Thread

NAME

thread - runnable / computation entity

SYNOPSIS

TODO

DESCRIPTION

The thread object is the construct that represents a time-shared CPU execution context. Thread objects live associated to a particular Process Object which provides the memory and the handles to other objects necessary for I/O and computation.

Lifetime

Threads are created by calling zx_thread_create(), but only start executing when either zx_thread_start() or zx_process_start() are called. Both syscalls take as an argument the entrypoint of the initial routine to execute.

The thread passed to zx_process_start() should be the first thread to start execution on a process.

A thread terminates execution:

• by calling zx_thread_exit()
• by calling zx_vmar_unmap_handle_close_thread_exit()
• by calling zx_futex_wake_handle_close_thread_exit()
• when the parent process terminates
• by calling zx_task_kill() with the thread's handle
• after generating an exception for which there is no handler or the handler decides to terminate the thread.

Returning from the entrypoint routine does not terminate execution. The last action of the entrypoint should be to call zx_thread_exit() or one of the above mentioned _exit() variants.

Closing the last handle to a thread does not terminate execution. In order to forcefully kill a thread for which there is no available handle, use zx_object_get_child() to obtain a handle to the thread. This method is strongly discouraged. Killing a thread that is executing might leave the process in a corrupt state.

Fuchsia native threads are always detached. That is, there is no join() operation needed to do a clean termination. However, some runtimes above the kernel, such as C11 or POSIX might require threads to be joined.

Signals

Threads provide the following signals:

• ZX_THREAD_TERMINATED
• ZX_THREAD_SUSPENDED
• ZX_THREAD_RUNNING

When a thread is started ZX_THREAD_RUNNING is asserted. When it is suspended ZX_THREAD_RUNNING is deasserted, and ZX_THREAD_SUSPENDED is asserted. When the thread is resumed ZX_THREAD_SUSPENDED is deasserted and ZX_THREAD_RUNNING is asserted. When a thread terminates both ZX_THREAD_RUNNING and ZX_THREAD_SUSPENDED are deasserted and ZX_THREAD_TERMINATED is asserted.

Note that signals are OR'd into the state maintained by the zx_object_wait_*() family of functions thus you may see any combination of requested signals when they return.

SYSCALLS

• zx_thread_create() - create a new thread within a process
• zx_thread_exit() - exit the current thread
• zx_thread_read_state() - read register state from a thread
• zx_thread_start() - cause a new thread to start executing
• zx_thread_write_state() - modify register state of a thread

• zx_task_create_exception_channel() - listen for task exceptions
• zx_task_kill() - cause a task to stop running