diff options
Diffstat (limited to 'contrib/perl5/pod/perlfaq8.pod')
| -rw-r--r-- | contrib/perl5/pod/perlfaq8.pod | 1075 |
1 files changed, 1075 insertions, 0 deletions
diff --git a/contrib/perl5/pod/perlfaq8.pod b/contrib/perl5/pod/perlfaq8.pod new file mode 100644 index 0000000..c4036ff --- /dev/null +++ b/contrib/perl5/pod/perlfaq8.pod @@ -0,0 +1,1075 @@ +=head1 NAME + +perlfaq8 - System Interaction ($Revision: 1.26 $, $Date: 1998/08/05 12:20:28 $) + +=head1 DESCRIPTION + +This section of the Perl FAQ covers questions involving operating +system interaction. This involves interprocess communication (IPC), +control over the user-interface (keyboard, screen and pointing +devices), and most anything else not related to data manipulation. + +Read the FAQs and documentation specific to the port of perl to your +operating system (eg, L<perlvms>, L<perlplan9>, ...). These should +contain more detailed information on the vagaries of your perl. + +=head2 How do I find out which operating system I'm running under? + +The $^O variable ($OSNAME if you use English) contains the operating +system that your perl binary was built for. + +=head2 How come exec() doesn't return? + +Because that's what it does: it replaces your currently running +program with a different one. If you want to keep going (as is +probably the case if you're asking this question) use system() +instead. + +=head2 How do I do fancy stuff with the keyboard/screen/mouse? + +How you access/control keyboards, screens, and pointing devices +("mice") is system-dependent. Try the following modules: + +=over 4 + +=item Keyboard + + Term::Cap Standard perl distribution + Term::ReadKey CPAN + Term::ReadLine::Gnu CPAN + Term::ReadLine::Perl CPAN + Term::Screen CPAN + +=item Screen + + Term::Cap Standard perl distribution + Curses CPAN + Term::ANSIColor CPAN + +=item Mouse + + Tk CPAN + +=back + +Some of these specific cases are shown below. + +=head2 How do I print something out in color? + +In general, you don't, because you don't know whether +the recipient has a color-aware display device. If you +know that they have an ANSI terminal that understands +color, you can use the Term::ANSIColor module from CPAN: + + use Term::ANSIColor; + print color("red"), "Stop!\n", color("reset"); + print color("green"), "Go!\n", color("reset"); + +Or like this: + + use Term::ANSIColor qw(:constants); + print RED, "Stop!\n", RESET; + print GREEN, "Go!\n", RESET; + +=head2 How do I read just one key without waiting for a return key? + +Controlling input buffering is a remarkably system-dependent matter. +If most systems, you can just use the B<stty> command as shown in +L<perlfunc/getc>, but as you see, that's already getting you into +portability snags. + + open(TTY, "+</dev/tty") or die "no tty: $!"; + system "stty cbreak </dev/tty >/dev/tty 2>&1"; + $key = getc(TTY); # perhaps this works + # OR ELSE + sysread(TTY, $key, 1); # probably this does + system "stty -cbreak </dev/tty >/dev/tty 2>&1"; + +The Term::ReadKey module from CPAN offers an easy-to-use interface that +should be more efficient than shelling out to B<stty> for each key. +It even includes limited support for Windows. + + use Term::ReadKey; + ReadMode('cbreak'); + $key = ReadKey(0); + ReadMode('normal'); + +However, that requires that you have a working C compiler and can use it +to build and install a CPAN module. Here's a solution using +the standard POSIX module, which is already on your systems (assuming +your system supports POSIX). + + use HotKey; + $key = readkey(); + +And here's the HotKey module, which hides the somewhat mystifying calls +to manipulate the POSIX termios structures. + + # HotKey.pm + package HotKey; + + @ISA = qw(Exporter); + @EXPORT = qw(cbreak cooked readkey); + + use strict; + use POSIX qw(:termios_h); + my ($term, $oterm, $echo, $noecho, $fd_stdin); + + $fd_stdin = fileno(STDIN); + $term = POSIX::Termios->new(); + $term->getattr($fd_stdin); + $oterm = $term->getlflag(); + + $echo = ECHO | ECHOK | ICANON; + $noecho = $oterm & ~$echo; + + sub cbreak { + $term->setlflag($noecho); # ok, so i don't want echo either + $term->setcc(VTIME, 1); + $term->setattr($fd_stdin, TCSANOW); + } + + sub cooked { + $term->setlflag($oterm); + $term->setcc(VTIME, 0); + $term->setattr($fd_stdin, TCSANOW); + } + + sub readkey { + my $key = ''; + cbreak(); + sysread(STDIN, $key, 1); + cooked(); + return $key; + } + + END { cooked() } + + 1; + +=head2 How do I check whether input is ready on the keyboard? + +The easiest way to do this is to read a key in nonblocking mode with the +Term::ReadKey module from