Perl supports network programming in a way that is very familiar to those who have written network code in C programs. In fact, most of the Perl functions that provide network access have the same names and similar parameters as their C counterparts. We can’t teach a complete course on network programming in this chapter, but let’s take a look at one of the task fragments to see how it’s done in Perl.
One of the things you need to find out is the network address that
goes with a network name, or vice versa. In C, you use the
gethostbyname
routine to convert a network name to a network address. You then use
this address to create a connection from your program to another
program somewhere else.
The Perl function to translate a host name to an address has the same name and similar parameters as the C routine, and looks like this:
($name, $aliases, $addrtype, $length, @addrs) = gethostbyname($name); # generic form of gethostbyname
The parameter to this function is a hostname—for example, slate.bedrock.com. The return value is a list of four or more parameters, depending on how many addresses are associated with the name. If the hostname is not valid, the function returns an empty list.
If gethostbyname
is called in a scalar context,
only the (first) address is returned.
When gethostbyname()
completes successfully,
$name
is the
canonical
name, which differs from the input name if the input name
is an alias. $aliases
is a list of space-separated
names by which the host is also known. $addrtype
gives a coded value to indicate the form of the addresses. In this
case, for
slate.bedrock.com,
we can presume that the value indicates an IP address, usually
represented as four numbers under 256, separated by dots.
$length
gives the number of addresses, which is
actually redundant information because you can look at the length of
@addrs
anyway.
But the useful part of the return value is @addrs
.
Each element of the list is a separate IP address, stored in an
internal format, handled in Perl as a four-character
string.[92]
While this four-character string is exactly what other Perl
networking functions are looking for, suppose we wanted to print out
the result for the user to see. In this case, we need to convert the
return value into a human-readable format with the assistance of the
unpack
function and a little additional massaging. Here’s some code
that prints one of
slate.bedrock.com’ s IP
addresses:
($addr) = (gethostbyname("slate.bedrock.com"))[4]; print "Slate's address is ", join(".",unpack("C4", $addr)), " ";
The unpack
takes the four-byte string and returns
four numbers. This just happens to be in the right order for the
join
to glue in a dot between each pair of numbers
to make the human-readable form. See Appendix C,
for information about building simple networking clients.