# Waiting and retrying

Source: https://libtmux.org/en/cxx/latest/topics/waiting-and-retry/

> Polling a condition instead of guessing a sleep, and tmux's own wait-for signal channel as the alternative to polling.

After sending input or starting a process, wait for the state your next step
requires. [Pane
interaction](https://libtmux.org/en/cxx/latest/topics/pane-interaction/#waiting-for-something-to-finish) covers
waiting for screen text. This page covers arbitrary conditions and tmux's named
[`wait-for`](https://libtmux.org/en/tmux/latest/manual/wait-for/) signal channels.

## Polling a condition

Polling checks a condition repeatedly until it succeeds or a deadline expires.
Set a deadline and choose an interval that limits unnecessary tmux commands.

**Helper:** Not listed.

**Where it lives:** no generic condition-poll helper found in the public
library; a [`wait_until`](<https://libtmux.org/en/cxx/latest/reference/libtmux-commandoperation-wait_until/>) exists only in the private `testing` component, for
waiting on a spawned child process, not on tmux state

### Examples

Use a loop with a deadline and interval when a specific wait API does not fit.
[Pane interaction](https://libtmux.org/en/cxx/latest/topics/pane-interaction/#waiting-for-something-to-finish)
covers output waits.

## tmux's own [wait-for](https://libtmux.org/en/tmux/latest/manual/wait-for/) channel

Use [`tmux wait-for -S <channel>`](https://libtmux.org/en/tmux/latest/manual/wait-for/) to signal and [`tmux wait-for <channel>`](https://libtmux.org/en/tmux/latest/manual/wait-for/) to
block until signalled. This avoids repeated screen captures when the command can
announce its own completion:

[`Server.signal`](<https://libtmux.org/en/cxx/latest/reference/libtmux-server-signal/>) signals a channel. [`Server.wait_for`](<https://libtmux.org/en/cxx/latest/reference/libtmux-server-wait_for/>) waits for the
channel with a timeout.

```cpp
server.signal("built");
server.wait_for("built", std::chrono::seconds{5});
```

tmux remembers a signal sent before a waiter starts. The next wait on that
channel returns immediately, so completion is not lost when the command finishes
first.

A raw [`wait-for`](https://libtmux.org/en/tmux/latest/manual/wait-for/) client can exit zero when the server dies, as well as when the
channel is signalled. Verify server liveness when a lost server must be treated
as a failed task.

Use a channel name specific to the task. A remembered signal can otherwise
satisfy an unrelated later wait.

<details>
<summary>tmux manual and source</summary>

The tmux manual describes [completion channels](https://github.com/tmux/tmux/blob/94796f6b1182507efac8a272fc309a79e22e58a5/tmux.1#L8715).
The [channel implementation](https://github.com/tmux/tmux/blob/94796f6b1182507efac8a272fc309a79e22e58a5/cmd-wait-for.c#L388)
retains an early signal until a waiter consumes it. Use a fresh channel name
for each operation.

</details>