---
name: "Tcl_DetachPids"
section: "3tcl"
platform: "linux"
package: "tcl8.6-doc"
version: "8.6.16+dfsg-1"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/Tcl_DetachPids/3tcl"
---

# Tcl\_DetachPids(3tcl)

manage child processes in background

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```

## NAME

Tcl\_DetachPids, Tcl\_ReapDetachedProcs, Tcl\_WaitPid - manage child processes in background

## SYNOPSIS

```
#include <tcl.h>

Tcl_DetachPids(numPids, pidPtr)

Tcl_ReapDetachedProcs()

Tcl_Pid
Tcl_WaitPid(pid, statusPtr, options)
```

## ARGUMENTS

- int *numPids* (in)

  Number of process ids contained in the array pointed to by *pidPtr*.

- int *\*pidPtr* (in)

  Address of array containing *numPids* process ids.

- Tcl\_Pid *pid* (in)

  The id of the process (pipe) to wait for.

- int *\*statusPtr* (out)

  The result of waiting on a process (pipe). Either 0 or ECHILD.

- int *options* (in)

  The options controlling the wait. WNOHANG specifies not to wait when checking the process.

  ```

  ```

## DESCRIPTION

**Tcl\_DetachPids** and **Tcl\_ReapDetachedProcs** provide a mechanism for managing subprocesses that are running in background. These procedures are needed because the parent of a process must eventually invoke the **waitpid** kernel call (or one of a few other similar kernel calls) to wait for the child to exit.  Until the parent waits for the child, the child's state cannot be completely reclaimed by the system.  If a parent continually creates children and doesn't wait on them, the system's process table will eventually overflow, even if all the children have exited.

**Tcl\_DetachPids** may be called to ask Tcl to take responsibility for one or more processes whose process ids are contained in the *pidPtr* array passed as argument.  The caller presumably has started these processes running in background and does not want to have to deal with them again.

**Tcl\_ReapDetachedProcs** invokes the **waitpid** kernel call on each of the background processes so that its state can be cleaned up if it has exited.  If the process has not exited yet, **Tcl\_ReapDetachedProcs** does not wait for it to exit;  it will check again the next time it is invoked. Tcl automatically calls **Tcl\_ReapDetachedProcs** each time the **exec** command is executed, so in most cases it is not necessary for any code outside of Tcl to invoke **Tcl\_ReapDetachedProcs**. However, if you call **Tcl\_DetachPids** in situations where the **exec** command may never get executed, you may wish to call **Tcl\_ReapDetachedProcs** from time to time so that background processes can be cleaned up.

**Tcl\_WaitPid** is a thin wrapper around the facilities provided by the operating system to wait on the end of a spawned process and to check a whether spawned process is still running. It is used by **Tcl\_ReapDetachedProcs** and the channel system to portably access the operating system.

## KEYWORDS

background, child, detach, process, wait