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diff --git a/contrib/perl5/pod/perlopentut.pod b/contrib/perl5/pod/perlopentut.pod deleted file mode 100644 index b4003f4..0000000 --- a/contrib/perl5/pod/perlopentut.pod +++ /dev/null @@ -1,864 +0,0 @@ -=head1 NAME - -perlopentut - tutorial on opening things in Perl - -=head1 DESCRIPTION - -Perl has two simple, built-in ways to open files: the shell way for -convenience, and the C way for precision. The choice is yours. - -=head1 Open E<agrave> la shell - -Perl's C<open> function was designed to mimic the way command-line -redirection in the shell works. Here are some basic examples -from the shell: - - $ myprogram file1 file2 file3 - $ myprogram < inputfile - $ myprogram > outputfile - $ myprogram >> outputfile - $ myprogram | otherprogram - $ otherprogram | myprogram - -And here are some more advanced examples: - - $ otherprogram | myprogram f1 - f2 - $ otherprogram 2>&1 | myprogram - - $ myprogram <&3 - $ myprogram >&4 - -Programmers accustomed to constructs like those above can take comfort -in learning that Perl directly supports these familiar constructs using -virtually the same syntax as the shell. - -=head2 Simple Opens - -The C<open> function takes two arguments: the first is a filehandle, -and the second is a single string comprising both what to open and how -to open it. C<open> returns true when it works, and when it fails, -returns a false value and sets the special variable $! to reflect -the system error. If the filehandle was previously opened, it will -be implicitly closed first. - -For example: - - open(INFO, "datafile") || die("can't open datafile: $!"); - open(INFO, "< datafile") || die("can't open datafile: $!"); - open(RESULTS,"> runstats") || die("can't open runstats: $!"); - open(LOG, ">> logfile ") || die("can't open logfile: $!"); - -If you prefer the low-punctuation version, you could write that this way: - - open INFO, "< datafile" or die "can't open datafile: $!"; - open RESULTS,"> runstats" or die "can't open runstats: $!"; - open LOG, ">> logfile " or die "can't open logfile: $!"; - -A few things to notice. First, the leading less-than is optional. -If omitted, Perl assumes that you want to open the file for reading. - -The other important thing to notice is that, just as in the shell, -any white space before or after the filename is ignored. This is good, -because you wouldn't want these to do different things: - - open INFO, "<datafile" - open INFO, "< datafile" - open INFO, "< datafile" - -Ignoring surround whitespace also helps for when you read a filename in -from a different file, and forget to trim it before opening: - - $filename = <INFO>; # oops, \n still there - open(EXTRA, "< $filename") || die "can't open $filename: $!"; - -This is not a bug, but a feature. Because C<open> mimics the shell in -its style of using redirection arrows to specify how to open the file, it -also does so with respect to extra white space around the filename itself -as well. For accessing files with naughty names, see -L<"Dispelling the Dweomer">. - -=head2 Pipe Opens - -In C, when you want to open a file using the standard I/O library, -you use the C<fopen> function, but when opening a pipe, you use the -C<popen> function. But in the shell, you just use a different redirection -character. That's also the case for Perl. The C<open> call -remains the same--just its argument differs. - -If the leading character is a pipe symbol, C<open> starts up a new -command and open a write-only filehandle leading into that command. -This lets you write into that handle and have what you write show up on -that command's standard input. For example: - - open(PRINTER, "| lpr -Plp1") || die "cannot fork: $!"; - print PRINTER "stuff\n"; - close(PRINTER) || die "can't close lpr: $!"; - -If the trailing character is a pipe, you start up a new command and open a -read-only filehandle leading out of that command. This lets whatever that -command writes to its standard output show up on your handle for reading. -For example: - - open(NET, "netstat -i -n |") || die "cannot fork: $!"; - while (<NET>) { } # do something with input - close(NET) || die "can't close netstat: $!"; - -What happens if you try to open a pipe to or from a non-existent command? -In most systems, such an C<open> will not return an error. That's -because in the traditional C<fork>/C<exec> model, running the other -program happens only in the forked child process, which means that -the failed C<exec> can't be reflected in the return value of C<open>. -Only a failed C<fork> shows up there. See -L<perlfaq8/"Why doesn't open() return an error when a pipe open fails?"