Creating special files

There are several types of special files:

A character special file is a file that provides access to an input/output device. Examples of character special files are: a terminal file, a NULL file, a file descriptor file, or a system console file. Each character special file has a device major number, which identifies the device type, and a device minor number, which identifies a specific device of a given device type. Character special files are customarily defined in /dev; these files are defined with the mknod command. You must have UID(0) to create a character special file. The best way to obtain UID(0) is to be defined to BPX.SUPERUSER FACILITY class. Then issue the su command to switch to UID(0) before issuing the mknod command.
You cannot share character special files in read/write mode among systems participating in a shared file system in a sysplex.
A pipe is a way to communicate in first-in-first-out (FIFO) order from one or more processes to one or more processes. Pipes are treated as though they were files.
Figure 1 shows how a pipe works.
Figure 1. How a pipe works

A pipe sends data from one process to another or back to itself. By forking processes, a pipe can be shared by a number of processes; for example, written to by three processes and read by seven.

A program creates a pipe with a pipe() function. The pipe vanishes when the last process closes it. A pipe does not have a name in the file system; a pipe is also called an unnamed pipe.
A FIFO special file sends data from one process to another so that the receiving process reads the data first-in-first-out (FIFO). A FIFO special file is also called a named pipe, or a FIFO. A FIFO special file can also be shared by a number of processes that were not created by forks. A FIFO special file can be written into and read by the same process using multiple threads.
FIFO special files can be shared between systems that use shared file systems. For more information about shared file systems, see Sharing file systems in a sysplex.

A program creates a FIFO special file with a mkfifo command or a mkfifo() function. The name is maintained in the file system until the named pipe is deleted by an rm command or an unlink() function.
A UNIX domain socket address file represents socket addresses in the UNIX domain.
These files cannot be shared in read/write mode among systems participating in a shared file system in a sysplex.

To prepare for using AF_UNIX (local) sockets, the AF_UNIX physical file system (PFS) creates a socket address file in the file hierarchy during the bind() function call. The files are defined as specified by the program that calls bind() and are typically in the user's home directory, the root directory, or in /tmp.

Because they are part of the file system, be careful not to delete any of these socket address files by accident. If you do delete them, programs will not be able to connect to or send datagrams to the program that created the file.

Pipes and FIFO special files are created by programs and users; character special files are typically created by the system programmer.