🧩 What is the TCB (Thread Control Block)?
The TCB (Thread Control Block) is a small, per-thread data structure used internally by the runtime to store metadata about each thread.
It is typically located at the beginning or end of the thread’s TLS (Thread-Local Storage) block.
📦 What’s Inside?
The exact contents are platform-specific, but usually include:
-
A pointer to the DTV (Dynamic Thread Vector)
-
errno(per-thread error value) -
Thread ID or pointer to the thread descriptor (
struct pthread) -
Guards or padding for alignment
📍 Where is it?
-
On x86_64, the TCB is typically placed at the end of the TLS block.
-
The thread pointer (
TP) register (e.g.,%fs) points to the TCB.
🧭 Memory Layout Example
mmap memory block (e.g., allocated with mmap or _dl_early_allocate)
0x00000000 ┌────────────────────────────────────────────┐
│ Thread Stack │
├────────────────────────────────────────────┤
│ TLS (.tdata/.tbss) │
├────────────────────────────────────────────┤
0x00001230 │ TCB (Thread Control Block) │ ← %fs points here (TLS_TCB_AT_TP)
└────────────────────────────────────────────┘Note: On systems with
TLS_TCB_AT_TP, the TCB is at the end of the TLS block, and the%fsregister points to it directly.
🔧 Why It Matters
-
Allows fast access to
__threadvariables -
Needed for per-thread libc functions (like
errno) -
Fundamental for thread creation and context switching
🔍 Summary
The TCB is the anchor of thread-local state. It sits in TLS and is accessed via the thread pointer register. It’s set up early by functions like __libc_setup_tls.