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### 27.4 Access to the Controlling Terminal

Processes in the foreground job of a controlling terminal have unrestricted access to that terminal; background processes do not. This section describes in more detail what happens when a process in a background job tries to access its controlling terminal.

When a process in a background job tries to read from its controlling terminal, the process group is usually sent a `SIGTTIN` signal. This normally causes all of the processes in that group to stop (unless they handle the signal and don't stop themselves). However, if the reading process is ignoring or blocking this signal, then `read` fails with an `EIO` error instead.

Similarly, when a process in a background job tries to write to its controlling terminal, the default behavior is to send a `SIGTTOU` signal to the process group. However, the behavior is modified by the `TOSTOP` bit of the local modes flags (see Local Modes). If this bit is not set (which is the default), then writing to the controlling terminal is always permitted without sending a signal. Writing is also permitted if the `SIGTTOU` signal is being ignored or blocked by the writing process.

Most other terminal operations that a program can do are treated as reading or as writing. (The description of each operation should say which.)

For more information about the primitive `read` and `write` functions, see I/O Primitives.