CPAN, passing it an argument of -1 to indicate +not to block: + + use Term::ReadKey; + + ReadMode('cbreak'); + + if (defined ($char = ReadKey(-1)) ) { + # input was waiting and it was $char + } else { + # no input was waiting + } + + ReadMode('normal'); # restore normal tty settings + +=head2 How do I clear the screen? + +If you only have to so infrequently, use C<system>: + + system("clear"); + +If you have to do this a lot, save the clear string +so you can print it 100 times without calling a program +100 times: + + $clear_string = `clear`; + print $clear_string; + +If you're planning on doing other screen manipulations, like cursor +positions, etc, you might wish to use Term::Cap module: + + use Term::Cap; + $terminal = Term::Cap->Tgetent( {OSPEED => 9600} ); + $clear_string = $terminal->Tputs('cl'); + +=head2 How do I get the screen size? + +If you have Term::ReadKey module installed from CPAN, +you can use it to fetch the width and height in characters +and in pixels: + + use Term::ReadKey; + ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize(); + +This is more portable than the raw C<ioctl>, but not as +illustrative: + + require 'sys/ioctl.ph'; + die "no TIOCGWINSZ " unless defined &TIOCGWINSZ; + open(TTY, "+</dev/tty") or die "No tty: $!"; + unless (ioctl(TTY, &TIOCGWINSZ, $winsize='')) { + die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\n", &TIOCGWINSZ; + } + ($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize); + print "(row,col) = ($row,$col)"; + print " (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel; + print "\n"; + +=head2 How do I ask the user for a password? + +(This question has nothing to do with the web. See a different +FAQ for that.) + +There's an example of this in L<perlfunc/crypt>). First, you put +the terminal into "no echo" mode, then just read the password +normally. You may do this with an old-style ioctl() function, POSIX +terminal control (see L<POSIX>, and Chapter 7 of the Camel), or a call +to the B<stty> program, with varying degrees of portability. + +You can also do this for most systems using the Term::ReadKey module +from CPAN, which is easier to use and in theory more portable. + + use Term::ReadKey; + + ReadMode('noecho'); + $password = ReadLine(0); + +=head2 How do I read and write the serial port? + +This depends on which operating system your program is running on. In +the case of Unix, the serial ports will be accessible through files in +/dev; on other systems, the devices names will doubtless differ. +Several problem areas common to all device interaction are the +following + +=over 4 + +=item lockfiles + +Your system may use lockfiles to control multiple access. Make sure +you follow the correct protocol. Unpredictable behaviour can result +from multiple processes reading from one device. + +=item open mode + +If you expect to use both read and write operations on the device, +you'll have to open it for update (see L<perlfunc/"open"> for +details). You may wish to open it without running the risk of +blocking by using sysopen() and C<O_RDWR|O_NDELAY|O_NOCTTY> from the +Fcntl module (part of the standard perl distribution). See +L<perlfunc/"sysopen"> for more on this approach. + +=item end of line + +Some devices will be expecting a "\r" at the end of each line rather +than a "\n". In some ports of perl, "\r" and "\n" are different from +their usual (Unix) ASCII values of "\012" and "\015". You may have to +give the numeric values you want directly, using octal ("\015"), hex +("0x0D"), or as a control-character specification ("\cM"). + + print DEV "atv1\012"; # wrong, for some devices + print DEV "atv1\015"; # right, for some devices + +Even though with normal text files, a "\n" will do the trick, there is +still no unified scheme for terminating a line that is portable +between Unix, DOS/Win, and Macintosh, except to terminate I<ALL> line +ends with "\015\012", and strip what you don't need from the output. +This applies especially to socket I/O and autoflushing, discussed +next. + +=item flushing output + +If you expect characters to get to your device when you print() them, +you'll want to autoflush that filehandle. You can use select() +and the C<$|> variable to control autoflushing (see L<perlvar/$|> +and L<perlfunc/select>): + + $oldh = select(DEV); + $| = 1; + select($oldh); + +You'll also see code that does this without a temporary variable, as in + + select((select(DEV), $| = 1)[0]); + +Or if you don't mind pulling in a few thousand lines +of code just because you're afraid of a little $| variable: + + use IO::Handle; + DEV->autoflush(1); + +As