### 15.3 FIFO Special Files

A FIFO special file is similar to a pipe, except that it is created in a different way. Instead of being an anonymous communications channel, a FIFO special file is entered into the file system by calling `mkfifo`.

Once you have created a FIFO special file in this way, any process can open it for reading or writing, in the same way as an ordinary file. However, it has to be open at both ends simultaneously before you can proceed to do any input or output operations on it. Opening a FIFO for reading normally blocks until some other process opens the same FIFO for writing, and vice versa.

The `mkfifo` function is declared in the header file `sys/stat.h`.

— Function: int mkfifo (const char *filename, mode_t mode)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> The `mkfifo` function makes a FIFO special file with name filename. The mode argument is used to set the file's permissions; see Setting Permissions.

> The normal, successful return value from `mkfifo` is `0`. In the case of an error, `-1` is returned. In addition to the usual file name errors (see File Name Errors), the following `errno` error conditions are defined for this function:

> `EEXIST`
>   The named file already exists.
> `ENOSPC`
>   The directory or file system cannot be extended.
> `EROFS`
>   The directory that would contain the file resides on a read-only file system.