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__GET_TCB(2) System Calls Manual __GET_TCB(2)

NAME

__get_tcb, __set_tcbget and set the address of the thread control block of the current thread

SYNOPSIS

void *
__get_tcb(void);
void
__set_tcb(void *);

DESCRIPTION

The __get_tcb() and __set_tcb() 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 system calls.
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.

RETURN VALUES

__get_tcb() returns the address of the thread control block of the current thread.

SEE ALSO

__tfork(3)

HISTORY

The __get_tcb() and __set_tcb() system calls appeared in OpenBSD 5.1.
March 30, 2016 OpenBSD-current