> -to see how to cope with this. There's also an explanation in L<perlipc>. - -If you would like to open a bidirectional pipe, the IPC::Open2 -library will handle this for you. Check out -L<perlipc/"Bidirectional Communication with Another Process"> - -=head2 The Minus File - -Again following the lead of the standard shell utilities, Perl's -C<open> function treats a file whose name is a single minus, "-", in a -special way. If you open minus for reading, it really means to access -the standard input. If you open minus for writing, it really means to -access the standard output. - -If minus can be used as the default input or default output, what happens -if you open a pipe into or out of minus? What's the default command it -would run? The same script as you're currently running! This is actually -a stealth C<fork> hidden inside an C<open> call. See -L<perlipc/"Safe Pipe Opens"> for details. - -=head2 Mixing Reads and Writes - -It is possible to specify both read and write access. All you do is -add a "+" symbol in front of the redirection. But as in the shell, -using a less-than on a file never creates a new file; it only opens an -existing one. On the other hand, using a greater-than always clobbers -(truncates to zero length) an existing file, or creates a brand-new one -if there isn't an old one. Adding a "+" for read-write doesn't affect -whether it only works on existing files or always clobbers existing ones. - - open(WTMP, "+< /usr/adm/wtmp") - || die "can't open /usr/adm/wtmp: $!"; - - open(SCREEN, "+> /tmp/lkscreen") - || die "can't open /tmp/lkscreen: $!"; - - open(LOGFILE, "+>> /tmp/applog" - || die "can't open /tmp/applog: $!"; - -The first one won't create a new file, and the second one will always -clobber an old one. The third one will create a new file if necessary -and not clobber an old one, and it will allow you to read at any point -in the file, but all writes will always go to the end. In short, -the first case is substantially more common than the second and third -cases, which are almost always wrong. (If you know C, the plus in -Perl's C<open> is historically derived from the one in C's fopen(3S), -which it ultimately calls.) - -In fact, when it comes to updating a file, unless you're working on -a binary file as in the WTMP case above, you probably don't want to -use this approach for updating. Instead, Perl's B<-i> flag comes to -the rescue. The following command takes all the C, C++, or yacc source -or header files and changes all their foo's to bar's, leaving -the old version in the original file name with a ".orig" tacked -on the end: - - $ perl -i.orig -pe 's/\bfoo\b/bar/g' *.[Cchy] - -This is a short cut for some renaming games that are really -the best way to update textfiles. See the second question in -L<perlfaq5> for more details. - -=head2 Filters - -One of the most common uses for C<open> is one you never -even notice. When you process the ARGV filehandle using -C<< <ARGV> >>, Perl actually does an implicit open -on each file in @ARGV. Thus a program called like this: - - $ myprogram file1 file2 file3 - -Can have all its files opened and processed one at a time -using a construct no more complex than: - - while (<>) { - # do something with $_ - } - -If @ARGV is empty when the loop first begins, Perl pretends you've opened -up minus, that is, the standard input. In fact, $ARGV, the currently -open file during C<< <ARGV> >> processing, is even set to "-" -in these circumstances. - -You are welcome to pre-process your @ARGV before starting the loop to -make sure it's to your liking. One reason to do this might be to remove -command options beginning with a minus. While you can always roll the -simple ones by hand, the Getopts modules are good for