=head1 NAME
-perlfaq5 - Files and Formats ($Revision: 1.20 $, $Date: 1997/03/19 17:24:51 $)
+perlfaq5 - Files and Formats ($Revision: 1.30 $, $Date: 2003/11/23 08:07:46 $)
=head1 DESCRIPTION
This section deals with I/O and the "f" issues: filehandles, flushing,
formats, and footers.
-=head2 How do I flush/unbuffer a filehandle? Why must I do this?
-
-The C standard I/O library (stdio) normally buffers characters sent to
-devices. This is done for efficiency reasons, so that there isn't a
-system call for each byte. Any time you use print() or write() in
-Perl, you go though this buffering. syswrite() circumvents stdio and
-buffering.
+=head2 How do I flush/unbuffer an output filehandle? Why must I do this?
+
+Perl does not support truly unbuffered output (except
+insofar as you can C<syswrite(OUT, $char, 1)>), although it
+does support is "command buffering", in which a physical
+write is performed after every output command.
+
+The C standard I/O library (stdio) normally buffers
+characters sent to devices so that there isn't a system call
+for each byte. In most stdio implementations, the type of
+output buffering and the size of the buffer varies according
+to the type of device. Perl's print() and write() functions
+normally buffer output, while syswrite() bypasses buffering
+all together.
+
+If you want your output to be sent immediately when you
+execute print() or write() (for instance, for some network
+protocols), you must set the handle's autoflush flag. This
+flag is the Perl variable $| and when it is set to a true
+value, Perl will flush the handle's buffer after each
+print() or write(). Setting $| affects buffering only for
+the currently selected default file handle. You choose this
+handle with the one argument select() call (see
+L<perlvar/$E<verbar>> and L<perlfunc/select>).
+
+Use select() to choose the desired handle, then set its
+per-filehandle variables.
+
+ $old_fh = select(OUTPUT_HANDLE);
+ $| = 1;
+ select($old_fh);
-In most stdio implementations, the type of buffering and the size of
-the buffer varies according to the type of device. Disk files are block
-buffered, often with a buffer size of more than 2k. Pipes and sockets
-are often buffered with a buffer size between 1/2 and 2k. Serial devices
-(e.g. modems, terminals) are normally line-buffered, and stdio sends
-the entire line when it gets the newline.
+Some idioms can handle this in a single statement:
-Perl does not support truly unbuffered output (except insofar as you can
-C<syswrite(OUT, $char, 1)>). What it does instead support is "command
-buffering", in which a physical write is performed after every output
-command. This isn't as hard on your system as unbuffering, but does
-get the output where you want it when you want it.
+ select((select(OUTPUT_HANDLE), $| = 1)[0]);
-If you expect characters to get to your device when you print them there,
-you'll want to autoflush its handle, as in the older:
+ $| = 1, select $_ for select OUTPUT_HANDLE;
- use FileHandle;
- open(DEV, "<+/dev/tty"); # ceci n'est pas une pipe
- DEV->autoflush(1);
-
-or the newer IO::* modules:
+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);
-or even this:
+or IO::Socket:
use IO::Socket; # this one is kinda a pipe?
- $sock = IO::Socket::INET->new(PeerAddr => 'www.perl.com',
- PeerPort => 'http(80)',
- Proto => 'tcp');
- die "$!" unless $sock;
+ my $sock = IO::Socket::INET->new( 'www.example.com:80' ) ;
$sock->autoflush();
- $sock->print("GET /\015\012");
- $document = join('', $sock->getlines());
- print "DOC IS: $document\n";
-
-Note the hardcoded carriage return and newline in their octal
-equivalents. This is the ONLY way (currently) to assure a proper
-flush on all platforms, including Macintosh.
-
-You can use select() and the C<$|> 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), $| = 1)[0]);
=head2 How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?
-Although humans have an easy time thinking of a text file as being a
-sequence of lines that operates much like a stack of playing cards --
-or punch cards -- computers usually see the text file as a sequence of
-bytes. In general, there's no direct way for Perl to seek to a
-particular line of a file, insert text into a file, or remove text
-from a file.
-
-(There are exceptions in special circumstances. Replacing a sequence
-of bytes with another sequence of the same length is one. Another is
-using the C<$DB_RECNO> array bindings as documented in L<DB_File>.
