Doing Non-Blocking I/O
Problem
You want to read from or write to a filehandle without the system blocking your process until the program, file, socket, or device at the other end is ready. This is desired less often of regular files than of special files.
Solution
Open the file with sysopen,
and specify the
O_NONBLOCK
option:
use Fcntl; sysopen(MODEM, "/dev/cua0", O_NONBLOCK|O_RDWR) or die "Can't open modem: $!\n";
If you already have a filehandle, use
fcntl
to change the flags:
use Fcntl; $flags = ''; fcntl(HANDLE, F_GETFL, $flags) or die "Couldn't get flags for HANDLE : $!\n"; $flags |= O_NONBLOCK; fcntl(HANDLE, F_SETFL, $flags) or die "Couldn't set flags for HANDLE: $!\n";
Once a filehandle is set for non-blocking I/O, the
sysread
or syswrite
calls that
would block will instead return undef
and set
$!
to EAGAIN:
use POSIX qw(:errno_h); $rv = syswrite(HANDLE, $buffer, length $buffer); if (!defined($rv) && $! == EAGAIN) { # would block } elsif ($rv != length $buffer) { # incomplete write } else { # successfully wrote } $rv = sysread(HANDLE, $buffer, $BUFSIZ); if (!defined($rv) && $! == EAGAIN) { # would block } else { # successfully read $rv bytes from HANDLE }
Discussion
The O_NONBLOCK
constant is part of the POSIX
standard, so most machines should support it. We use the POSIX module
to get the numeric value for the error EAGAIN.
See Also
The sysopen
and fcntl
functions
in perlfunc
(1) and in Chapter 3 of
Programming Perl; the documentation for the standard POSIX module; your system’s ...
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