How to tell if a character special file would block if read from

Question

I would like to be able to tell if a character special file would block if a character were read from it without actually reading a character from it. Can this be done?

Answer 1

You can do this from bash using a 0 timeout to read.

if read -t 0
then read data
fi

To test a file descriptor other than stdin, say 3, use -u 3. To find how many chars are ready on stdin you can use a small perl script:

#!/usr/bin/perl
require 'sys/ioctl.ph';
$size = pack("L", 0);
ioctl(*STDIN, FIONREAD(), $size) or die "ioctl fail: $!\n";
$size = unpack("L", $size);
print "stdin ready: $size\n";

Answer 2

POSIX allows non-blocking, zero-character reads which "may detect and return errors", including that the read would block:

• open(2) the device in O_NONBLOCKing mode;
• read(2) with a count of 0.

If this read operation checks for errors in such cases, if a read would block then read will return -1 and set errno to EAGAIN (or possibly EWOULDBLOCK for sockets, but you're asking about character devices).

Answer 3

From C or C++ you would normally use pselect() to test if there is data ready for reading. You can do that without having to set the file descriptor mode to non-blocking.