch. it reimplements threading primitives like pthread_create, pthread_join, and pthread_exit, along with basic semaphore support. this project definitely helped me explore how user-level threading and synchronization can be manually orchestrated in a unix-like environment.

architecture

the core of the system revolves around a thread control block (TCB) array that can manage up to 128 threads. each TCB stores metadata like thread id, execution status (ready, running, blocked, exited), a jmp_buf for context switching, and a pointer to its allocated stack. there is also a global variable tracks the currently running thread and a signal-based timer (sigalrm) triggers context switches every 50ms via a scheduler function.

threading model

• pthread_create: initializes the TCB for a new thread, sets up its stack, and prepares its context to jump into the specified start routine. the first call also sets up the scheduler and signal handler.
• pthread_exit: marks the current thread as exited, frees its resources, and invokes the scheduler to switch to the next ready thread.
• pthread_join: allows one thread to wait for another to finish. if the target thread hasn’t exited, the calling thread is blocked until it does.

synchronization

basic semaphore functionality is implemented using a custom sema struct. semaphores maintain a count and a queue of blocked threads. sem_wait decrements the count or blocks the thread if the count is zero, while sem_post increments the count and unblocks waiting threads. sem_init and sem_destroy handle initialization and cleanup.