get or set the address of the thread
control block of the current thread
functions are for use by libpthread and other parts of the C runtime to
retrieve and set the address of the current thread's thread control block
(TCB). This is used to locate per-thread data such as
errno. Each kernel-level thread in a process has a
separate value for this address, which can be obtained and changed via these
The initial thread of a new process created using fork(2) or vfork(2) inherits the TCB address of the thread that created it. A thread created using __tfork(3) starts with the TCB address specified in that call. execve(2) resets the thread's TCB address to zero.
On some platforms, this address is also directly mapped to a CPU register which can be accessed from userspace.
__get_tcb() returns the address of the
thread control block of the current thread.
__set_tcb() system calls appeared in