this. - - use Getopt::Std; - - # -v, -D, -o ARG, sets $opt_v, $opt_D, $opt_o - getopts("vDo:"); - - # -v, -D, -o ARG, sets $args{v}, $args{D}, $args{o} - getopts("vDo:", \%args); - -Or the standard Getopt::Long module to permit named arguments: - - use Getopt::Long; - GetOptions( "verbose" => \$verbose, # --verbose - "Debug" => \$debug, # --Debug - "output=s" => \$output ); - # --output=somestring or --output somestring - -Another reason for preprocessing arguments is to make an empty -argument list default to all files: - - @ARGV = glob("*") unless @ARGV; - -You could even filter out all but plain, text files. This is a bit -silent, of course, and you might prefer to mention them on the way. - - @ARGV = grep { -f && -T } @ARGV; - -If you're using the B<-n> or B<-p> command-line options, you -should put changes to @ARGV in a C<BEGIN{}> block. - -Remember that a normal C<open> has special properties, in that it might -call fopen(3S) or it might called popen(3S), depending on what its -argument looks like; that's why it's sometimes called "magic open". -Here's an example: - - $pwdinfo = `domainname` =~ /^(\(none\))?$/ - ? '< /etc/passwd' - : 'ypcat passwd |'; - - open(PWD, $pwdinfo) - or die "can't open $pwdinfo: $!"; - -This sort of thing also comes into play in filter processing. Because -C<< <ARGV> >> processing employs the normal, shell-style Perl C<open>, -it respects all the special things we've already seen: - - $ myprogram f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile - -That program will read from the file F<f1>, the process F<cmd1>, standard -input (F<tmpfile> in this case), the F<f2> file, the F<cmd2> command, -and finally the F<f3> file. - -Yes, this also means that if you have a file named "-" (and so on) in -your directory, that they won't be processed as literal files by C<open>. -You'll need to pass them as "./-" much as you would for the I<rm> program. -Or you could use C<sysopen> as described below. - -One of the more interesting applications is to change files of a certain -name into pipes. For example, to autoprocess gzipped or compressed -files by decompressing them with I<gzip>: - - @ARGV = map { /^\.(gz|Z)$/ ? "gzip -dc $_ |" : $_ } @ARGV; - -Or, if you have the I<GET> program installed from LWP, -you can fetch URLs before processing them: - - @ARGV = map { m#^\w+://# ? "GET $_ |" : $_ } @ARGV; - -It's not for nothing that this is called magic C<< <ARGV> >>. -Pretty nifty, eh? - -=head1 Open E<agrave> la C - -If you want the convenience of the shell, then Perl's C<open> is -definitely the way to go. On the other hand, if you want finer precision -than C's simplistic fopen(3S) provides, then you should look to Perl's -C<sysopen>, which is a direct hook into the open(2) system call. -That does mean it's a bit more involved, but that's the price of -precision. - -C<sysopen> takes 3 (or 4) arguments. - - sysopen HANDLE, PATH, FLAGS, [MASK] - -The HANDLE argument is a filehandle just as with C<open>. The PATH is -a literal path, one that doesn't pay attention to any greater-thans or -less-thans or pipes or minuses, nor ignore white space. If it's there, -it's part of the path. The FLAGS argument contains one or more values -derived from the Fcntl module that have been or'd together using the -bitwise "|" operator. The final argument, the MASK, is optional; if -present, it is combined with the user's current umask for the creation -mode of the file. You should usually omit this. - -Although the traditional values of read-only, write-only, and read-write -are 0, 1, and 2 respectively, this is known not to hold true on some -systems. Instead, it's best to load in the appropriate constants first -from the Fcntl module, which supplies the following standard flags: - - O_RDONLY Read only - O_WRONLY Write only - O_RDWR Read and write - O_CREAT Create the file if it doesn't exist - O_EXCL Fail if the file already exists - O_APPEND Append to the file - O_TRUNC Truncate the file - O_NONBLOCK Non-blocking access - -Less common flags that are sometimes available on some operating -systems include C<O_BINARY>, C<O_TEXT>, C<O_SHLOCK>, C<O_EXLOCK>, -C<O_DEFER>, C<O_SYNC>, C<O_ASYNC>, C<O_DSYNC>, C<O_RSYNC>, -C<O_NOCTTY>, C<O_NDELAY> and C<O_LARGEFILE>. Consult your open(2) -manpage or its local equivalent for details. (Note: starting from -Perl release 5.6 the O_LARGEFILE flag, if available, is automatically -added to the sysopen() flags because large files are the default.) - -Here's how to use C<sysopen> to emulate the simple C<open> calls we had -before. We'll omit the C<|| die $!