cvs commit: perlfaq perlfaq1.pod perlfaq3.pod perlfaq4.pod perlfaq5.pod
[email protected] (brian d foy) 1 Feb 2006 01:37:21 -0000
| Newsgroups | perl.cvs.perlfaq |
|---|---|
| Message-ID | <[email protected]> |
cvsuser 06/01/31 17:37:21
Modified: . perlfaq1.pod perlfaq3.pod perlfaq4.pod perlfaq5.pod
Log:
* perlfaq1: updated date we expect Perl 6 (I put 2008 just to be safe)
* perlfaq3: How do I debug my Perl programs?
+ Rewrote to give it a friendly tone
+ Acctually answered the question
+ Added a paragraph about Devel::ebug
+ Added a list of commercial products
+ perlfaq4: How do I get a random number between X and Y?
+ Cleansed whitespace
+ Fixed up function name to describe what function does
+ Should we get rid of the prototype?
+ perlfaq5: How can I make a filehandle local to a subroutine? How do I pass filehandles between subroutines? How do I make an array of filehandles?
+ lots and lots of whitespace fixes. The indention got all messed
up somewhere
+ answered the part of the question dealing with an array of
filehandles. that part had been skipped in earlier versions.
Revision Changes Path
1.20 +2 -2 perlfaq/perlfaq1.pod
Index: perlfaq1.pod
===================================================================
RCS file: /cvs/public/perlfaq/perlfaq1.pod,v
retrieving revision 1.19
retrieving revision 1.20
diff -u -r1.19 -r1.20
--- perlfaq1.pod 31 Dec 2005 00:54:37 -0000 1.19
+++ perlfaq1.pod 1 Feb 2006 01:37:21 -0000 1.20
@@ -111,7 +111,7 @@
There is no perl6.x for the next couple of years. Stay tuned,
but don't worry that you'll have to change major versions of Perl
-soon (i.e. before 2006).
+soon (i.e. before 2008).
=item *
1.57 +35 -22 perlfaq/perlfaq3.pod
Index: perlfaq3.pod
===================================================================
RCS file: /cvs/public/perlfaq/perlfaq3.pod,v
retrieving revision 1.56
retrieving revision 1.57
diff -u -r1.56 -r1.57
--- perlfaq3.pod 31 Dec 2005 00:54:37 -0000 1.56
+++ perlfaq3.pod 1 Feb 2006 01:37:21 -0000 1.57
@@ -110,28 +110,41 @@
=head2 How do I debug my Perl programs?
-Have you tried C<use warnings> or used C<-w>? They enable warnings
-to detect dubious practices.
+(contributed by brian d foy)
+
+Before you do anything else, you can help yourself by ensuring that
+you let Perl tell you about problem areas in your code. By turning
+on warnings and strictures, you can head off many problems before
+they get too big. You can find out more about these in L<strict>
+and L<warnings>.
+
+ #!/usr/bin/perl
+ use strict;
+ use warnings;
+
+Beyond that, the simplest debugger is the C<print> function. Use it
+to look at values as you run your program:
+
+ print STDERR "The value is [$value]\n";
+
+The C<Data::Dumper> module can pretty-print Perl data structures:
+
+ use Data::Dumper( Dump );
+ print STDERR "The hash is " . Dump( \%hash ) . "\n";
+
+Perl comes with an interactive debugger, which you can start with the
+C<-d> switch. It's fully explained in L<perldebug>.
+
+If you'd like a graphical user interface and you have Tk, you can use
+C<ptkdb>. It's on CPAN and available for free.
+
+If you need something much more sophisicated and controllable, Leon
+Brocard's Devel::ebug (which you can call with the -D switch as -Debug)
+gives you the programmatic hooks into everything you need to write your
+own (without too much pain and suffering).
-Have you tried C<use strict>? It prevents you from using symbolic
-references, makes you predeclare any subroutines that you call as bare
-words, and (probably most importantly) forces you to predeclare your
-variables with C<my>, C<our>, or C<use vars>.
-
-Did you check the return values of each and every system call? The operating
-system (and thus Perl) tells you whether they worked, and if not
-why.
-
- open(FH, "> /etc/cantwrite")
- or die "Couldn't write to /etc/cantwrite: $!\n";
-
-Did you read L<perltrap>? It's full of gotchas for old and new Perl
-programmers and even has sections for those of you who are upgrading
-from languages like I<awk> and I<C>.
