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.

An example usage of this call is:

#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:

recvfrom: If no messages are available to be received, recvfrom returns the value -1 and the EWOULDBLOCK error. This is also true for recv and read.
sendto: If there is no available message space for the message to be transmitted, sendto returns the value -1 and the EWOULDBLOCK error.

The O_NDELAY flag for fcntl(2) is also supported. If you use this flag and there is no message available to be received on a recv, recvfrom, 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, and write if there is not enough buffer space to complete the send.

The O_NDELAY and O_NONBLOCK flags have precedence over the FIOSNBIO flag. Setting both the O_DELAY and O_NONBLOCK flags is not allowed.

Nonblocking I/O

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