-Yet another is manipulating files with all lines the same length.)
-
-The general solution is to create a temporary copy of the text file with
-the changes you want, then copy that over the original.
-
- $old = $file;
- $new = "$file.tmp.$$";
- $bak = "$file.bak";
-
- open(OLD, "< $old") or die "can't open $old: $!";
- open(NEW, "> $new") or die "can't open $new: $!";
-
- # Correct typos, preserving case
- while (<OLD>) {
- s/\b(p)earl\b/${1}erl/i;
- (print NEW $_) or die "can't write to $new: $!";
- }
-
- close(OLD) or die "can't close $old: $!";
- close(NEW) or die "can't close $new: $!";
-
- rename($old, $bak) or die "can't rename $old to $bak: $!";
- rename($new, $old) or die "can't rename $new to $old: $!";
-
-Perl can do this sort of thing for you automatically with the C<-i>
-command-line switch or the closely-related C<$^I> variable (see
-L<perlrun> for more details). Note that
-C<-i> may require a suffix on some non-Unix systems; see the
-platform-specific documentation that came with your port.
-
- # Renumber a series of tests from the command line
- perl -pi -e 's/(^\s+test\s+)\d+/ $1 . ++$count /e' t/op/taint.t
-
- # form a script
- local($^I, @ARGV) = ('.bak', 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 $.
- }
-
-If you need to seek to an arbitrary line of a file that changes
-infrequently, you could build up an index of byte positions of where
-the line ends are in the file. If the file is large, an index of
-every tenth or hundredth line end would allow you to seek and read
-fairly efficiently. If the file is sorted, try the look.pl library
-(part of the standard perl distribution).
-
-In the unique case of deleting lines at the end of a file, you
-can use tell() and truncate(). The following code snippet deletes
-the last line of a file without making a copy or reading the
-whole file into memory:
-
- open (FH, "+< $file");
- while ( <FH> ) { $addr = tell(FH) unless eof(FH) }
- truncate(FH, $addr);
-
-Error checking is left as an exercise for the reader.
+Use the Tie::File module, which is included in the standard
+distribution since Perl 5.8.0.
=head2 How do I count the number of lines in a file?
}
close FILE;
+This assumes no funny games with newline translations.
+
+=head2 How can I use Perl's C<-i> option from within a program?
+
+C<-i> sets the value of Perl's C<$^I> variable, which in turn affects
+the behavior of C<< <> >>; see L<perlrun> for more details. By
+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
+
+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
+C<.c.orig> file.
+
=head2 How do I make a temporary file name?
-Use the process ID and/or the current time-value. If you need to have
-many temporary files in one process, use a counter:
+Use the File::Temp module, see L<File::Temp> for more information.
+
+ use File::Temp qw/ tempfile tempdir /;
+
+ $dir = tempdir( CLEANUP => 1 );
+ ($fh, $filename) = tempfile( DIR => $dir );
+
+ # or if you don't need to know the filename
+
+ $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()
+ 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 {
- use IO::File;
use Fcntl;
- my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMP} || $ENV{TEMP};
+ my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP};
my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time());
sub temp_file {
- my $fh = undef;
+ local *FH;
my $count = 0;
- until (defined($fh) || $count > 100) {
+ until (defined(fileno(FH)) || $count++ > 100) {
$base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
- $fh = IO::File->new($base_name, O_WRONLY|O_EXCL|O_CREAT, 0644)
+ # O_EXCL is required for security reasons.
+ sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT);
}
- if (defined($fh)) {
- return ($fh, $base_name);
+ if (defined(fileno(FH))
+ return (*FH, $base_name);
} else {
return ();
}
}
}
-Or you could simply use IO::Handle::new_tmpfile.
-
=head2 How can I manipulate fixed-record-length files?
-The most efficient way is using pack() and unpack(). This is faster
-than using substr(). Here is a sample chunk of code to break up and
-put back together again some fixed-format input lines, in this case
-from the output of a normal, Berkeley-style ps:
+The most efficient way is using L<pack()|perlfunc/"pack"> and
+L<unpack()|perlfunc/"unpack">. This is faster than using
+L<substr()|perlfunc/"substr"> when taking many, many strings. It is
+slower for just a few.