-
-Have you tried the Perl debugger, described in L<perldebug>? You can
-step through your program and see what it's doing and thus work out
-why what it's doing isn't what it should be doing.
+You can also use a commercial debugger such as Affrus (Mac OS X), Komodo
+from Activestate (Windows and Mac OS X), or EPIC (most platforms).
=head2 How do I profile my Perl programs?
1.74 +13 -10 perlfaq/perlfaq4.pod
Index: perlfaq4.pod
===================================================================
RCS file: /cvs/public/perlfaq/perlfaq4.pod,v
retrieving revision 1.73
retrieving revision 1.74
diff -u -r1.73 -r1.74
--- perlfaq4.pod 31 Dec 2005 00:54:37 -0000 1.73
+++ perlfaq4.pod 1 Feb 2006 01:37:21 -0000 1.74
@@ -362,6 +362,9 @@
=head2 How do I get a random number between X and Y?
+To get a random number between two values, you can use the
+C<rand()> builtin to get a random number between 0 and
+
C<rand($x)> returns a number such that
C<< 0 <= rand($x) < $x >>. Thus what you want to have perl
figure out is a random number in the range from 0 to the
@@ -371,19 +374,19 @@
want a random number between 0 and 5 that you can then add
to 10.
- my $number = 10 + int rand( 15-10+1 );
+ my $number = 10 + int rand( 15-10+1 );
Hence you derive the following simple function to abstract
that. It selects a random integer between the two given
-integers (inclusive), For example: C<random_int_in(50,120)>.
+integers (inclusive), For example: C<random_int_between(50,120)>.
- sub random_int_in ($$) {
- my($min, $max) = @_;
- # Assumes that the two arguments are integers themselves!
- return $min if $min == $max;
- ($min, $max) = ($max, $min) if $min > $max;
- return $min + int rand(1 + $max - $min);
- }
+ sub random_int_between ($$) {
+ my($min, $max) = @_;
+ # Assumes that the two arguments are integers themselves!
+ return $min if $min == $max;
+ ($min, $max) = ($max, $min) if $min > $max;
+ return $min + int rand(1 + $max - $min);
+ }
=head1 Data: Dates
1.43 +265 -249 perlfaq/perlfaq5.pod
Index: perlfaq5.pod
===================================================================
RCS file: /cvs/public/perlfaq/perlfaq5.pod,v
retrieving revision 1.42
retrieving revision 1.43
diff -u -r1.42 -r1.43
--- perlfaq5.pod 31 Dec 2005 00:54:37 -0000 1.42
+++ perlfaq5.pod 1 Feb 2006 01:37:21 -0000 1.43
@@ -36,30 +36,30 @@
Use select() to choose the desired handle, then set its
per-filehandle variables.
- $old_fh = select(OUTPUT_HANDLE);
- $| = 1;
- select($old_fh);
+ $old_fh = select(OUTPUT_HANDLE);
+ $| = 1;
+ select($old_fh);
Some idioms can handle this in a single statement:
- select((select(OUTPUT_HANDLE), $| = 1)[0]);
+ select((select(OUTPUT_HANDLE), $| = 1)[0]);
- $| = 1, select $_ for select OUTPUT_HANDLE;
+ $| = 1, select $_ for select OUTPUT_HANDLE;
Some modules offer object-oriented access to handles and their
variables, although they may be overkill if this is the only
thing you do with them. You can use IO::Handle:
- use IO::Handle;
- open(DEV, ">/dev/printer"); # but is this?
- DEV->autoflush(1);
+ use IO::Handle;
+ open(DEV, ">/dev/printer"); # but is this?
+ DEV->autoflush(1);
or IO::Socket:
- use IO::Socket; # this one is kinda a pipe?
+ use IO::Socket; # this one is kinda a pipe?
my $sock = IO::Socket::INET->new( 'www.example.com:80' );
- $sock->autoflush();
+ $sock->autoflush();
=head2 How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?
X<file, editing>
@@ -75,12 +75,12 @@
If your text file doesn't end with a newline, then it's not really a
proper text file, so this may report one fewer line than you expect.
