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<HTML><HEAD><TITLE>Recipe 7.22. Program: lockarea (Perl Cookbook)</TITLE><METANAME="DC.title"CONTENT="Perl Cookbook"><METANAME="DC.creator"CONTENT="Tom Christiansen & Nathan Torkington"><METANAME="DC.publisher"CONTENT="O'Reilly & Associates, Inc."><METANAME="DC.date"CONTENT="1999-07-02T01:37:48Z"><METANAME="DC.type"CONTENT="Text.Monograph"><METANAME="DC.format"CONTENT="text/html"SCHEME="MIME"><METANAME="DC.source"CONTENT="1-56592-243-3"SCHEME="ISBN"><METANAME="DC.language"CONTENT="en-US"><METANAME="generator"CONTENT="Jade 1.1/O'Reilly DocBook 3.0 to HTML 4.0"><LINKREV="made"HREF="mailto:online-books@oreilly.com"TITLE="Online Books Comments"><LINKREL="up"HREF="ch07_01.htm"TITLE="7. File Access"><LINKREL="prev"HREF="ch07_22.htm"TITLE="7.21. Program: netlock"><LINKREL="next"HREF="ch08_01.htm"TITLE="8. 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File Contents"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch07-18319">7.22. Program: lockarea</A></H2><PCLASS="para"><ACLASS="indexterm"NAME="ch07-idx-1000009810-0"></A><ACLASS="indexterm"NAME="ch07-idx-1000009810-1"></A><ACLASS="indexterm"NAME="ch07-idx-1000009810-2"></A><ACLASS="indexterm"NAME="ch07-idx-1000009810-3"></A>Perl's <CODECLASS="literal">flock</CODE> function only locks complete files, not regions of the file. Although <CODECLASS="literal">fcntl</CODE> supports locking of a file's regions, this is difficult to access from Perl, largely because no one has written an XS module that portably packs up the necessary structure.</P><PCLASS="para">The program in <ACLASS="xref"HREF="ch07_23.htm#ch07-39765"TITLE="lockarea">Example 7.11</A> implements <ICLASS="filename">fcntl</I>, but only for the three architectures it already knows about: SunOS, BSD, and Linux. If you're running something else, you'll have to figure out the layout of the <CODECLASS="literal">flock</CODE> structure. We did this by eyeballing the C-language <EMCLASS="emphasis">sys/fcntl.h</EM> <CODECLASS="literal">#include</CODE> file - and running the <EMCLASS="emphasis">c2ph</EM> program to figure out alignment and typing. This program, while included with Perl, only works on systems with a strong Berkeley heritage, like those listed above. As with Unix - or Perl itself - you don't <EMCLASS="emphasis">have</EM> to use <EMCLASS="emphasis">c2ph</EM>, but it sure makes life easier if you can.</P><PCLASS="para">The <CODECLASS="literal">struct_flock</CODE><ACLASS="indexterm"NAME="ch07-idx-1000009816-0"></A> function in the <EMCLASS="emphasis">lockarea</EM> program packs and unpacks in the proper format for the current architectures by consulting the <CODECLASS="literal">$^O</CODE> variable, which contains your current operating system name. There is no <CODECLASS="literal">struct_flock</CODE> function declaration. It's just aliased to the architecture-specific version. Function aliasing is discussed in <ACLASS="xref"HREF="ch10_15.htm"TITLE="Redefining a Function">Recipe 10.14</A>.</P><PCLASS="para">The <EMCLASS="emphasis">lockarea</EM> program opens a temporary file, clobbering any existing contents and writing a screenful (80 by 23) of blanks. Each line is the same length.