STREAMS-UX Programmer's Guide (February 2007)

STREAMS Mechanism and System Calls

Creating a Stream

Chapter 2

Creating a Stream

open(2) Opening a STREAMS Device

A user application creates a stream by opening a STREAMS device with the open (2) call. This call is part of

the standard UNIX I/O. It opens a UNIX file for reading, writing, or both.

Synopsis

#include <fcntl.h>

int open(const char *path, int oflag, ... /* [mode_t mode] */ );

Arguments

path A pointer to a path name of the STREAMS device file being opened.

oflag The bitwise inclusive OR of O_NDELAY or O_NONBLOCK inclusive OR-ed with one of the

read-write flags. Other flag values are not applicable to STREAMS devices and have no

effect on them.

mode An optional mode argument.

Read-write flags specify the mode in which the STREAMS device is to be opened. The mode can be read-only,

write-only, or read-write. Determine the value for this flag by considering the type of driver and the nature of

the STREAMS application accessing the driver.

Only one of the following values must be used to specify the value of oflag:

O_RDONLY Open for read only

O_WRONLY Open for write only

O_RDWR Open for read-write

If none or more than one value is used, the behavior is undefined.

O_NDELAY and O_NONBLOCK are the only general flags relevant to STREAMS devices. The values of O_NDELAY

and O_NONBLOCK affect the operation of STREAMS drivers and certain system calls. Refer to read (2), getmsg

(2), putmsg (2), and write (2) for more information on these flags.

For drivers, the implementation of O_NDELAY and O_NONBLOCK is device specific. Each STREAMS device

driver may treat these options differently. For example, while opening a STREAMS device file associated with

a communication line, if either O_NDELAY or O_NONBLOCK is set, the open() returns without waiting for

carrier. Otherwise, it does not return until the carrier is present.

Return Values

open (2) returns an integer file descriptor upon successful completion, and a -1 in case of failure. The

corresponding errno is set appropriately, as with the standard UNIX File System I/O.

Creating a STREAMS-Based pipe with pipe(2)

A STREAMS-based pipe is a special case of a stream. Two streams are created and connected together when

a thread executes a pipe (2) system call. The data flow is bi-directional in a STREAMS-based pipe. Both

stream heads can be used for read and write operations. Figure 2-1, “A STREAMS-Based Pipe,” illustrates the

bi-directional data flow in a STREAMS-based pipe.