- $lines = 0;
- open(FILE, $filename) or die "Can't open `$filename': $!";
- while (sysread FILE, $buffer, 4096) {
- $lines += ($buffer =~ tr/\n//);
- }
- close FILE;
+ $lines = 0;
+ open(FILE, $filename) or die "Can't open `$filename': $!";
+ while (sysread FILE, $buffer, 4096) {
+ $lines += ($buffer =~ tr/\n//);
+ }
+ close FILE;
This assumes no funny games with newline translations.
@@ -92,19 +92,19 @@
modifying the appropriate variables directly, you can get the same
behavior within a larger program. For example:
- # ...
- {
- local($^I, @ARGV) = ('.orig', glob("*.c"));
- while (<>) {
- if ($. == 1) {
- print "This line should appear at the top of each file\n";
- }
- s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case
- print;
- close ARGV if eof; # Reset $.
- }
- }
- # $^I and @ARGV return to their old values here
+ # ...
+ {
+ local($^I, @ARGV) = ('.orig', glob("*.c"));
+ while (<>) {
+ if ($. == 1) {
+ print "This line should appear at the top of each file\n";
+ }
+ s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case
+ print;
+ close ARGV if eof; # Reset $.
+ }
+ }
+ # $^I and @ARGV return to their old values here
This block modifies all the C<.c> files in the current directory,
leaving a backup of the original data from each file in a new
@@ -138,47 +138,50 @@
Otherwise, you can use the File::Temp module.
- use File::Temp qw/ tempfile tempdir /;
+ use File::Temp qw/ tempfile tempdir /;
- $dir = tempdir( CLEANUP => 1 );
- ($fh, $filename) = tempfile( DIR => $dir );
+ $dir = tempdir( CLEANUP => 1 );
+ ($fh, $filename) = tempfile( DIR => $dir );
- # or if you don't need to know the filename
+ # or if you don't need to know the filename
- $fh = tempfile( DIR => $dir );
+ $fh = tempfile( DIR => $dir );
The File::Temp has been a standard module since Perl 5.6.1. If you
don't have a modern enough Perl installed, use the C<new_tmpfile>
class method from the IO::File module to get a filehandle opened for
reading and writing. Use it if you don't need to know the file's name:
- use IO::File;
- $fh = IO::File->new_tmpfile()
+ use IO::File;
+ $fh = IO::File->new_tmpfile()
or die "Unable to make new temporary file: $!";
If you're committed to creating a temporary file by hand, use the
process ID and/or the current time-value. If you need to have many
temporary files in one process, use a counter:
- BEGIN {
+ BEGIN {
use Fcntl;
my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP};
my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
+
sub temp_file {
- local *FH;
- my $count = 0;
- until (defined(fileno(FH)) || $count++ > 100) {
+ local *FH;
+ my $count = 0;
+ until (defined(fileno(FH)) || $count++ > 100) {
$base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
# O_EXCL is required for security reasons.
sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT);
- }
- if (defined(fileno(FH))
+ }
+
+ if (defined(fileno(FH))
return (*FH, $base_name);
- } else {
+ }
+ else {
return ();
}
}
- }
+ }
=head2 How can I manipulate fixed-record-length files?
X<fixed-length> X<file, fixed-length records>
@@ -192,27 +195,27 @@
some fixed-format input lines, in this case from the output of a normal,
Berkeley-style ps:
- # sample input line:
- # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
- my $PS_T = 'A6 A4 A7 A5 A*';
- open my $ps, '-|', 'ps';
- print scalar <$ps>;
- my @fields = qw( pid tt stat time command );
- while (<$ps>) {
- my %process;
- @process{@fields} = unpack($PS_T, $_);
+ # sample input line:
+ # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
+ my $PS_T = 'A6 A4 A7 A5 A*';
+ open my $ps, '-|', 'ps';
+ print scalar <$ps>;
+ my @fields = qw( pid tt stat time command );
+ while (<$ps>) {
+ my %process;
+ @process{@fields} = unpack($PS_T, $_);
for my $field ( @fields ) {
- print "$field: <$process{$field}>\n";
+ print "$field: <$process{$field}>\n";
}
print 'line=', pack($PS_T, @process{@fields} ), "\n";
- }
+ }
We've used a hash slice in order to easily handle the fields of each row.
Storing the keys in an array means it's easy to operate on them as a
group or loop over them with for. It also avoids polluting the program
with global variables and using symbolic references.
