These routines are the same as their counterparts in the system C library. In list context, the return values from the various get routines are as follows:
# 0 1 2 3 4
my ( $name, $passwd, $gid, $members ) = getgr*
my ( $name, $aliases, $addrtype, $net ) = getnet*
my ( $name, $aliases, $port, $proto ) = getserv*
my ( $name, $aliases, $proto ) = getproto*
my ( $name, $aliases, $addrtype, $length, @addrs ) = gethost*
my ( $name, $passwd, $uid, $gid, $quota,
$comment, $gcos, $dir, $shell, $expire ) = getpw*
# 5 6 7 8 9(If the entry doesn't exist, the return value is a single meaningless true value.)
The exact meaning of the $gcos field varies but it usually contains the real name of the user (as opposed to the login name) and other information pertaining to the user. Beware, however, that in many system users are able to change this information and therefore it cannot be trusted and therefore the $gcos is tainted (see perlsec). The $passwd and $shell, user's encrypted password and login shell, are also tainted, for the same reason.
In scalar context, you get the name, unless the function was a lookup by name, in which case you get the other thing, whatever it is. (If the entry doesn't exist you get the undefined value.) For example:
my $uid = getpwnam($name);
my $name = getpwuid($num);
my $name = getpwent();
my $gid = getgrnam($name);
my $name = getgrgid($num);
my $name = getgrent();
# etc.In getpw*() the fields $quota, $comment, and $expire are special in that they are unsupported on many systems. If the $quota is unsupported, it is an empty scalar. If it is supported, it usually encodes the disk quota. If the $comment field is unsupported, it is an empty scalar. If it is supported it usually encodes some administrative comment about the user. In some systems the $quota field may be $change or $age, fields that have to do with password aging. In some systems the $comment field may be $class. The $expire field, if present, encodes the expiration period of the account or the password. For the availability and the exact meaning of these fields in your system, please consult getpwnam(3) and your system's pwd.h file. You can also find out from within Perl what your $quota and $comment fields mean and whether you have the $expire field by using the Config module and the values d_pwquota, d_pwage, d_pwchange, d_pwcomment, and d_pwexpire. Shadow password files are supported only if your vendor has implemented them in the intuitive fashion that calling the regular C library routines gets the shadow versions if you're running under privilege or if there exists the shadow(3) functions as found in System V (this includes Solaris and Linux). Those systems that implement a proprietary shadow password facility are unlikely to be supported.
The $members value returned by getgr*() is a space-separated list of the login names of the members of the group.
For the gethost*() functions, if the h_errno variable is supported in C, it will be returned to you via $? if the function call fails. The @addrs value returned by a successful call is a list of raw addresses returned by the corresponding library call. In the Internet domain, each address is four bytes long; you can unpack it by saying something like:
my ($w,$x,$y,$z) = unpack('W4',$addr[0]);The Socket library makes this slightly easier:
use Socket;
my $iaddr = inet_aton("127.1"); # or whatever address
my $name = gethostbyaddr($iaddr, AF_INET);
# or going the other way
my $straddr = inet_ntoa($iaddr);In the opposite way, to resolve a hostname to the IP address you can write this:
use Socket;
my $packed_ip = gethostbyname("www.perl.org");
my $ip_address;
if (defined $packed_ip) {
$ip_address = inet_ntoa($packed_ip);
}Make sure gethostbyname is called in SCALAR context and that its return value is checked for definedness.
The getprotobynumber function, even though it only takes one argument, has the precedence of a list operator, so beware:
getprotobynumber $number eq 'icmp' # WRONG
getprotobynumber($number eq 'icmp') # actually means this
getprotobynumber($number) eq 'icmp' # better this wayIf you get tired of remembering which element of the return list contains which return value, by-name interfaces are provided in standard modules: File::stat, Net::hostent, Net::netent, Net::protoent, Net::servent, Time::gmtime, Time::localtime, and User::grent. These override the normal built-ins, supplying versions that return objects with the appropriate names for each field. For example:
use File::stat;
use User::pwent;
my $is_his = (stat($filename)->uid == pwent($whoever)->uid);Even though it looks as though they're the same method calls (uid), they aren't, because a File::stat object is different from a User::pwent object.
Many of these functions are not safe in a multi-threaded environment where more than one thread can be using them. In particular, functions like getpwent() iterate per-process and not per-thread, so if two threads are simultaneously iterating, neither will get all the records.
Some systems have thread-safe versions of some of the functions, such as getpwnam_r() instead of getpwnam(). There, Perl automatically and invisibly substitutes the thread-safe version, without notice. This means that code that safely runs on some systems can fail on others that lack the thread-safe versions.
Portability issues: "getpwnam" in perlport to "endservent" in perlport.