> checks for clarity, but make sure -you always check the return values in real code. These aren't quite -the same, since C<open> will trim leading and trailing white space, -but you'll get the idea: - -To open a file for reading: - - open(FH, "< $path"); - sysopen(FH, $path, O_RDONLY); - -To open a file for writing, creating a new file if needed or else truncating -an old file: - - open(FH, "> $path"); - sysopen(FH, $path, O_WRONLY | O_TRUNC | O_CREAT); - -To open a file for appending, creating one if necessary: - - open(FH, ">> $path"); - sysopen(FH, $path, O_WRONLY | O_APPEND | O_CREAT); - -To open a file for update, where the file must already exist: - - open(FH, "+< $path"); - sysopen(FH, $path, O_RDWR); - -And here are things you can do with C<sysopen> that you cannot do with -a regular C<open>. As you see, it's just a matter of controlling the -flags in the third argument. - -To open a file for writing, creating a new file which must not previously -exist: - - sysopen(FH, $path, O_WRONLY | O_EXCL | O_CREAT); - -To open a file for appending, where that file must already exist: - - sysopen(FH, $path, O_WRONLY | O_APPEND); - -To open a file for update, creating a new file if necessary: - - sysopen(FH, $path, O_RDWR | O_CREAT); - -To open a file for update, where that file must not already exist: - - sysopen(FH, $path, O_RDWR | O_EXCL | O_CREAT); - -To open a file without blocking, creating one if necessary: - - sysopen(FH, $path, O_WRONLY | O_NONBLOCK | O_CREAT); - -=head2 Permissions E<agrave> la mode - -If you omit the MASK argument to C<sysopen>, Perl uses the octal value -0666. The normal MASK to use for executables and directories should -be 0777, and for anything else, 0666. - -Why so permissive? Well, it isn't really. The MASK will be modified -by your process's current C<umask>. A umask is a number representing -I<disabled> permissions bits; that is, bits that will not be turned on -in the created files' permissions field. - -For example, if your C<umask> were 027, then the 020 part would -disable the group from writing, and the 007 part would disable others -from reading, writing, or executing. Under these conditions, passing -C<sysopen> 0666 would create a file with mode 0640, since C<0666 &~ 027> -is 0640. - -You should seldom use the MASK argument to C<sysopen()>. That takes -away the user's freedom to choose what permission new files will have. -Denying choice is almost always a bad thing. One exception would be for -cases where sensitive or private data is being stored, such as with mail -folders, cookie files, and internal temporary files. - -=head1 Obscure Open Tricks - -=head2 Re-Opening Files (dups) - -Sometimes you already have a filehandle open, and want to make another -handle that's a duplicate of the first one. In the shell, we place an -ampersand in front of a file descriptor number when doing redirections. -For example, C<< 2>&1 >> makes descriptor 2 (that's STDERR in Perl) -be redirected into descriptor 1 (which is usually Perl's STDOUT). -The same is essentially true in Perl: a filename that begins with an -ampersand is treated instead as a file descriptor if a number, or as a -filehandle if a string. - - open(SAVEOUT, ">&SAVEERR") || die "couldn't dup SAVEERR: $!"; - open(MHCONTEXT, "<&4") || die "couldn't dup fd4: $!"; - -That means that if a function is expecting a filename, but you don't -want to give it a filename because you already have the file open, you -can just pass the filehandle with a leading ampersand. It's best to -use a fully qualified handle though, just in case the function happens -to be in a different package: - - somefunction("&main::LOGFILE"); - -This way if somefunction() is planning on opening its argument, it can -just use the already opened handle. This differs from passing a handle, -because with a handle, you