-=head2 How can I make a filehandle local to a subroutine? How do I pass filehandles between subroutines? How do I make an array of filehandles?
+=head2 How can I make a filehandle local to a subroutine? How do I pass filehandles between subroutines? How do I make an array of filehandles?
X<filehandle, local> X<filehandle, passing> X<filehandle, reference>
As of perl5.6, open() autovivifies file and directory handles
@@ -228,6 +231,19 @@
process_file( $fh );
+If you like, you can store these filehandles in an array or a hash.
+If you access them directly, they aren't simple scalars and you
+need to give C<print> a little help by placing the filehandle
+reference in braces. Perl can only figure it out on its own when
+the filehandle reference is a simple scalar.
+
+ my @fhs = ( $fh1, $fh2, $fh3 );
+
+ for( $i = 0; $i <= $#fhs; $i++ ) {
+ print {$fhs[$i]} "just another Perl answer, \n";
+ }
+
+
Before perl5.6, you had to deal with various typeglob idioms
which you may see in older code.
@@ -248,18 +264,18 @@
in a place that a filehandle is expected. Here are ways
to get indirect filehandles:
- $fh = SOME_FH; # bareword is strict-subs hostile
- $fh = "SOME_FH"; # strict-refs hostile; same package only
- $fh = *SOME_FH; # typeglob
- $fh = \*SOME_FH; # ref to typeglob (bless-able)
- $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
+ $fh = SOME_FH; # bareword is strict-subs hostile
+ $fh = "SOME_FH"; # strict-refs hostile; same package only
+ $fh = *SOME_FH; # typeglob
+ $fh = \*SOME_FH; # ref to typeglob (bless-able)
+ $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
Or, you can use the C<new> method from one of the IO::* modules to
create an anonymous filehandle, store that in a scalar variable,
and use it as though it were a normal filehandle.
- use IO::Handle; # 5.004 or higher
- $fh = IO::Handle->new();
+ use IO::Handle; # 5.004 or higher
+ $fh = IO::Handle->new();
Then use any of those as you would a normal filehandle. Anywhere that
Perl is expecting a filehandle, an indirect filehandle may be used
@@ -268,32 +284,32 @@
the C<< <FH> >> diamond operator will accept either a named filehandle
or a scalar variable containing one:
- ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
- print $ofh "Type it: ";
- $got = <$ifh>
- print $efh "What was that: $got";
+ ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
+ print $ofh "Type it: ";
+ $got = <$ifh>
+ print $efh "What was that: $got";
If you're passing a filehandle to a function, you can write
the function in two ways:
- sub accept_fh {
- my $fh = shift;
- print $fh "Sending to indirect filehandle\n";
- }
+ sub accept_fh {
+ my $fh = shift;
+ print $fh "Sending to indirect filehandle\n";
+ }
Or it can localize a typeglob and use the filehandle directly:
- sub accept_fh {
- local *FH = shift;
- print FH "Sending to localized filehandle\n";
- }
+ sub accept_fh {
+ local *FH = shift;
+ print FH "Sending to localized filehandle\n";
+ }
Both styles work with either objects or typeglobs of real filehandles.
(They might also work with strings under some circumstances, but this
is risky.)
- accept_fh(*STDOUT);
- accept_fh($handle);
+ accept_fh(*STDOUT);
+ accept_fh($handle);
In the examples above, we assigned the filehandle to a scalar variable
before using it. That is because only simple scalar variables, not
@@ -302,24 +318,24 @@
something other than a simple scalar variable as a filehandle is
illegal and won't even compile:
- @fd = (*STDIN, *STDOUT, *STDERR);
- print $fd[1] "Type it: "; # WRONG
- $got = <$fd[0]> # WRONG
- print $fd[2] "What was that: $got"; # WRONG
+ @fd = (*STDIN, *STDOUT, *STDERR);
+ print $fd[1] "Type it: "; # WRONG
+ $got = <$fd[0]> # WRONG
+ print $fd[2] "What was that: $got"; # WRONG
With C<print> and C<printf>, you get around this by using a block and
an expression where you would place the filehandle:
- print { $fd[1] } "funny stuff\n";
- printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
- # Pity the poor deadbeef.
+ print { $fd[1] } "funny stuff\n";
+ printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
+ # Pity the poor deadbeef.