</P><PCLASS="para">The program then forks one or more times and lets all the child processes try to update the file at the same time. The first argument, <EMCLASS="emphasis">N</EM>, is the number of times to fork to produce <CODECLASS="literal">2</CODE> <CODECLASS="literal">**</CODE> <CODECLASS="literal">N</CODE> processes. So <EMCLASS="emphasis">lockarea 1</EM> makes two children, <EMCLASS="emphasis">lockarea 2</EM> makes four, <EMCLASS="emphasis">lockarea 3</EM> makes eight, <EMCLASS="emphasis">lockarea 4</EM> makes sixteen, etc. The more kids, the more contention for the locks.</P><PCLASS="para">Each process picks a random line in the file, locks just that line, and updates it. It writes its process ID into the line, prepended with a count of how many times the line has been updated:</P><PRECLASS="programlisting">4: 18584 was just here</PRE><PCLASS="para">If the line was already locked, when the lock is finally granted, the line is updated with a message indicating the process that was in the way of this process:</P><PRECLASS="programlisting">29: 24652 ZAPPED 24656</PRE><PCLASS="para">A fun demo is to run the <EMCLASS="emphasis">lockarea</EM> program in the background and the <EMCLASS="emphasis">rep</EM> program from <ACLASS="xref"HREF="ch15_01.htm"TITLE="User Interfaces">Chapter 15</A> watching the file change. Think of it as a video game for systems programmers.</P><PRECLASS="programlisting">% lockarea 5 &% rep -1 'cat /tmp/lkscreen'</PRE><PCLASS="para">When you interrupt the original parent, usually with Ctrl-C or by sending it a SIGINT from the command line, it kills all its children by sending its entire <ACLASS="indexterm"NAME="ch07-idx-1000012068-0"></A><ACLASS="indexterm"NAME="ch07-idx-1000012068-1"></A>process group a signal.</P><DIVCLASS="example"><H4CLASS="example"><ACLASS="title"NAME="ch07-39765">Example 7.11: lockarea</A></H4><PRECLASS="programlisting">#!/usr/bin/perl -w# lockarea - demo record locking with fcntluse strict;my $FORKS = shift || 1;my $SLEEP = shift || 1;use Fcntl;use POSIX qw(:unistd_h :errno_h);my $COLS = 80;my $ROWS = 23;# when's the last time you saw *this* mode used correctly?open(FH, "+> /tmp/lkscreen") or die $!;select(FH);$| = 1;select STDOUT;# clear screenfor (1 .. $ROWS) { print FH " " x $COLS, "\n";}my $progenitor = $$;fork while $FORKS-- > 0;print "hello from $$\n";if ($progenitor == $$) { $SIG{INT} = \&genocide;} else { $SIG{INT} = sub { die "goodbye from $$" };}while (1) { my $line_num = int rand($ROWS); my $line; my $n; # move to line seek(FH, $n = $line_num * ($COLS+1), SEEK_SET) or next; # get lock my $place = tell(FH); my $him; next unless defined($him = lock(*FH, $place, $COLS)); # read line read(FH, $line, $COLS) == $COLS or next; my $count = ($line =~ /(\d+)/) ? $1 : 0; $count++; # update line seek(FH, $place, 0) or die $!; my $update = sprintf($him ? "%6d: %d ZAPPED %d" : "%6d: %d was just here", $count, $$, $him); my $start = int(rand($COLS - length($update))); die "XXX" if $start + length($update) > $COLS; printf FH "%*.*s\n", -$COLS, $COLS, " " x $start . $update; # release lock and go to sleep unlock(*FH, $place, $COLS); sleep $SLEEP if $SLEEP;}die "NOT REACHED"; # just in case# lock($handle, $offset, $timeout) - get an fcntl locksub lock { my ($fh, $start, $till) = @_; ##print "$$: Locking $start, $till\n"; my $lock = struct_flock(F_WRLCK, SEEK_SET, $start, $till, 0); my $blocker = 0; unless (fcntl($fh, F_SETLK, $lock)) { die "F_SETLK $$ @_: $!" unless $! == EAGAIN || $! == EDEADLK; fcntl($fh, F_GETLK, $lock) or die "F_GETLK $$ @_: $!"; $blocker = (struct_flock($lock))[-1]; ##print "lock $$ @_: waiting for $blocker\n"; $lock = struct_flock(F_WRLCK, SEEK_SET, $start, $till, 0); unless (fcntl($fh, F_SETLKW, $lock)) { warn "F_SETLKW $$ @_: $!