perl function fcntl

The fcntl function in Perl is used to perform various operations on file descriptors. It is often used to set or get file descriptor flags or to perform other low-level file I/O operations. Here is an example:

use Fcntl qw(:DEFAULT :flock);

my $filename = 'test.txt';

# Open the file for reading and writing
open(my $fh, "+<", $filename) or die "Cannot open $filename: $!";

# Lock the file for exclusive access
fcntl($fh, F_SETLK, LOCK_EX) or die "Cannot lock $filename: $!";

# Read from the file
my $data;
while (my $line = <$fh>) {
  $data .= $line;
}

# Write to the file
print $fh "New data\n";

# Release the lock
fcntl($fh, F_SETLK, LOCK_UN) or die "Cannot unlock $filename: $!";

# Close the file
close($fh);

In this example, the fcntl function is used to lock a file for exclusive access using file locking (LOCK_EX). The file is then read from and written to, and the lock is released (LOCK_UN) when the program is finished. The fcntl function is also used to get or set other file descriptor flags or perform other file I/O operations, such as setting the non-blocking I/O flag (O_NONBLOCK).

Note that the fcntl function requires the Fcntl module to be loaded, which provides constants for the various flags and operations that can be performed. The :DEFAULT and :flock tags are used to import the commonly used flags and operations.