That block is a proper block like any other, so you can put more
complicated code there. This sends the message out to one of two places:
- $ok = -x "/bin/cat";
- print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
- print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n";
+ $ok = -x "/bin/cat";
+ print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
+ print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n";
This approach of treating C<print> and C<printf> like object methods
calls doesn't work for the diamond operator. That's because it's a
@@ -330,7 +346,7 @@
would work, but only because readline() requires a typeglob. It doesn't
work with objects or strings, which might be a bug we haven't fixed yet.
- $got = readline($fd[0]);
+ $got = readline($fd[0]);
Let it be noted that the flakiness of indirect filehandles is not
related to whether they're strings, typeglobs, objects, or anything else.
@@ -361,28 +377,28 @@
This subroutine will add commas to your number:
sub commify {
- local $_ = shift;
- 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
- return $_;
- }
+ local $_ = shift;
+ 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
+ return $_;
+ }
This regex from Benjamin Goldberg will add commas to numbers:
- s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
+ s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
It is easier to see with comments:
- s/(
- ^[-+]? # beginning of number.
- \d+? # first digits before first comma
- (?= # followed by, (but not included in the match) :
- (?>(?:\d{3})+) # some positive multiple of three digits.
- (?!\d) # an *exact* multiple, not x * 3 + 1 or whatever.
- )
- | # or:
- \G\d{3} # after the last group, get three digits
- (?=\d) # but they have to have more digits after them.
- )/$1,/xg;
+ s/(
+ ^[-+]? # beginning of number.
+ \d+? # first digits before first comma
+ (?= # followed by, (but not included in the match) :
+ (?>(?:\d{3})+) # some positive multiple of three digits.
+ (?!\d) # an *exact* multiple, not x * 3 + 1 or whatever.
+ )
+ | # or:
+ \G\d{3} # after the last group, get three digits
+ (?=\d) # but they have to have more digits after them.
+ )/$1,/xg;
=head2 How can I translate tildes (~) in a filename?
X<tilde> X<tilde expansion>
@@ -413,12 +429,12 @@
Because you're using something like this, which truncates the file and
I<then> gives you read-write access:
- open(FH, "+> /path/name"); # WRONG (almost always)
+ open(FH, "+> /path/name"); # WRONG (almost always)
Whoops. You should instead use this, which will fail if the file
doesn't exist.
- open(FH, "+< /path/name"); # open for update
+ open(FH, "+< /path/name"); # open for update
Using ">" always clobbers or creates. Using "<" never does
either. The "+" doesn't change this.
@@ -426,52 +442,52 @@
Here are examples of many kinds of file opens. Those using sysopen()
all assume
- use Fcntl;
+ use Fcntl;
To open file for reading:
- open(FH, "< $path") || die $!;
- sysopen(FH, $path, O_RDONLY) || die $!;
+ open(FH, "< $path") || die $!;
+ sysopen(FH, $path, O_RDONLY) || die $!;
To open file for writing, create new file if needed or else truncate old file:
- open(FH, "> $path") || die $!;
- sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!;
- sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!;
+ open(FH, "> $path") || die $!;
+ sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!;
+ sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!;
To open file for writing, create new file, file must not exist:
- sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!;
- sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!;
+ sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!;
+ sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!;
To open file for appending, create if necessary:
- open(FH, ">> $path") || die $!;
- sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!;
- sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;
+ open(FH, ">> $path") || die $!;
+ sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!;
+ sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;
To open file for appending, file must exist:
- sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!;
+ sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!;
To open file for update, file must exist:
- open(FH, "+< $path") || die $!;
- sysopen(FH, $path, O_RDWR) || die $!;
+ open(FH, "+< $path") || die $!;
+ sysopen(FH, $path, O_RDWR) || die $!;
To open file for update, create file if necessary:
- sysopen(FH, $path, O_RDWR|O_CREAT) || die $!;
- sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!;
+ sysopen(FH, $path, O_RDWR|O_CREAT) || die $!;
+ sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!;
To open file for update, file must not exist:
- sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!;
- sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!;
+ sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!;
+ sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!;
To open a file without blocking, creating if necessary:
- sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT)
+ sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT)
or die "can't open /foo/somefile: $!":
Be warned that neither creation nor deletion of files is guaranteed to
@@ -526,7 +542,7 @@
If your operating system supports a proper mv(1) utility or its
functional equivalent, this works:
- rename($old, $new) or system("mv", $old, $new);
+ rename($old, $new) or system("mv", $old, $new);
It may be more portable to use the File::Copy module instead.