+
+Here is a sample chunk of code to break up and put back together again
+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
- $PS_T = 'A6 A4 A7 A5 A*';
- open(PS, "ps|");
- $_ = <PS>; print;
- while (<PS>) {
- ($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
- for $var (qw!pid tt stat time command!) {
- print "$var: <$$var>\n";
+ 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 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),
- "\n";
+ print 'line=', pack($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?
-You may have some success with typeglobs, as we always had to use
-in days of old:
+As of perl5.6, open() autovivifies file and directory handles
+as references if you pass it an uninitialized scalar variable.
+You can then pass these references just like any other scalar,
+and use them in the place of named handles.
- local(*FH);
+ open my $fh, $file_name;
-But while still supported, that isn't the best to go about getting
-local filehandles. Typeglobs have their drawbacks. You may well want
-to use the C<FileHandle> module, which creates new filehandles for you
-(see L<FileHandle>):
+ open local $fh, $file_name;
- use FileHandle;
- sub findme {
- my $fh = FileHandle->new();
- open($fh, "</etc/hosts") or die "no /etc/hosts: $!";
- while (<$fh>) {
- print if /\b127\.(0\.0\.)?1\b/;
- }
- # $fh automatically closes/disappears here
+ print $fh "Hello World!\n";
+
+ process_file( $fh );
+
+Before perl5.6, you had to deal with various typeglob idioms
+which you may see in older code.
+
+ open FILE, "> $filename";
+ process_typeglob( *FILE );
+ process_reference( \*FILE );
+
+ sub process_typeglob { local *FH = shift; print FH "Typeglob!" }
+ sub process_reference { local $fh = shift; print $fh "Reference!" }
+
+If you want to create many anonymous handles, you should
+check out the Symbol or IO::Handle modules.
+
+=head2 How can I use a filehandle indirectly?
+
+An indirect filehandle is using something other than a symbol
+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
+
+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
+ $fh = IO::Handle->new();
+
+Then use any of those as you would a normal filehandle. Anywhere that
+Perl is expecting a filehandle, an indirect filehandle may be used
+instead. An indirect filehandle is just a scalar variable that contains
+a filehandle. Functions like C<print>, C<open>, C<seek>, or
+the C<< <FH> >> diamond operator will accept either a named filehandle
+or a scalar variable containing one:
+
+ ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
+ print $ofh "Type it: ";
+ $got = <$ifh>
+ print $efh "What was that: $got";
+
+If you're passing a filehandle to a function, you can write
+the function in two ways:
+
+ sub accept_fh {
+ my $fh = shift;
+ print $fh "Sending to indirect filehandle\n";
}
-Internally, Perl believes filehandles to be of class IO::Handle. You
-may use that module directly if you'd like (see L<IO::Handle>), or
-one of its more specific derived classes.
+Or it can localize a typeglob and use the filehandle directly:
+
+ sub accept_fh {
+ local *FH = shift;
+ print FH "Sending to localized filehandle\n";
+ }
+
+Both styles work with either objects or typeglobs of real filehandles.
+(They might also work with strings under some circumstances, but this
+is risky.)
+
+ accept_fh(*STDOUT);
+ accept_fh($handle);
+
+In the examples above, we assigned the filehandle to a scalar variable
+before using it. That is because only simple scalar variables, not
+expressions or subscripts of hashes or arrays, can be used with
+built-ins like C<print>, C<printf>, or the diamond operator. Using
+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
+
+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.
+
+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";
+
+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
+real operator, not just a function with a comma-less argument. Assuming
+you've been storing typeglobs in your structure as we did above, you
+can use the built-in function named C<readline> to read a record just
+as C<< <> >> does. Given the initialization shown above for @fd, this
+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]);
+
+Let it be noted that the flakiness of indirect filehandles is not
+related to whether they're strings, typeglobs, objects, or anything else.
+It's the syntax of the fundamental operators. Playing the object
+game doesn't help you at all here.
=head2 How can I set up a footer format to be used with write()?
-There's no built-in way to do this, but L<perlform> has a couple of
+There's no builtin way to do this, but L<perlform> has a couple of
techniques to make it possible for the intrepid hacker.