don't open the file. Here you have something -you can pass to open. - -If you have one of those tricky, newfangled I/O objects that the C++ -folks are raving about, then this doesn't work because those aren't a -proper filehandle in the native Perl sense. You'll have to use fileno() -to pull out the proper descriptor number, assuming you can: - - use IO::Socket; - $handle = IO::Socket::INET->new("www.perl.com:80"); - $fd = $handle->fileno; - somefunction("&$fd"); # not an indirect function call - -It can be easier (and certainly will be faster) just to use real -filehandles though: - - use IO::Socket; - local *REMOTE = IO::Socket::INET->new("www.perl.com:80"); - die "can't connect" unless defined(fileno(REMOTE)); - somefunction("&main::REMOTE"); - -If the filehandle or descriptor number is preceded not just with a simple -"&" but rather with a "&=" combination, then Perl will not create a -completely new descriptor opened to the same place using the dup(2) -system call. Instead, it will just make something of an alias to the -existing one using the fdopen(3S) library call This is slightly more -parsimonious of systems resources, although this is less a concern -these days. Here's an example of that: - - $fd = $ENV{"MHCONTEXTFD"}; - open(MHCONTEXT, "<&=$fd") or die "couldn't fdopen $fd: $!"; - -If you're using magic C<< <ARGV> >>, you could even pass in as a -command line argument in @ARGV something like C<"<&=$MHCONTEXTFD">, -but we've never seen anyone actually do this. - -=head2 Dispelling the Dweomer - -Perl is more of a DWIMmer language than something like Java--where DWIM -is an acronym for "do what I mean". But this principle sometimes leads -to more hidden magic than one knows what to do with. In this way, Perl -is also filled with I<dweomer>, an obscure word meaning an enchantment. -Sometimes, Perl's DWIMmer is just too much like dweomer for comfort. - -If magic C<open> is a bit too magical for you, you don't have to turn -to C<sysopen>. To open a file with arbitrary weird characters in -it, it's necessary to protect any leading and trailing whitespace. -Leading whitespace is protected by inserting a C<"./"> in front of a -filename that starts with whitespace. Trailing whitespace is protected -by appending an ASCII NUL byte (C<"\0">) at the end off the string. - - $file =~ s#^(\s)#./$1#; - open(FH, "< $file\0") || die "can't open $file: $!"; - -This assumes, of course, that your system considers dot the current -working directory, slash the directory separator, and disallows ASCII -NULs within a valid filename. Most systems follow these conventions, -including all POSIX systems as well as proprietary Microsoft systems. -The only vaguely popular system that doesn't work this way is the -proprietary Macintosh system, which uses a colon where the rest of us -use a slash. Maybe C<sysopen> isn't such a bad idea after all. - -If you want to use C<< <ARGV> >> processing in a totally boring -and non-magical way, you could do this first: - - # "Sam sat on the ground and put his head in his hands. - # 'I wish I had never come here, and I don't want to see - # no more magic,' he said, and fell silent." - for (@ARGV) { - s#^([^./])#./$1#; - $_ .= "\0"; - } - while (<>) { - # now process $_ - } - -But be warned that users will not appreciate being unable to use "-" -to mean standard input, per the standard convention. - -=head2 Paths as Opens - -You've probably noticed how Perl's C<warn> and C<die> functions can -produce messages like: - - Some warning at scriptname line 29, <FH> line 7. - -That's because you opened a filehandle FH, and had read in seven records -from it. But what was the name of the file, not the handle? - -If you aren't running with C<strict refs>, or if you've turn them off -temporarily, then all you have to do is this: - - open($path, "< $path") || die "can't open $path: $!"; - while (<$path>) { - # whatever - } - -Since you're using the pathname of the file as its handle, -you'll get warnings more like - - Some warning at scriptname line 29, </etc/motd> line 7. - -=head2 Single Argument Open - -Remember how we said that Perl's open took two arguments? That was a -passive prevarication. You see, it can also