mentioned in the previous item, this still doesn't work when using +socket I/O between Unix and Macintosh. You'll need to hardcode your +line terminators, in that case. + +=item non-blocking input + +If you are doing a blocking read() or sysread(), you'll have to +arrange for an alarm handler to provide a timeout (see +L<perlfunc/alarm>). If you have a non-blocking open, you'll likely +have a non-blocking read, which means you may have to use a 4-arg +select() to determine whether I/O is ready on that device (see +L<perlfunc/"select">. + +=back + +While trying to read from his caller-id box, the notorious Jamie Zawinski +<jwz@netscape.com>, after much gnashing of teeth and fighting with sysread, +sysopen, POSIX's tcgetattr business, and various other functions that +go bump in the night, finally came up with this: + + sub open_modem { + use IPC::Open2; + my $stty = `/bin/stty -g`; + open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1"); + # starting cu hoses /dev/tty's stty settings, even when it has + # been opened on a pipe... + system("/bin/stty $stty"); + $_ = <MODEM_IN>; + chop; + if ( !m/^Connected/ ) { + print STDERR "$0: cu printed `$_' instead of `Connected'\n"; + } + } + + +=head2 How do I decode encrypted password files? + +You spend lots and lots of money on dedicated hardware, but this is +bound to get you talked about. + +Seriously, you can't if they are Unix password files - the Unix +password system employs one-way encryption. It's more like hashing than +encryption. The best you can check is whether something else hashes to +the same string. You can't turn a hash back into the original string. +Programs like Crack +can forcibly (and intelligently) try to guess passwords, but don't +(can't) guarantee quick success. + +If you're worried about users selecting bad passwords, you should +proactively check when they try to change their password (by modifying +passwd(1), for example). + +=head2 How do I start a process in the background? + +You could use + + system("cmd &") + +or you could use fork as documented in L<perlfunc/"fork">, with +further examples in L<perlipc>. Some things to be aware of, if you're +on a Unix-like system: + +=over 4 + +=item STDIN, STDOUT, and STDERR are shared + +Both the main process and the backgrounded one (the "child" process) +share the same STDIN, STDOUT and STDERR filehandles. If both try to +access them at once, strange things can happen. You may want to close +or reopen these for the child. You can get around this with +C<open>ing a pipe (see L<perlfunc/"open">) but on some systems this +means that the child process cannot outlive the parent. + +=item Signals + +You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too. +SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is +sent when you write to a filehandle whose child process has closed (an +untrapped SIGPIPE can cause your program to silently die). This is +not an issue with C<system("cmd&")>. + +=item Zombies + +You have to be prepared to "reap" the child process when it finishes + + $SIG{CHLD} = sub { wait }; + +See L<perlipc/"Signals"> for other examples of code to do this. +Zombies are not an issue with C<system("prog &")>. + +=back + +=head2 How do I trap control characters/signals? + +You don't actually "trap" a control character. Instead, that character +generates a signal which is sent to your terminal's currently +foregrounded process group, which you then trap in your process. +Signals are documented in L<perlipc/"Signals"> and chapter 6 of the Camel. + +Be warned that very few C libraries are re-entrant. Therefore, if you +attempt to print() in a handler that got invoked during another stdio +operation your internal structures will likely be in an +inconsistent state, and your program will dump core. You can +sometimes avoid this by using syswrite() instead of print(). + +Unless you're exceedingly careful, the only safe things to do inside a +signal handler are: set a variable and exit. And in the first case, +you should only set a variable in such a way that malloc() is not +called (eg, by setting a variable that already has a value). + +For example: + + $Interrupted = 0; # to ensure it has a value + $SIG{INT} = sub { + $Interrupted++; + syswrite(STDERR, "ouch\n", 5); + } + +However, because syscalls restart by default, you'll find that if +you're in a "slow" call, such as E<lt>FHE<gt>, read(), connect(), or +wait(), that the only way to terminate them is by "longjumping" out; +that is, by raising an exception. See the time-out handler for a +blocking flock() in L<perlipc/"Signals"> or