You just copy to the new file to the new name (checking return
@@ -589,14 +605,14 @@
A common bit of code B<NOT TO USE> is this:
- sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
- open(LCK, "> file.lock"); # THIS BROKEN CODE
+ sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
+ open(LCK, "> file.lock"); # THIS BROKEN CODE
This is a classic race condition: you take two steps to do something
which must be done in one. That's why computer hardware provides an
atomic test-and-set instruction. In theory, this "ought" to work:
- sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
+ sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
or die "can't open file.lock: $!";
except that lamentably, file creation (and deletion) is not atomic
@@ -614,18 +630,18 @@
Anyway, this is what you can do if you can't help yourself.
- use Fcntl qw(:DEFAULT :flock);
- sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
- flock(FH, LOCK_EX) or die "can't flock numfile: $!";
- $num = <FH> || 0;
- seek(FH, 0, 0) or die "can't rewind numfile: $!";
- truncate(FH, 0) or die "can't truncate numfile: $!";
- (print FH $num+1, "\n") or die "can't write numfile: $!";
- close FH or die "can't close numfile: $!";
+ use Fcntl qw(:DEFAULT :flock);
+ sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
+ flock(FH, LOCK_EX) or die "can't flock numfile: $!";
+ $num = <FH> || 0;
+ seek(FH, 0, 0) or die "can't rewind numfile: $!";
+ truncate(FH, 0) or die "can't truncate numfile: $!";
+ (print FH $num+1, "\n") or die "can't write numfile: $!";
+ close FH or die "can't close numfile: $!";
Here's a much better web-page hit counter:
- $hits = int( (time() - 850_000_000) / rand(1_000) );
+ $hits = int( (time() - 850_000_000) / rand(1_000) );
If the count doesn't impress your friends, then the code might. :-)
@@ -664,20 +680,20 @@
If you're just trying to patch a binary, in many cases something as
simple as this works:
- perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
+ perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
However, if you have fixed sized records, then you might do something more
like this:
- $RECSIZE = 220; # size of record, in bytes
- $recno = 37; # which record to update
- open(FH, "+<somewhere") || die "can't update somewhere: $!";
- seek(FH, $recno * $RECSIZE, 0);
- read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
- # munge the record
- seek(FH, -$RECSIZE, 1);
- print FH $record;
- close FH;
+ $RECSIZE = 220; # size of record, in bytes
+ $recno = 37; # which record to update
+ open(FH, "+<somewhere") || die "can't update somewhere: $!";
+ seek(FH, $recno * $RECSIZE, 0);
+ read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
+ # munge the record
+ seek(FH, -$RECSIZE, 1);
+ print FH $record;
+ close FH;
Locking and error checking are left as an exercise for the reader.
Don't forget them or you'll be quite sorry.
@@ -699,18 +715,18 @@
Here's an example:
- $write_secs = (stat($file))[9];
- printf "file %s updated at %s\n", $file,
+ $write_secs = (stat($file))[9];
+ printf "file %s updated at %s\n", $file,
scalar localtime($write_secs);
If you prefer something more legible, use the File::stat module
(part of the standard distribution in version 5.004 and later):
- # error checking left as an exercise for reader.
- use File::stat;
- use Time::localtime;
- $date_string = ctime(stat($file)->mtime);
- print "file $file updated at $date_string\n";
+ # error checking left as an exercise for reader.
+ use File::stat;
+ use Time::localtime;
+ $date_string = ctime(stat($file)->mtime);
+ print "file $file updated at $date_string\n";
The POSIX::strftime() approach has the benefit of being,
in theory, independent of the current locale. See L<perllocale>
@@ -724,12 +740,12 @@
read and write times from its first argument to all the rest
of them.
- if (@ARGV < 2) {
- die "usage: cptimes timestamp_file other_files ...\n";
- }
- $timestamp = shift;
- ($atime, $mtime) = (stat($timestamp))[8,9];
- utime $atime, $mtime, @ARGV;
+ if (@ARGV < 2) {
+ die "usage: cptimes timestamp_file other_files ...\n";
+ }
+ $timestamp = shift;
+ ($atime, $mtime) = (stat($timestamp))[8,9];
+ utime $atime, $mtime, @ARGV;
Error checking is, as usual, left as an exercise for the reader.