=head2 How can I write() into a string?
-See L<perlform> for an swrite() function.
+See L<perlform/"Accessing Formatting Internals"> for an swrite() function.
=head2 How can I output my numbers with commas added?
-This one will do it for you:
-
- sub commify {
- local $_ = shift;
- 1 while s/^(-?\d+)(\d{3})/$1,$2/;
- return $_;
- }
+This subroutine will add commas to your number:
- $n = 23659019423.2331;
- print "GOT: ", commify($n), "\n";
+ sub commify {
+ local $_ = shift;
+ 1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
+ return $_;
+ }
- GOT: 23,659,019,423.2331
+This regex from Benjamin Goldberg will add commas to numbers:
-You can't just:
+ s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;
- s/^(-?\d+)(\d{3})/$1,$2/g;
+It is easier to see with comments:
-because you have to put the comma in and then recalculate your
-position.
+ s/(
+ ^[-+]? # beginning of number.
+ \d{1,3}? # 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?
-Use the E<lt>E<gt> (glob()) operator, documented in L<perlfunc>. This
-requires that you have a shell installed that groks tildes, meaning
-csh or tcsh or (some versions of) ksh, and thus may have portability
-problems. The Glob::KGlob module (available from CPAN) gives more
-portable glob functionality.
+Use the <> (glob()) operator, documented in L<perlfunc>. Older
+versions of Perl require that you have a shell installed that groks
+tildes. Recent perl versions have this feature built in. The
+File::KGlob module (available from CPAN) gives more portable glob
+functionality.
Within Perl, you may use this directly:
: ( $ENV{HOME} || $ENV{LOGDIR} )
}ex;
-=head2 How come when I open the file read-write it wipes it out?
+=head2 How come when I open a file read-write it wipes it out?
Because you're using something like this, which truncates the file and
I<then> gives you read-write access:
- open(FH, "+> /path/name"); # WRONG
+ 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.
+
+Here are examples of many kinds of file opens. Those using sysopen()
+all assume
+
+ use Fcntl;
+
+To open file for reading:
+
+ 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 $!;
+
+To open file for writing, create new file, file must not exist:
-If this is an issue, try:
+ sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!;
+ sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!;
- sysopen(FH, "/path/name", O_RDWR|O_CREAT, 0644);
+To open file for appending, create if necessary:
-Error checking is left as an exercise for the reader.
+ 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 $!;
-=head2 Why do I sometimes get an "Argument list too long" when I use <*>?
+To open file for appending, file must exist:
-The C<E<lt>E<gt>> operator performs a globbing operation (see above).
-By default glob() forks csh(1) to do the actual glob expansion, but
+ 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 $!;
+
+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 $!;
+
+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 $!;
+
+To open a file without blocking, creating if necessary:
+
+ 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
+be an atomic operation over NFS. That is, two processes might both
+successfully create or unlink the same file! Therefore O_EXCL
+isn't as exclusive as you might wish.
+
+See also the new L<perlopentut> if you have it (new for 5.6).
+
+=head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
+
+The C<< <> >> operator performs a globbing operation (see above).
+In Perl versions earlier than v5.6.0, the internal glob() operator forks
+csh(1) to do the actual glob expansion, but
csh can't handle more than 127 items and so gives the error message
C<Argument list too long>. People who installed tcsh as csh won't
have this problem, but their users may be surprised by it.
-To get around this, either do the glob yourself with C<Dirhandle>s and
-patterns, or use a module like Glob::KGlob, one that doesn't use the
-shell to do globbing.
+To get around this, either upgrade to Perl v5.6.0 or later, do the glob
+yourself with readdir() and patterns, or use a module like File::KGlob,
+one that doesn't use the shell to do globbing.
=head2 Is there a leak/bug in glob()?
Due to the current implementation on some operating systems, when you
use the glob() function or its angle-bracket alias in a scalar
-context, you may cause a leak and/or unpredictable behavior. It's
+context, you may cause a memory leak and/or unpredictable behavior. It's
best therefore to use glob() only in list context.
-=head2 How can I open a file with a leading "E<gt>" or trailing blanks?
+=head2 How can I open a file with a leading ">" or trailing blanks?
