perl function fileno

The fileno function in Perl is used to get the file descriptor number of a filehandle. Here is an example:

my $filename = 'test.txt';

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

# Get the file descriptor number
my $fd = fileno($fh);

# Print the file descriptor number
print "File descriptor number for $filename: $fd\n";

# Close the file
close($fh);

In this example, the fileno function is used to get the file descriptor number of the filehandle $fh. The file is opened for reading ("<") and the file descriptor number is printed to the console. Finally, the file is closed.

The file descriptor number is a low-level identifier for an open file that is used by the operating system to perform I/O operations. It is an integer value that is usually greater than or equal to 0. The file descriptor number can be used with various system calls, such as fcntl, select, and poll, to perform low-level file I/O operations.

Note that the fileno function can only be used with open filehandles. If the filehandle is not open, or if it is not a valid filehandle, the function will return undef.