@@ -751,7 +767,7 @@
If you only have to do this once, you can print individually
to each filehandle.
- for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
+ for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
=head2 How can I read in an entire file all at once?
X<slurp> X<file, slurping>
@@ -761,24 +777,24 @@
use File::Slurp;
$all_of_it = read_file($filename); # entire file in scalar
- @all_lines = read_file($filename); # one line perl element
+ @all_lines = read_file($filename); # one line perl element
The customary Perl approach for processing all the lines in a file is to
do so one line at a time:
- open (INPUT, $file) || die "can't open $file: $!";
- while (<INPUT>) {
- chomp;
- # do something with $_
- }
- close(INPUT) || die "can't close $file: $!";
+ open (INPUT, $file) || die "can't open $file: $!";
+ while (<INPUT>) {
+ chomp;
+ # do something with $_
+ }
+ close(INPUT) || die "can't close $file: $!";
This is tremendously more efficient than reading the entire file into
memory as an array of lines and then processing it one element at a time,
which is often--if not almost always--the wrong approach. Whenever
you see someone do this:
- @lines = <INPUT>;
+ @lines = <INPUT>;
you should think long and hard about why you need everything loaded at
once. It's just not a scalable solution. You might also find it more
@@ -789,16 +805,16 @@
You can read the entire filehandle contents into a scalar.
- {
+ {
local(*INPUT, $/);
open (INPUT, $file) || die "can't open $file: $!";
$var = <INPUT>;
- }
+ }
That temporarily undefs your record separator, and will automatically
close the file at block exit. If the file is already open, just use this:
- $var = do { local $/; <INPUT> };
+ $var = do { local $/; <INPUT> };
For ordinary files you can also use the read function.
@@ -830,18 +846,18 @@
interface (POSIX), you can use the following code, which you'll note
turns off echo processing as well.
- #!/usr/bin/perl -w
- use strict;
- $| = 1;
- for (1..4) {
- my $got;
- print "gimme: ";
- $got = getone();
- print "--> $got\n";
- }
+ #!/usr/bin/perl -w
+ use strict;
+ $| = 1;
+ for (1..4) {
+ my $got;
+ print "gimme: ";
+ $got = getone();
+ print "--> $got\n";
+ }
exit;
- BEGIN {
+ BEGIN {
use POSIX qw(:termios_h);
my ($term, $oterm, $echo, $noecho, $fd_stdin);
@@ -856,40 +872,40 @@
$noecho = $oterm & ~$echo;
sub cbreak {
- $term->setlflag($noecho);
- $term->setcc(VTIME, 1);
- $term->setattr($fd_stdin, TCSANOW);
- }
-
+ $term->setlflag($noecho);
+ $term->setcc(VTIME, 1);
+ $term->setattr($fd_stdin, TCSANOW);
+ }
+
sub cooked {
- $term->setlflag($oterm);
- $term->setcc(VTIME, 0);
- $term->setattr($fd_stdin, TCSANOW);
- }
+ $term->setlflag($oterm);
+ $term->setcc(VTIME, 0);
+ $term->setattr($fd_stdin, TCSANOW);
+ }
sub getone {
- my $key = '';
- cbreak();
- sysread(STDIN, $key, 1);
- cooked();
- return $key;
- }
+ my $key = '';
+ cbreak();
+ sysread(STDIN, $key, 1);
+ cooked();
+ return $key;
+ }
- }
+ }
- END { cooked() }
+ END { cooked() }
The Term::ReadKey module from CPAN may be easier to use. Recent versions
include also support for non-portable systems as well.
- use Term::ReadKey;
- open(TTY, "</dev/tty");
- print "Gimme a char: ";
- ReadMode "raw";
- $key = ReadKey 0, *TTY;
- ReadMode "normal";
- printf "\nYou said %s, char number %03d\n",
- $key, ord $key;
+ use Term::ReadKey;
+ open(TTY, "</dev/tty");
+ print "Gimme a char: ";
+ ReadMode "raw";
+ $key = ReadKey 0, *TTY;
+ ReadMode "normal";
+ printf "\nYou said %s, char number %03d\n",
+ $key, ord $key;
=head2 How can I tell whether there's a character waiting on a filehandle?
