Advanced Programming in the UNIX® Environment

ptg10805159

Section 16.8 Nonblocking and Asynchronous I/O 627

16.8 Nonblocking and Asynchronous I/O

Normally,the recvfunctions will block when no data is immediately available.

Similarly,thesendfunctions will block when there is not enough room in the socket’s

output queue to send the message. This behavior changes when the socket is in

nonblocking mode. In this case, these functions will fail instead of blocking, setting

errnoto eitherEWOULDBLOCKorEAGAIN.When this happens, we can use eitherpoll

orselectto determine when we can receive or transmit data.

The Single UNIX Specification includes support for a general asynchronous I/O

mechanism (recall Section 14.5). The socket mechanism has its own way of handling

asynchronous I/O, but this isn’t standardized in the Single UNIX Specification. Some

texts refer to the classic socket-based asynchronous I/O mechanism as ‘‘signal-based

I/O’’ to distinguish it from the general asynchronous I/O mechanism found in the

Single UNIX Specification.

With socket-based asynchronous I/O, we can arrange to be sent theSIGIOsignal

when we can read data from a socket or when space becomes available in a socket’s

write queue. Enabling asynchronous I/O is a two-step process.

1. Establish socket ownership so signals can be delivered to the proper processes.


2. Inform the socket that we want it to signal us when I/O operations won’t block.
   We can accomplish the first step in three ways.


3. Use theF_SETOWNcommand withfcntl.


4. Use theFIOSETOWNcommand withioctl.


5. Use theSIOCSPGRPcommand withioctl.
   To accomplish the second step, we have two choices.


6. Use theF_SETFLcommand withfcntland enable theO_ASYNCfile flag.


7. Use theFIOASYNCcommand withioctl.
   We have several options, but they arenot universally supported. Figure16.23
   summarizes the support for these options provided by the platforms discussed in this
   text.

FreeBSD Linux Mac OS X Solaris

Mechanism POSIX.1 8.0 3.2.0 10.6.8 10

fcntl(fd, F_SETOWN, pid) •••• •

ioctl(fd, FIOSETOWN, pid) •• ••

ioctl(fd, SIOCSPGRP, pid) •• ••

fcntl(fd, F_SETFL, flags|O_ASYNC) •• •

ioctl(fd, FIOASYNC, &n); •• ••

Figure 16.23 Socket asynchronous I/O management commands