Normally perl ignores trailing blanks in filenames, and interprets
certain leading characters (or a trailing "|") to mean something
-special. To avoid this, you might want to use a routine like this.
-It makes incomplete pathnames into explicit relative ones, and tacks a
-trailing null byte on the name to make perl leave it alone:
-
- sub safe_filename {
- local $_ = shift;
- return m#^/#
- ? "$_\0"
- : "./$_\0";
- }
+special.
- $fn = safe_filename("<<<something really wicked ");
- open(FH, "> $fn") or "couldn't open $fn: $!";
+The three argument form of open() lets you specify the mode
+separately from the filename. The open() function treats
+special mode characters and whitespace in the filename as
+literals
-You could also use the sysopen() function (see L<perlfunc/sysopen>).
+ open FILE, "<", " file "; # filename is " file "
+ open FILE, ">", ">file"; # filename is ">file"
+
+It may be a lot clearer to use sysopen(), though:
+
+ use Fcntl;
+ $badpath = "<<<something really wicked ";
+ sysopen (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC)
+ or die "can't open $badpath: $!";
=head2 How can I reliably rename a file?
-Well, usually you just use Perl's rename() function. But that may
-not work everywhere, in particular, renaming files across file systems.
-If your operating system supports a mv(1) program or its moral equivalent,
-this works:
+If your operating system supports a proper mv(1) utility or its
+functional equivalent, this works:
rename($old, $new) or system("mv", $old, $new);
-It may be more compelling to use the File::Copy module instead. You
-just copy to the new file to the new name (checking return values),
-then delete the old one. This isn't really the same semantics as a
-real rename(), though, which preserves metainformation like
+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
+values), then delete the old one. This isn't really the same
+semantically as a rename(), which preserves meta-information like
permissions, timestamps, inode info, etc.
+Newer versions of File::Copy export a move() function.
+
=head2 How can I lock a file?
-Perl's built-in flock() function (see L<perlfunc> for details) will call
+Perl's builtin flock() function (see L<perlfunc> for details) will call
flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and
later), and lockf(3) if neither of the two previous system calls exists.
On some systems, it may even use a different form of native locking.
=item 3
-Some versions of flock() can't lock files over a network (e.g. on NFS
-file systems), so you'd need to force the use of fcntl(2) when you
-build Perl. See the flock entry of L<perlfunc>, and the F<INSTALL>
-file in the source distribution for information on building Perl to do
-this.
+Some versions of flock() can't lock files over a network (e.g. on NFS file
+systems), so you'd need to force the use of fcntl(2) when you build Perl.
+But even this is dubious at best. See the flock entry of L<perlfunc>
+and the F<INSTALL> file in the source distribution for information on
+building Perl to do this.
+
+Two potentially non-obvious but traditional flock semantics are that
+it waits indefinitely until the lock is granted, and that its locks are
+I<merely advisory>. Such discretionary locks are more flexible, but
+offer fewer guarantees. This means that files locked with flock() may
+be modified by programs that do not also use flock(). Cars that stop
+for red lights get on well with each other, but not with cars that don't
+stop for red lights. See the perlport manpage, your port's specific
+documentation, or your system-specific local manpages for details. It's
+best to assume traditional behavior if you're writing portable programs.
+(If you're not, you should as always feel perfectly free to write
+for your own system's idiosyncrasies (sometimes called "features").
+Slavish adherence to portability concerns shouldn't get in the way of
+your getting your job done.)
+
+For more information on file locking, see also
+L<perlopentut/"File Locking"> if you have it (new for 5.6).
=back
-The CPAN module File::Lock offers similar functionality and (if you
-have dynamic loading) won't require you to rebuild perl if your
-flock() can't lock network files.
-
-=head2 What can't I just open(FH, ">file.lock")?
+=head2 Why can't I just open(FH, "E<gt>file.lock")?
A common bit of code B<NOT TO USE> is this:
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, 0644)
- or die "can't open file.lock: $!":
+ 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
over NFS, so this won't work (at least, not every time) over the net.
-Various schemes involving involving link() have been suggested, but
+Various schemes involving link() have been suggested, but
these tend to involve busy-wait, which is also subdesirable.