chapter 6 of the Camel. + +=head2 How do I modify the shadow password file on a Unix system? + +If perl was installed correctly, and your shadow library was written +properly, the getpw*() functions described in L<perlfunc> should in +theory provide (read-only) access to entries in the shadow password +file. To change the file, make a new shadow password file (the format +varies from system to system - see L<passwd(5)> for specifics) and use +pwd_mkdb(8) to install it (see L<pwd_mkdb(5)> for more details). + +=head2 How do I set the time and date? + +Assuming you're running under sufficient permissions, you should be +able to set the system-wide date and time by running the date(1) +program. (There is no way to set the time and date on a per-process +basis.) This mechanism will work for Unix, MS-DOS, Windows, and NT; +the VMS equivalent is C<set time>. + +However, if all you want to do is change your timezone, you can +probably get away with setting an environment variable: + + $ENV{TZ} = "MST7MDT"; # unixish + $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms + system "trn comp.lang.perl.misc"; + +=head2 How can I sleep() or alarm() for under a second? + +If you want finer granularity than the 1 second that the sleep() +function provides, the easiest way is to use the select() function as +documented in L<perlfunc/"select">. If your system has itimers and +syscall() support, you can check out the old example in +http://www.perl.com/CPAN/doc/misc/ancient/tutorial/eg/itimers.pl . + +=head2 How can I measure time under a second? + +In general, you may not be able to. The Time::HiRes module (available +from CPAN) provides this functionality for some systems. + +In general, you may not be able to. But if your system supports both the +syscall() function in Perl as well as a system call like gettimeofday(2), +then you may be able to do something like this: + + require 'sys/syscall.ph'; + + $TIMEVAL_T = "LL"; + + $done = $start = pack($TIMEVAL_T, ()); + + syscall( &SYS_gettimeofday, $start, 0)) != -1 + or die "gettimeofday: $!"; + + ########################## + # DO YOUR OPERATION HERE # + ########################## + + syscall( &SYS_gettimeofday, $done, 0) != -1 + or die "gettimeofday: $!"; + + @start = unpack($TIMEVAL_T, $start); + @done = unpack($TIMEVAL_T, $done); + + # fix microseconds + for ($done[1], $start[1]) { $_ /= 1_000_000 } + + $delta_time = sprintf "%.4f", ($done[0] + $done[1] ) + - + ($start[0] + $start[1] ); + +=head2 How can I do an atexit() or setjmp()/longjmp()? (Exception handling) + +Release 5 of Perl added the END block, which can be used to simulate +atexit(). Each package's END block is called when the program or +thread ends (see L<perlmod> manpage for more details). + +For example, you can use this to make sure your filter program +managed to finish its output without filling up the disk: + + END { + close(STDOUT) || die "stdout close failed: $!"; + } + +The END block isn't called when untrapped signals kill the program, though, so if +you use END blocks you should also use + + use sigtrap qw(die normal-signals); + +Perl's exception-handling mechanism is its eval() operator. You can +use eval() as setjmp and die() as longjmp. For details of this, see +the section on signals, especially the time-out handler for a blocking +flock() in L<perlipc/"Signals"> and chapter 6 of the Camel. + +If exception handling is all you're interested in, try the +exceptions.pl library (part of the standard perl distribution). + +If you want the atexit() syntax (and an rmexit() as well), try the +AtExit module available from CPAN. + +=head2 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean? + +Some Sys-V based systems, notably Solaris 2.X, redefined some of the +standard socket constants. Since these were constant across all +architectures, they were often hardwired into perl code. The proper +way to deal with this is to "use Socket" to get the correct values. + +Note that even though SunOS and Solaris are binary compatible, these +values are different. Go figure. + +=head2 How can I call my system's unique C functions from Perl? + +In most cases, you write an external module to do it - see the answer +to "Where can I learn about linking C with Perl? [h2xs, xsubpp]". +However, if the function is a system call, and your system supports +syscall(), you can use the syscall function (documented in +L<perlfunc>). + +Remember to check the modules that came with your distribution, and +CPAN as well - someone may already have written a module to do it. + +=head2 Where do I get the include files