STREAMS Programming Guide

STREAMS Interfaces

The stream head provides the interface between the stream and an application program. After a stream has been opened, STREAMS-related system calls let a user process insert and delete (push and pop) modules. That process can then communicate with and control the operation of the stream head, modules, and drivers. The stream head handles most system calls so that the related processing does not have to be incorporated in a module or driver.

STREAMS System Calls

Table 2-1 offers an overview of some basic STREAMS-related system calls.

Table 2-1 Summary of Basic STREAMS-related System Calls




Opens a stream 


Closes a stream 


Reads data from a stream 


Writes data to a stream 


Controls a stream  


Receives a message at the stream head 


Receives a priority message at the stream head 


Sends a message downstream 


Sends a priority message downstream 


Identifies files on which a user can send or receive messages,  

or on which certain events have occurred (not restricted to 

STREAMS, although historically it was) 


Creates a bidirectional channel that provides a communication 

path between multiple processes  

Note -

Sections 1, 2, 3, 7, and 9 of the online manual pages (man pages) contain all the STREAMS information.

Action Summary

The open(2) system call recognizes a STREAMS special file and creates a stream to the specified driver. A user process can receive and send data on STREAMS files using read(2) and write(2) in the same way as with traditional character files. ioctl(2) lets users perform functions specific to a particular device. STREAMS ioctl(2) commands (see the streamio(7I) man page) support a variety of functions to access and control streams. The final close(2) on a stream dismantles it.

The poll(2) system call provides a mechanism for multiplexing input/output over a set of file descriptors that reference open files. putmsg(2) and getmsg(2) and the putpmsg(2) and getpmsg(2) send and receive STREAMS messages, and can act on STREAMS modules and drivers through a service interface.