-=head2 I still don't get locking. I just want to increment the number
-in the file. How can I do this?
+=head2 I still don't get locking. I just want to increment the number in the file. How can I do this?
Didn't anyone ever tell you web-page hit counters were useless?
+They don't count number of hits, they're a waste of time, and they serve
+only to stroke the writer's vanity. It's better to pick a random number;
+they're more realistic.
-Anyway, this is what to do:
+Anyway, this is what you can do if you can't help yourself.
- use Fcntl;
- sysopen(FH, "numfile", O_RDWR|O_CREAT, 0644) or die "can't open numfile: $!";
- flock(FH, 2) or die "can't flock 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: $!";
- # DO NOT UNLOCK THIS UNTIL YOU CLOSE
close FH or die "can't close numfile: $!";
Here's a much better web-page hit counter:
If the count doesn't impress your friends, then the code might. :-)
+=head2 All I want to do is append a small amount of text to the end of a file. Do I still have to use locking?
+
+If you are on a system that correctly implements flock() and you use the
+example appending code from "perldoc -f flock" everything will be OK
+even if the OS you are on doesn't implement append mode correctly (if
+such a system exists.) So if you are happy to restrict yourself to OSs
+that implement flock() (and that's not really much of a restriction)
+then that is what you should do.
+
+If you know you are only going to use a system that does correctly
+implement appending (i.e. not Win32) then you can omit the seek() from
+the above code.
+
+If you know you are only writing code to run on an OS and filesystem that
+does implement append mode correctly (a local filesystem on a modern
+Unix for example), and you keep the file in block-buffered mode and you
+write less than one buffer-full of output between each manual flushing
+of the buffer then each bufferload is almost guaranteed to be written to
+the end of the file in one chunk without getting intermingled with
+anyone else's output. You can also use the syswrite() function which is
+simply a wrapper around your systems write(2) system call.
+
+There is still a small theoretical chance that a signal will interrupt
+the system level write() operation before completion. There is also a
+possibility that some STDIO implementations may call multiple system
+level write()s even if the buffer was empty to start. There may be some
+systems where this probability is reduced to zero.
+
=head2 How do I randomly update a binary file?
If you're just trying to patch a binary, in many cases something as
seek(FH, $recno * $RECSIZE, 0);
read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
# munge the record
- seek(FH, $recno * $RECSIZE, 0);
+ seek(FH, -$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.
-
-Don't forget to set binmode() under DOS-like platforms when operating
-on files that have anything other than straight text in them. See the
-docs on open() and on binmode() for more details.
+Don't forget them or you'll be quite sorry.
=head2 How do I get a file's timestamp in perl?
-If you want to retrieve the time at which the file was last read,
-written, or had its meta-data (owner, etc) changed, you use the B<-M>,
-B<-A>, or B<-C> filetest operations as documented in L<perlfunc>. These
-retrieve the age of the file (measured against the start-time of your
-program) in days as a floating point number. To retrieve the "raw"
-time in seconds since the epoch, you would call the stat function,
-then use localtime(), gmtime(), or POSIX::strftime() to convert this
-into human-readable form.
+If you want to retrieve the time at which the file was last
+read, written, or had its meta-data (owner, etc) changed,
+you use the B<-M>, B<-A>, or B<-C> file test operations as
+documented in L<perlfunc>. These retrieve the age of the
+file (measured against the start-time of your program) in
+days as a floating point number. Some platforms may not have
+all of these times. See L<perlport> for details. To
+retrieve the "raw" time in seconds since the epoch, you
+would call the stat function, then use localtime(),
+gmtime(), or POSIX::strftime() to convert this into
+human-readable form.
Here's an example:
$write_secs = (stat($file))[9];
- print "file $file updated at ", scalar(localtime($file)), "\n";
+ 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 $date_string\n";
-Error checking is left as an exercise for the reader.
+The POSIX::strftime() approach has the benefit of being,
+in theory, independent of the current locale. See L<perllocale>
+for details.
=head2 How do I set a file's timestamp in perl?
($atime, $mtime) = (stat($timestamp))[8,9];
utime $atime, $mtime, @ARGV;
-Error checking is left as an exercise for the reader.
+Error checking is, as usual, left as an exercise for the reader.