to do ioctl() or syscall()? + +Historically, these would be generated by the h2ph tool, part of the +standard perl distribution. This program converts cpp(1) directives +in C header files to files containing subroutine definitions, like +&SYS_getitimer, which you can use as arguments to your functions. +It doesn't work perfectly, but it usually gets most of the job done. +Simple files like F<errno.h>, F<syscall.h>, and F<socket.h> were fine, +but the hard ones like F<ioctl.h> nearly always need to hand-edited. +Here's how to install the *.ph files: + + 1. become super-user + 2. cd /usr/include + 3. h2ph *.h */*.h + +If your system supports dynamic loading, for reasons of portability and +sanity you probably ought to use h2xs (also part of the standard perl +distribution). This tool converts C header files to Perl extensions. +See L<perlxstut> for how to get started with h2xs. + +If your system doesn't support dynamic loading, you still probably +ought to use h2xs. See L<perlxstut> and L<ExtUtils::MakeMaker> for +more information (in brief, just use B<make perl> instead of a plain +B<make> to rebuild perl with a new static extension). + +=head2 Why do setuid perl scripts complain about kernel problems? + +Some operating systems have bugs in the kernel that make setuid +scripts inherently insecure. Perl gives you a number of options +(described in L<perlsec>) to work around such systems. + +=head2 How can I open a pipe both to and from a command? + +The IPC::Open2 module (part of the standard perl distribution) is an +easy-to-use approach that internally uses pipe(), fork(), and exec() to do +the job. Make sure you read the deadlock warnings in its documentation, +though (see L<IPC::Open2>). See L<perlipc/"Bidirectional Communication +with Another Process"> and L<perlipc/"Bidirectional Communication with +Yourself"> + +You may also use the IPC::Open3 module (part of the standard perl +distribution), but be warned that it has a different order of +arguments from IPC::Open2 (see L<IPC::Open3>). + +=head2 Why can't I get the output of a command with system()? + +You're confusing the purpose of system() and backticks (``). system() +runs a command and returns exit status information (as a 16 bit value: +the low 7 bits are the signal the process died from, if any, and +the high 8 bits are the actual exit value). Backticks (``) run a +command and return what it sent to STDOUT. + + $exit_status = system("mail-users"); + $output_string = `ls`; + +=head2 How can I capture STDERR from an external command? + +There are three basic ways of running external commands: + + system $cmd; # using system() + $output = `$cmd`; # using backticks (``) + open (PIPE, "cmd |"); # using open() + +With system(), both STDOUT and STDERR will go the same place as the +script's versions of these, unless the command redirects them. +Backticks and open() read B<only> the STDOUT of your command. + +With any of these, you can change file descriptors before the call: + + open(STDOUT, ">logfile"); + system("ls"); + +or you can use Bourne shell file-descriptor redirection: + + $output = `$cmd 2>some_file`; + open (PIPE, "cmd 2>some_file |"); + +You can also use file-descriptor redirection to make STDERR a +duplicate of STDOUT: + + $output = `$cmd 2>&1`; + open (PIPE, "cmd 2>&1 |"); + +Note that you I<cannot> simply open STDERR to be a dup of STDOUT +in your Perl program and avoid calling the shell to do the redirection. +This doesn't work: + + open(STDERR, ">&STDOUT"); + $alloutput = `cmd args`; # stderr still escapes + +This fails because the open() makes STDERR go to where STDOUT was +going at the time of the open(). The backticks then make STDOUT go to +a string, but don't change STDERR (which still goes to the old +STDOUT). + +Note that you I<must> use Bourne shell (sh(1)) redirection syntax in +backticks, not csh(1)! Details on why Perl's system() and backtick +and pipe opens all use the Bourne shell are in +http://www.perl.com/CPAN/doc/FMTEYEWTK/versus/csh.whynot . +To capture a command's STDERR and STDOUT together: + + $output = `cmd 2>&1`; # either with backticks + $pid = open(PH, "cmd 2>&1 |"); # or with an open pipe + while (<PH>) { } # plus a read + +To capture a command's STDOUT but discard its STDERR: + + $output = `cmd 2>/dev/null`; # either with backticks + $pid = open(PH, "cmd 2>/dev/null |"); # or with an open pipe + while (<PH>) { } # plus a read + +To capture a command's STDERR but discard its STDOUT: + + $output = `cmd 2>&1 1>/dev/null`; # either with backticks + $pid = open(PH, "cmd 2>&1 1>/dev/null |"); # or with an open pipe + while (<PH>) { } # plus a read + +To exchange a command's STDOUT and STDERR in order to capture the STDERR +but leave its STDOUT to come out our old STDERR: + + $output = `cmd 3>&1 1>&2 2>&3 3>&-`; # either with backticks + $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe + while (<PH>) { } # plus a read + +To read both a command's STDOUT and its STDERR separately, it's easiest +and safest to redirect them separately to files, and then read from those +files when the program is done: + + system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr"); + +Ordering is important in all these examples. That's because the shell +processes file descriptor redirections in strictly left to right order. + + system("prog args 1>tmpfile 2>&1"); + system("prog args 2>&1 1>tmpfile"); + +The first command sends both standard out and standard error to the +temporary file. The second command sends only the old standard output +there, and the old standard error shows up on the old standard out. + +=head2 Why doesn't open() return an error when a pipe open fails? + +It does, but probably not how you expect it to. On systems that +follow the standard fork()/exec() paradigm (such as Unix), it works like +this: open() causes a fork(). In the parent, open() returns with the +process ID of the child. The child exec()s the command to be piped +to/from. The parent can't know whether the exec() was successful or +not - all it can return is whether the fork() succeeded or not. To +find out if the command succeeded, you have to catch SIGCHLD and +wait() to get the exit status. You should also catch SIGPIPE if +you're writing to the child -- you may not have found out the exec() +failed by the time you write. This is documented in L<perlipc>. + +On systems that follow the spawn() paradigm, open() I<might> do what +you expect - unless perl uses a shell to start your command. In this +case the fork()/exec() description still applies. + +=head2 What's wrong with using backticks in a void context? + +Strictly speaking, nothing. Stylistically speaking, it's not a good +way to write maintainable code because backticks have a (potentially +humungous) return value, and you're ignoring it. It's may also not be very +efficient, because you have to read in all the lines of output, allocate +memory for them, and then throw it away. Too often people are lulled +to writing: + + `cp file file.bak`; + +And now they think "Hey, I'll just always use backticks to run programs." +Bad idea: backticks are for capturing a program's output; the system() +function is for running programs. + +Consider this line: + + `cat /etc/termcap`; + +You haven't assigned the output anywhere, so it just wastes memory +(for a little while). Plus you forgot to check C<$?> to see whether +the program even ran correctly. Even if you wrote + + print `cat /etc/termcap`; + +In most cases, this could and probably should be written as + + system("cat /etc/termcap") == 0 + or die "cat program failed!"; + +Which will get the output quickly (as its generated, instead of only +at the end) and also check the return value. + +system() also provides direct control over whether shell wildcard +processing may take place, whereas backticks do not. + +=head2 How can I call backticks without shell processing? + +This is a bit tricky. Instead of writing + + @ok = `grep @opts '$search_string' @filenames`; + +You have to do this: + + my @ok = (); + if (open(GREP, "-|")) { + while (<GREP>) { + chomp; + push(@ok, $_); + } + close GREP; + } else { + exec 'grep', @opts, $search_string, @filenames; + } + +Just as with system(), no shell escapes happen when you exec() a list. + +There are more examples of this L<perlipc/"Safe Pipe Opens">. + +=head2 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)? + +Because some stdio's set error and eof flags that need clearing. The +POSIX module defines clearerr() that you can use. That is the +technically correct way to do it. Here are some less reliable +workarounds: + +=over 4 + +=item 1 + +Try keeping around the seekpointer and go there, like this: + + $where = tell(LOG); + seek(LOG, $where, 0); + +=item 2 + +If that doesn't work, try seeking to a different part of the file and +then back. + +=item 3 + +If that doesn't work, try seeking to a different part of +the file, reading something, and then seeking back. + +=item 4 + +If that doesn't work, give up on your stdio package and use sysread. + +=back + +=head2 How can I convert my shell script to perl? + +Learn Perl and rewrite it. Seriously, there's no simple converter. +Things that are awkward to do in the