BSD Sockets Interface Programmer's Guide

Chapter 3 77

Advanced Topics for Stream Sockets

Nonblocking I/O

Sockets are created in blocking mode I/O by default. You can specify that

a socket be put in nonblocking mode by using the ioctl system call

with the FIOSNBIO request. Here is an example:

#include <sys/ioctl.h>

...

ioctl(s, FIOSNBIO, &arg);

arg is a pointer to int:

• When int equals 0, the socket is changed to blocking mode.

• When int equals 1, the socket is changed to nonblocking mode.

If a socket is in nonblocking mode, the following calls are affected:

accept If no connection requests are present, accept returns

immediately with the EWOULDBLOCK error.

connect If the connection cannot be completed immediately,

connect returns with the EINPROGRESS error.

recv If no data are available to be received, recv returns

the value –1 and the EWOULDBLOCK error. This is

also true for read.

send If there is no available buffer space for the data to be

transmitted, send returns the value -1 and the

EWOULDBLOCK error. This is also true for write.

The O_NDELAY ﬂag for fcntl(2) is also supported. If you use this ﬂag

and there are no data available to be received on a recv, recvfrom ,

recvmsg, or read call, the call returns immediately with the value of 0.

If you use the O_NONBLOCK ﬂag, the call returns immediately with the

value of -1 and the EAGAIN error. This is the same as returning an

end-of-ﬁle condition. This is also true for send, sendto, sendmsg, and

write if there is not enough buffer space to complete the send.

NOTE The O_NDELAY and O_NONBLOCK ﬂags have precedence over the

FIOSNBIO ﬂag. Setting both the O_NDELAY and O_NONBLOCK ﬂags

is not allowed.