Note that utime() currently doesn't work correctly with Win95/NT
ports. A bug has been reported. Check it carefully before using
-it on those platforms.
+utime() on those platforms.
=head2 How do I print to more than one file at once?
-If you only have to do this once, you can do this:
+To connect one filehandle to several output filehandles,
+you can use the IO::Tee or Tie::FileHandle::Multiplex modules.
+
+If you only have to do this once, you can print individually
+to each filehandle.
for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
-To connect up to one filehandle to several output filehandles, it's
-easiest to use the tee(1) program if you have it, and let it take care
-of the multiplexing:
+=head2 How can I read in an entire file all at once?
+
+You can use the File::Slurp module to do it in one step.
+
+ use File::Slurp;
+
+ $all_of_it = read_file($filename); # entire file in scalar
+ @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: $!";
+
+This is tremendously more efficient than reading the entire file into
+memory as an array of lines and then processing it one element at a time,
+which is often--if not almost always--the wrong approach. Whenever
+you see someone do this:
+
+ @lines = <INPUT>;
+
+you should think long and hard about why you need everything loaded at
+once. It's just not a scalable solution. You might also find it more
+fun to use the standard Tie::File module, or the DB_File module's
+$DB_RECNO bindings, which allow you to tie an array to a file so that
+accessing an element the array actually accesses the corresponding
+line in the file.
+
+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> };
- open (FH, "| tee file1 file2 file3");
+For ordinary files you can also use the read function.
-Otherwise you'll have to write your own multiplexing print function --
-or your own tee program -- or use Tom Christiansen's, at
-http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz, which is
-written in Perl.
+ read( INPUT, $var, -s INPUT );
-In theory a IO::Tee class could be written, but to date we haven't
-seen such.
+The third argument tests the byte size of the data on the INPUT filehandle
+and reads that many bytes into the buffer $var.
=head2 How can I read in a file by paragraphs?
-Use the C<$\> variable (see L<perlvar> for details). You can either
+Use the C<$/> variable (see L<perlvar> for details). You can either
set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">,
for instance, gets treated as two paragraphs and not three), or
C<"\n\n"> to accept empty paragraphs.
+Note that a blank line must have no blanks in it. Thus
+S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two.
+
=head2 How can I read a single character from a file? From the keyboard?
You can use the builtin C<getc()> function for most filehandles, but
it won't (easily) work on a terminal device. For STDIN, either use
-the Term::ReadKey module from CPAN, or use the sample code in
+the Term::ReadKey module from CPAN or use the sample code in
L<perlfunc/getc>.
-If your system supports POSIX, you can use the following code, which
-you'll note turns off echo processing as well.
+If your system supports the portable operating system programming
+interface (POSIX), you can use the following code, which you'll note
+turns off echo processing as well.
#!/usr/bin/perl -w
use strict;
END { cooked() }
-The Term::ReadKey module from CPAN may be easier to use:
+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");
printf "\nYou said %s, char number %03d\n",
$key, ord $key;
-For DOS systems, Dan Carson <dbc@tc.fluke.COM> reports the following:
-
-To put the PC in "raw" mode, use ioctl with some magic numbers gleaned
-from msdos.c (Perl source file) and Ralf Brown's interrupt list (comes
-across the net every so often):
-
- $old_ioctl = ioctl(STDIN,0,0); # Gets device info
- $old_ioctl &= 0xff;
- ioctl(STDIN,1,$old_ioctl | 32); # Writes it back, setting bit 5
-
-Then to read a single character:
-
- sysread(STDIN,$c,1); # Read a single character
-
-And to put the PC back to "cooked" mode:
-
- ioctl(STDIN,1,$old_ioctl); # Sets it back to cooked mode.