\n"; return; # undef } } return $blocker;}# unlock($handle, $offset, $timeout) - release an fcntl locksub unlock { my ($fh, $start, $till) = @_; ##print "$$: Unlocking $start, $till\n"; my $lock = struct_flock(F_UNLCK, SEEK_SET, $start, $till, 0); fcntl($fh, F_SETLK, $lock) or die "F_UNLCK $$ @_: $!";}# OS-dependent flock structures# Linux struct flock# short l_type;# short l_whence;# off_t l_start;# off_t l_len;# pid_t l_pid;BEGIN { # c2ph says: typedef='s2 l2 i', sizeof=16 my $FLOCK_STRUCT = 's s l l i'; sub linux_flock { if (wantarray) { my ($type, $whence, $start, $len, $pid) = unpack($FLOCK_STRUCT, $_[0]); return ($type, $whence, $start, $len, $pid); } else { my ($type, $whence, $start, $len, $pid) = @_; return pack($FLOCK_STRUCT, $type, $whence, $start, $len, $pid); } }}# SunOS struct flock:# short l_type; /* F_RDLCK, F_WRLCK, or F_UNLCK */# short l_whence; /* flag to choose starting offset */# long l_start; /* relative offset, in bytes */# long l_len; /* length, in bytes; 0 means lock to EOF */# short l_pid; /* returned with F_GETLK */# short l_xxx; /* reserved for future use */BEGIN { # c2ph says: typedef='s2 l2 s2', sizeof=16 my $FLOCK_STRUCT = 's s l l s s'; sub sunos_flock { if (wantarray) { my ($type, $whence, $start, $len, $pid, $xxx) = unpack($FLOCK_STRUCT, $_[0]); return ($type, $whence, $start, $len, $pid); } else { my ($type, $whence, $start, $len, $pid) = @_; return pack($FLOCK_STRUCT, $type, $whence, $start, $len, $pid, 0); } }}# (Free)BSD struct flock:# off_t l_start; /* starting offset */# off_t l_len; /* len = 0 means until end of file */# pid_t l_pid; /* lock owner */# short l_type; /* lock type: read/write, etc. */# short l_whence; /* type of l_start */BEGIN { # c2ph says: typedef="q2 i s2", size=24 my $FLOCK_STRUCT = 'll ll i s s'; # XXX: q is ll sub bsd_flock { if (wantarray) { my ($xxstart, $start, $xxlen, $len, $pid, $type, $whence) = unpack($FLOCK_STRUCT, $_[0]); return ($type, $whence, $start, $len, $pid); } else { my ($type, $whence, $start, $len, $pid) = @_; my ($xxstart, $xxlen) = (0,0); return pack($FLOCK_STRUCT, $xxstart, $start, $xxlen, $len, $pid, $type, $whence); } }}# alias the fcntl structure at compile timeBEGIN { for ($^O) { *struct_flock = do { /bsd/ && \&bsd_flock || /linux/ && \&linux_flock || /sunos/ && \&sunos_flock || die "unknown operating system $^O, bailing out"; }; }}# install signal handler for childrenBEGIN { my $called = 0; sub genocide { exit if $called++; print "$$: Time to die, kiddies.\n" if $$ == $progenitor; my $job = getpgrp(); $SIG{INT} = 'IGNORE'; kill -2, $job if $job; # killpg(SIGINT, job) 1 while wait > 0; print "$$: My turn\n" if $$ == $progenitor; exit; }}END { &genocide }<ACLASS="indexterm"NAME="ch07-idx-1000010808-0"></A><ACLASS="indexterm"NAME="ch07-idx-1000010808-1"></A><ACLASS="indexterm"NAME="ch07-idx-1000010808-2"></A><ACLASS="indexterm"NAME="ch07-idx-1000010808-3"></A></PRE></DIV><PCLASS="para"></P></DIV><DIVCLASS="htmlnav"><P></P><HRALIGN="LEFT"WIDTH="684"TITLE="footer"><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch07_22.htm"TITLE="7.21. 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