take just one argument. -If and only if the variable is a global variable, not a lexical, you -can pass C<open> just one argument, the filehandle, and it will -get the path from the global scalar variable of the same name. - - $FILE = "/etc/motd"; - open FILE or die "can't open $FILE: $!"; - while (<FILE>) { - # whatever - } - -Why is this here? Someone has to cater to the hysterical porpoises. -It's something that's been in Perl since the very beginning, if not -before. - -=head2 Playing with STDIN and STDOUT - -One clever move with STDOUT is to explicitly close it when you're done -with the program. - - END { close(STDOUT) || die "can't close stdout: $!" } - -If you don't do this, and your program fills up the disk partition due -to a command line redirection, it won't report the error exit with a -failure status. - -You don't have to accept the STDIN and STDOUT you were given. You are -welcome to reopen them if you'd like. - - open(STDIN, "< datafile") - || die "can't open datafile: $!"; - - open(STDOUT, "> output") - || die "can't open output: $!"; - -And then these can be read directly or passed on to subprocesses. -This makes it look as though the program were initially invoked -with those redirections from the command line. - -It's probably more interesting to connect these to pipes. For example: - - $pager = $ENV{PAGER} || "(less || more)"; - open(STDOUT, "| $pager") - || die "can't fork a pager: $!"; - -This makes it appear as though your program were called with its stdout -already piped into your pager. You can also use this kind of thing -in conjunction with an implicit fork to yourself. You might do this -if you would rather handle the post processing in your own program, -just in a different process: - - head(100); - while (<>) { - print; - } - - sub head { - my $lines = shift || 20; - return unless $pid = open(STDOUT, "|-"); - die "cannot fork: $!" unless defined $pid; - while (<STDIN>) { - print; - last if --$lines < 0; - } - exit; - } - -This technique can be applied to repeatedly push as many filters on your -output stream as you wish. - -=head1 Other I/O Issues - -These topics aren't really arguments related to C<open> or C<sysopen>, -but they do affect what you do with your open files. - -=head2 Opening Non-File Files - -When is a file not a file? Well, you could say when it exists but -isn't a plain file. We'll check whether it's a symbolic link first, -just in case. - - if (-l $file || ! -f _) { - print "$file is not a plain file\n"; - } - -What other kinds of files are there than, well, files? Directories, -symbolic links, named pipes, Unix-domain sockets, and block and character -devices. Those are all files, too--just not I<plain> files. This isn't -the same issue as being a text file. Not all text files are plain files. -Not all plain files are textfiles. That's why there are separate C<-f> -and C<-T> file tests. - -To open a directory, you should use the C<opendir> function, then -process it with C<readdir>, carefully restoring the directory -name if necessary: - - opendir(DIR, $dirname) or die "can't opendir $dirname: $!"; - while (defined($file = readdir(DIR))) { - # do something with "$dirname/$file" - } - closedir(DIR); - -If you want to process directories recursively, it's better to use the -File::Find module. For example, this prints out all files recursively, -add adds a slash to their names if the file is a directory. - - @ARGV = qw(.) unless @ARGV; - use File::Find; - find sub { print $File::Find::name, -d && '/', "\n" }, @ARGV; - -This finds all bogus symbolic links beneath a particular directory: - - find sub { print "$File::Find::name\n" if -l && !-e }, $dir; - -As you see, with symbolic links, you can just pretend that it is -what it points to. Or, if you want to know I<what> it points to, then -C<readlink> is called for: - - if (-l $file) { - if (defined($whither = readlink($file))) { - print "$file points to $whither\n"; - } else { - print "$file points nowhere: $!