shell are easy to do in Perl, and +this very awkwardness is what would make a shell->perl converter +nigh-on impossible to write. By rewriting it, you'll think about what +you're really trying to do, and hopefully will escape the shell's +pipeline datastream paradigm, which while convenient for some matters, +causes many inefficiencies. + +=head2 Can I use perl to run a telnet or ftp session? + +Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from +CPAN). http://www.perl.com/CPAN/scripts/netstuff/telnet.emul.shar +will also help for emulating the telnet protocol, but Net::Telnet is +quite probably easier to use.. + +If all you want to do is pretend to be telnet but don't need +the initial telnet handshaking, then the standard dual-process +approach will suffice: + + use IO::Socket; # new in 5.004 + $handle = IO::Socket::INET->new('www.perl.com:80') + || die "can't connect to port 80 on www.perl.com: $!"; + $handle->autoflush(1); + if (fork()) { # XXX: undef means failure + select($handle); + print while <STDIN>; # everything from stdin to socket + } else { + print while <$handle>; # everything from socket to stdout + } + close $handle; + exit; + +=head2 How can I write expect in Perl? + +Once upon a time, there was a library called chat2.pl (part of the +standard perl distribution), which never really got finished. If you +find it somewhere, I<don't use it>. These days, your best bet is to +look at the Expect module available from CPAN, which also requires two +other modules from CPAN, IO::Pty and IO::Stty. + +=head2 Is there a way to hide perl's command line from programs such as "ps"? + +First of all note that if you're doing this for security reasons (to +avoid people seeing passwords, for example) then you should rewrite +your program so that critical information is never given as an +argument. Hiding the arguments won't make your program completely +secure. + +To actually alter the visible command line, you can assign to the +variable $0 as documented in L<perlvar>. This won't work on all +operating systems, though. Daemon programs like sendmail place their +state there, as in: + + $0 = "orcus [accepting connections]"; + +=head2 I {changed directory, modified my environment} in a perl script. How come the change disappeared when I exited the script? How do I get my changes to be visible? + +=over 4 + +=item Unix + +In the strictest sense, it can't be done -- the script executes as a +different process from the shell it was started from. Changes to a +process are not reflected in its parent, only in its own children +created after the change. There is shell magic that may allow you to +fake it by eval()ing the script's output in your shell; check out the +comp.unix.questions FAQ for details. + +=back + +=head2 How do I close a process's filehandle without waiting for it to complete? + +Assuming your system supports such things, just send an appropriate signal +to the process (see L<perlfunc/"kill">. It's common to first send a TERM +signal, wait a little bit, and then send a KILL signal to finish it off. + +=head2 How do I fork a daemon process? + +If by daemon process you mean one that's detached (disassociated from +its tty), then the following process is reported to work on most +Unixish systems. Non-Unix users should check their Your_OS::Process +module for other solutions. + +=over 4 + +=item * + +Open /dev/tty and use the the TIOCNOTTY ioctl on it. See L<tty(4)> +for details. Or better yet, you can just use the POSIX::setsid() +function, so you don't have to worry about process groups. + +=item * + +Change directory to / + +=item * + +Reopen STDIN, STDOUT, and STDERR so they're not connected to the old +tty. + +=item * + +Background yourself like this: + + fork && exit; + +=back + +=head2 How do I make my program run with sh and csh? + +See the F<eg/nih> script (part of the perl source distribution). + +=head2 How do I find out if I'm running interactively or not? + +Good question. Sometimes C<-t STDIN> and C<-t STDOUT> can give clues, +sometimes not. + + if (-t STDIN && -t STDOUT) { + print "Now what? "; + } + +On POSIX systems, you can test whether your own process group matches +the current process group of your controlling terminal as follows: + + use POSIX qw/getpgrp tcgetpgrp/; + open(TTY, "/dev/tty") or die $!; + $tpgrp = tcgetpgrp(TTY); + $pgrp = getpgrp(); + if ($tpgrp == $pgrp) { + print "foreground\n"; + } else { + print "background\n"; + } + +=head2 How do I timeout a slow event? + +Use the alarm() function, probably in conjunction with a signal +handler, as documented L<perlipc/"Signals"> and