🧵 What is TLS (Thread-Local Storage)?
TLS (Thread-Local Storage) is a mechanism that allows each thread in a multithreaded program to have its own copy of a variable, separate from other threads. This is essential for keeping thread-specific state like errno, or per-thread buffers. it is init in the gilibc in the function __libc_setup_tls
📦 Where is it Stored?
In ELF (Executable and Linkable Format) binaries, TLS data is stored in two sections:
-
.tdata: contains initialized thread-local variables (e.g.__thread int x = 5;) -
.tbss: contains uninitialized thread-local variables (like a thread-local__thread int y;that defaults to zero)
Each thread gets its own copy of these sections when it’s created.
🧠 How it Works
At program startup, glibc sets up TLS for the main thread. When a new thread is created (e.g. via pthread_create), glibc allocates memory for that thread’s TLS by copying .tdata and zeroing .tbss.
Each thread has a special register pointing to its TLS block (e.g., %fs on x86_64), so accesses to __thread variables go to the correct location.
you can learn more about it here