\n"; - } - } - -Named pipes are a different matter. You pretend they're regular files, -but their opens will normally block until there is both a reader and -a writer. You can read more about them in L<perlipc/"Named Pipes">. -Unix-domain sockets are rather different beasts as well; they're -described in L<perlipc/"Unix-Domain TCP Clients and Servers">. - -When it comes to opening devices, it can be easy and it can tricky. -We'll assume that if you're opening up a block device, you know what -you're doing. The character devices are more interesting. These are -typically used for modems, mice, and some kinds of printers. This is -described in L<perlfaq8/"How do I read and write the serial port?"> -It's often enough to open them carefully: - - sysopen(TTYIN, "/dev/ttyS1", O_RDWR | O_NDELAY | O_NOCTTY) - # (O_NOCTTY no longer needed on POSIX systems) - or die "can't open /dev/ttyS1: $!"; - open(TTYOUT, "+>&TTYIN") - or die "can't dup TTYIN: $!"; - - $ofh = select(TTYOUT); $| = 1; select($ofh); - - print TTYOUT "+++at\015"; - $answer = <TTYIN>; - -With descriptors that you haven't opened using C<sysopen>, such as a -socket, you can set them to be non-blocking using C<fcntl>: - - use Fcntl; - fcntl(Connection, F_SETFL, O_NONBLOCK) - or die "can't set non blocking: $!"; - -Rather than losing yourself in a morass of twisting, turning C<ioctl>s, -all dissimilar, if you're going to manipulate ttys, it's best to -make calls out to the stty(1) program if you have it, or else use the -portable POSIX interface. To figure this all out, you'll need to read the -termios(3) manpage, which describes the POSIX interface to tty devices, -and then L<POSIX>, which describes Perl's interface to POSIX. There are -also some high-level modules on CPAN that can help you with these games. -Check out Term::ReadKey and Term::ReadLine. - -What else can you open? To open a connection using sockets, you won't use -one of Perl's two open functions. See -L<perlipc/"Sockets: Client/Server Communication"> for that. Here's an -example. Once you have it, you can use FH as a bidirectional filehandle. - - use IO::Socket; - local *FH = IO::Socket::INET->new("www.perl.com:80"); - -For opening up a URL, the LWP modules from CPAN are just what -the doctor ordered. There's no filehandle interface, but -it's still easy to get the contents of a document: - - use LWP::Simple; - $doc = get('http://www.linpro.no/lwp/'); - -=head2 Binary Files - -On certain legacy systems with what could charitably be called terminally -convoluted (some would say broken) I/O models, a file isn't a file--at -least, not with respect to the C standard I/O library. On these old -systems whose libraries (but not kernels) distinguish between text and -binary streams, to get files to behave properly you'll have to bend over -backwards to avoid nasty problems. On such infelicitous systems, sockets -and pipes are already opened in binary mode, and there is currently no -way to turn that off. With files, you have more options. - -Another option is to use the C<binmode> function on the appropriate -handles before doing regular I/O on them: - - binmode(STDIN); - binmode(STDOUT); - while (<STDIN>) { print } - -Passing C<sysopen> a non-standard flag option will also open the file in -binary mode on those systems that support it. This is the equivalent of -opening the file normally, then calling C<binmode>ing on the handle. - - sysopen(BINDAT, "records.data", O_RDWR | O_BINARY) - || die "can't open records.data: $!"; - -Now you can use C<read> and C<print> on that handle without worrying -about the system non-standard I/O library breaking your data. It's not -a pretty picture, but then, legacy systems seldom are. CP/M will be -with us until the end of days, and after. - -On systems with exotic I/O systems, it turns out that, astonishingly -enough, even unbuffered I/O using C<sysread> and C<syswrite> might do -sneaky data mutilation behind your back. - - while (sysread(WHENCE, $buf, 1024)) { - syswrite(WHITHER, $buf, length($buf)); - } - -Depending on the vicissitudes of your runtime system, even these calls -may need C<binmode> or C<O_BINARY> first. Systems known to be free of -such difficulties include Unix, the Mac OS, Plan9, and Inferno. - -=head2 File Locking - -In a multitasking environment, you may need to be careful not to collide -with other processes who want to do I/O on the same files as others -are working on. You'll often need