@@ -903,11 +919,11 @@
It's very system dependent. Here's one solution that works on BSD
systems:
- sub key_ready {
- my($rin, $nfd);
- vec($rin, fileno(STDIN), 1) = 1;
- return $nfd = select($rin,undef,undef,0);
- }
+ sub key_ready {
+ my($rin, $nfd);
+ vec($rin, fileno(STDIN), 1) = 1;
+ return $nfd = select($rin,undef,undef,0);
+ }
If you want to find out how many characters are waiting, there's
also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that
@@ -915,37 +931,37 @@
can be C<require>d. FIONREAD ends up defined as a function in the
I<sys/ioctl.ph> file:
- require 'sys/ioctl.ph';
+ require 'sys/ioctl.ph';
- $size = pack("L", 0);
- ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
- $size = unpack("L", $size);
+ $size = pack("L", 0);
+ ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
+ $size = unpack("L", $size);
If I<h2ph> wasn't installed or doesn't work for you, you can
I<grep> the include files by hand:
- % grep FIONREAD /usr/include/*/*
- /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
+ % grep FIONREAD /usr/include/*/*
+ /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
Or write a small C program using the editor of champions:
- % cat > fionread.c
- #include <sys/ioctl.h>
- main() {
- printf("%#08x\n", FIONREAD);
- }
- ^D
- % cc -o fionread fionread.c
- % ./fionread
- 0x4004667f
+ % cat > fionread.c
+ #include <sys/ioctl.h>
+ main() {
+ printf("%#08x\n", FIONREAD);
+ }
+ ^D
+ % cc -o fionread fionread.c
+ % ./fionread
+ 0x4004667f
And then hard code it, leaving porting as an exercise to your successor.
- $FIONREAD = 0x4004667f; # XXX: opsys dependent
+ $FIONREAD = 0x4004667f; # XXX: opsys dependent
- $size = pack("L", 0);
- ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
- $size = unpack("L", $size);
+ $size = pack("L", 0);
+ ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
+ $size = unpack("L", $size);
FIONREAD requires a filehandle connected to a stream, meaning that sockets,
pipes, and tty devices work, but I<not> files.
@@ -955,7 +971,7 @@
First try
- seek(GWFILE, 0, 1);
+ seek(GWFILE, 0, 1);
The statement C<seek(GWFILE, 0, 1)> doesn't change the current position,
but it does clear the end-of-file condition on the handle, so that the
@@ -985,8 +1001,8 @@
If you check L<perlfunc/open>, you'll see that several of the ways
to call open() should do the trick. For example:
- open(LOG, ">>/foo/logfile");
- open(STDERR, ">&LOG");
+ open(LOG, ">>/foo/logfile");
+ open(STDERR, ">&LOG");
Or even with a literal numeric descriptor:
@@ -1008,17 +1024,17 @@
numeric descriptor as with MHCONTEXT above. But if you really have
to, you may be able to do this:
- require 'sys/syscall.ph';
- $rc = syscall(&SYS_close, $fd + 0); # must force numeric
- die "can't sysclose $fd: $!" unless $rc == -1;
+ require 'sys/syscall.ph';
+ $rc = syscall(&SYS_close, $fd + 0); # must force numeric
+ die "can't sysclose $fd: $!" unless $rc == -1;
Or, just use the fdopen(3S) feature of open():
- {
+ {
local *F;
open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
close F;
- }
+ }
=head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
X<filename, DOS issues>
@@ -1065,8 +1081,8 @@
Here's an algorithm from the Camel Book:
- srand;
- rand($.) < 1 && ($line = $_) while <>;
+ srand;
+ rand($.) < 1 && ($line = $_) while <>;
This has a significant advantage in space over reading the whole file
in. You can find a proof of this method in I<The Art of Computer
@@ -1085,24 +1101,24 @@
Saying
- print "@lines\n";
+ print "@lines\n";
joins together the elements of C<@lines> with a space between them.
If C<@lines> were C<("little", "fluffy", "clouds")> then the above
statement would print
- little fluffy clouds
+ little fluffy clouds
but if each element of C<@lines> was a line of text, ending a newline
character C<("little\n", "fluffy\n", "clouds\n")> then it would print:
- little
- fluffy
- clouds
+ little
+ fluffy
+ clouds
If your array contains lines, just print them:
- print @lines;
+ print @lines;
=head1 AUTHOR AND COPYRIGHT