chapter 6 of the +Camel. You may instead use the more flexible Sys::AlarmCall module +available from CPAN. + +=head2 How do I set CPU limits? + +Use the BSD::Resource module from CPAN. + +=head2 How do I avoid zombies on a Unix system? + +Use the reaper code from L<perlipc/"Signals"> to call wait() when a +SIGCHLD is received, or else use the double-fork technique described +in L<perlfunc/fork>. + +=head2 How do I use an SQL database? + +There are a number of excellent interfaces to SQL databases. See the +DBD::* modules available from +http://www.perl.com/CPAN/modules/dbperl/DBD . +A lot of information on this can be found at +http://www.hermetica.com/technologia/perl/DBI/index.html . + +=head2 How do I make a system() exit on control-C? + +You can't. You need to imitate the system() call (see L<perlipc> for +sample code) and then have a signal handler for the INT signal that +passes the signal on to the subprocess. Or you can check for it: + + $rc = system($cmd); + if ($rc & 127) { die "signal death" } + +=head2 How do I open a file without blocking? + +If you're lucky enough to be using a system that supports +non-blocking reads (most Unixish systems do), you need only to use the +O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with +sysopen(): + + use Fcntl; + sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644) + or die "can't open /tmp/somefile: $!": + +=head2 How do I install a CPAN module? + +The easiest way is to have the CPAN module do it for you. This module +comes with perl version 5.004 and later. To manually install the CPAN +module, or any well-behaved CPAN module for that matter, follow these +steps: + +=over 4 + +=item 1 + +Unpack the source into a temporary area. + +=item 2 + + perl Makefile.PL + +=item 3 + + make + +=item 4 + + make test + +=item 5 + + make install + +=back + +If your version of perl is compiled without dynamic loading, then you +just need to replace step 3 (B<make>) with B<make perl> and you will +get a new F<perl> binary with your extension linked in. + +See L<ExtUtils::MakeMaker> for more details on building extensions. +See also the next question. + +=head2 What's the difference between require and use? + +Perl offers several different ways to include code from one file into +another. Here are the deltas between the various inclusion constructs: + + 1) do $file is like eval `cat $file`, except the former: + 1.1: searches @INC and updates %INC. + 1.2: bequeaths an *unrelated* lexical scope on the eval'ed code. + + 2) require $file is like do $file, except the former: + 2.1: checks for redundant loading, skipping already loaded files. + 2.2: raises an exception on failure to find, compile, or execute $file. + + 3) require Module is like require "Module.pm", except the former: + 3.1: translates each "::" into your system's directory separator. + 3.2: primes the parser to disambiguate class Module as an indirect object. + + 4) use Module is like require Module, except the former: + 4.1: loads the module at compile time, not run-time. + 4.2: imports symbols and semantics from that package to the current one. + +In general, you usually want C<use> and a proper Perl module. + +=head2 How do I keep my own module/library directory? + +When you build modules, use the PREFIX option when generating +Makefiles: + + perl Makefile.PL PREFIX=/u/mydir/perl + +then either set the PERL5LIB environment variable before you run +scripts that use the modules/libraries (see L<perlrun>) or say + + use lib '/u/mydir/perl'; + +See Perl's L<lib> for more information. + +=head2 How do I add the directory my program lives in to the module/library search path? + + use FindBin; + use lib "$FindBin::Bin"; + use your_own_modules; + +=head2 How do I add a directory to my include path at runtime? + +Here are the suggested ways of modifying your include path: + + the PERLLIB environment variable + the PERL5LIB environment variable + the perl -Idir commpand line flag + the use lib pragma, as in + use lib "$ENV{HOME}/myown_perllib"; + +The latter is particularly useful because it knows about machine +dependent architectures. The lib.pm pragmatic module was first +included with the 5.002 release of Perl. + +=head1 AUTHOR AND COPYRIGHT + +Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington. +All rights reserved. + +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 I<outside> +of that package require that special arrangements be made with +copyright holder. + +Irrespective of its distribution, all code examples in this file +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. |