shared or exclusive locks -on files for reading and writing respectively. You might just -pretend that only exclusive locks exist. - -Never use the existence of a file C<-e $file> as a locking indication, -because there is a race condition between the test for the existence of -the file and its creation. Atomicity is critical. - -Perl's most portable locking interface is via the C<flock> function, -whose simplicity is emulated on systems that don't directly support it, -such as SysV or WindowsNT. The underlying semantics may affect how -it all works, so you should learn how C<flock> is implemented on your -system's port of Perl. - -File locking I<does not> lock out another process that would like to -do I/O. A file lock only locks out others trying to get a lock, not -processes trying to do I/O. Because locks are advisory, if one process -uses locking and another doesn't, all bets are off. - -By default, the C<flock> call will block until a lock is granted. -A request for a shared lock will be granted as soon as there is no -exclusive locker. A request for a exclusive lock will be granted as -soon as there is no locker of any kind. Locks are on file descriptors, -not file names. You can't lock a file until you open it, and you can't -hold on to a lock once the file has been closed. - -Here's how to get a blocking shared lock on a file, typically used -for reading: - - use 5.004; - use Fcntl qw(:DEFAULT :flock); - open(FH, "< filename") or die "can't open filename: $!"; - flock(FH, LOCK_SH) or die "can't lock filename: $!"; - # now read from FH - -You can get a non-blocking lock by using C<LOCK_NB>. - - flock(FH, LOCK_SH | LOCK_NB) - or die "can't lock filename: $!"; - -This can be useful for producing more user-friendly behaviour by warning -if you're going to be blocking: - - use 5.004; - use Fcntl qw(:DEFAULT :flock); - open(FH, "< filename") or die "can't open filename: $!"; - unless (flock(FH, LOCK_SH | LOCK_NB)) { - $| = 1; - print "Waiting for lock..."; - flock(FH, LOCK_SH) or die "can't lock filename: $!"; - print "got it.\n" - } - # now read from FH - -To get an exclusive lock, typically used for writing, you have to be -careful. We C<sysopen> the file so it can be locked before it gets -emptied. You can get a nonblocking version using C<LOCK_EX | LOCK_NB>. - - use 5.004; - use Fcntl qw(:DEFAULT :flock); - sysopen(FH, "filename", O_WRONLY | O_CREAT) - or die "can't open filename: $!"; - flock(FH, LOCK_EX) - or die "can't lock filename: $!"; - truncate(FH, 0) - or die "can't truncate filename: $!"; - # now write to FH - -Finally, due to the uncounted millions who cannot be dissuaded from -wasting cycles on useless vanity devices called hit counters, here's -how to increment a number in a file safely: - - use Fcntl qw(:DEFAULT :flock); - - sysopen(FH, "numfile", O_RDWR | O_CREAT) - or die "can't open numfile: $!"; - # autoflush FH - $ofh = select(FH); $| = 1; select ($ofh); - flock(FH, LOCK_EX) - or die "can't write-lock numfile: $!"; - - $num = <FH> || 0; - seek(FH, 0, 0) - or die "can't rewind numfile : $!"; - print FH $num+1, "\n" - or die "can't write numfile: $!"; - - truncate(FH, tell(FH)) - or die "can't truncate numfile: $!"; - close(FH) - or die "can't close numfile: $!"; - -=head1 SEE ALSO - -The C<open> and C<sysopen> function in perlfunc(1); -the standard open(2), dup(2), fopen(3), and fdopen(3) manpages; -the POSIX documentation. - -=head1 AUTHOR and COPYRIGHT - -Copyright 1998 Tom Christiansen. - -When included as part of the Standard Version of Perl, or as part of -its complete documentation whether printed or otherwise, this work may -be distributed only under the terms of Perl's Artistic License. Any -distribution of this file or derivatives thereof outside of that -package require that special arrangements be made with copyright -holder. - -Irrespective of its distribution, all code examples in these files are -hereby placed into the public domain. You are permitted and -encouraged to use this code in your own programs for fun or for profit -as you see fit. A simple comment in the code giving credit would be -courteous but is not required. - -=head1 HISTORY - -First release: Sat Jan 9 08:09:11 MST 1999 |