-
-So now you have $c. If C<ord($c) == 0>, you have a two byte code, which
-means you hit a special key. Read another byte with C<sysread(STDIN,$c,1)>,
-and that value tells you what combination it was according to this
-table:
-
- # PC 2-byte keycodes = ^@ + the following:
-
- # HEX KEYS
- # --- ----
- # 0F SHF TAB
- # 10-19 ALT QWERTYUIOP
- # 1E-26 ALT ASDFGHJKL
- # 2C-32 ALT ZXCVBNM
- # 3B-44 F1-F10
- # 47-49 HOME,UP,PgUp
- # 4B LEFT
- # 4D RIGHT
- # 4F-53 END,DOWN,PgDn,Ins,Del
- # 54-5D SHF F1-F10
- # 5E-67 CTR F1-F10
- # 68-71 ALT F1-F10
- # 73-77 CTR LEFT,RIGHT,END,PgDn,HOME
- # 78-83 ALT 1234567890-=
- # 84 CTR PgUp
+=head2 How can I tell whether there's a character waiting on a filehandle?
-This is all trial and error I did a long time ago, I hope I'm reading the
-file that worked.
+The very first thing you should do is look into getting the Term::ReadKey
+extension from CPAN. As we mentioned earlier, it now even has limited
+support for non-portable (read: not open systems, closed, proprietary,
+not POSIX, not Unix, etc) systems.
-=head2 How can I tell if there's a character waiting on a filehandle?
-
-You should check out the Frequently Asked Questions list in
+You should also check out the Frequently Asked Questions list in
comp.unix.* for things like this: the answer is essentially the same.
It's very system dependent. Here's one solution that works on BSD
systems:
return $nfd = select($rin,undef,undef,0);
}
-You should look into getting the Term::ReadKey extension from CPAN.
+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
+comes with Perl tries to convert C include files to Perl code, which
+can be C<require>d. FIONREAD ends up defined as a function in the
+I<sys/ioctl.ph> file:
-=head2 How do I open a file without blocking?
+ require 'sys/ioctl.ph';
-You need to use the O_NDELAY or O_NONBLOCK flag from the Fcntl module
-in conjunction with sysopen():
+ $size = pack("L", 0);
+ ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
+ $size = unpack("L", $size);
- use Fcntl;
- sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
- or die "can't open /tmp/somefile: $!":
+If I<h2ph> wasn't installed or doesn't work for you, you can
+I<grep> the include files by hand:
-=head2 How do I create a file only if it doesn't exist?
+ % grep FIONREAD /usr/include/*/*
+ /usr/include/asm/ioctls.h:#define FIONREAD 0x541B
-You need to use the O_CREAT and O_EXCL flags from the Fcntl module in
-conjunction with sysopen():
+Or write a small C program using the editor of champions:
- use Fcntl;
- sysopen(FH, "/tmp/somefile", O_WRONLY|O_EXCL|O_CREAT, 0644)
- or die "can't open /tmp/somefile: $!":
+ % cat > fionread.c
+ #include <sys/ioctl.h>
+ main() {
+ printf("%#08x\n", FIONREAD);
+ }
+ ^D
+ % cc -o fionread fionread.c
+ % ./fionread
+ 0x4004667f
-Be warned that neither creation nor deletion of files is guaranteed to
-be an atomic operation over NFS. That is, two processes might both
-successful create or unlink the same file!
+And then hard code it, leaving porting as an exercise to your successor.
+
+ $FIONREAD = 0x4004667f; # XXX: opsys dependent
+
+ $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.
=head2 How do I do a C<tail -f> in perl?
filehandle. The method: read until end of file, clearerr(), read some
more. Lather, rinse, repeat.
+There's also a File::Tail module from CPAN.
+
=head2 How do I dup() a filehandle in Perl?
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, ">>/tmp/logfile");
+ open(LOG, ">>/foo/logfile");
open(STDERR, ">&LOG");
Or even with a literal numeric descriptor:
$fd = $ENV{MHCONTEXTFD};
open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
-Error checking has been left as an exercise for the reader.
+Note that "<&STDIN" makes a copy, but "<&=STDIN" make
+an alias. That means if you close an aliased handle, all
+aliases become inaccessible. This is not true with
+a copied one.
+
+Error checking, as always, has been left as an exercise for the reader.
=head2 How do I close a file descriptor by number?
This should rarely be necessary, as the Perl close() function is to be
used for things that Perl opened itself, even if it was a dup of a
-numeric descriptor, as with MHCONTEXT above. But if you really have
+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 $rc == -1;
-=head2 Why can't I use "C:\temp\foo" in DOS paths? What doesn't `C:\temp\foo.exe` work?
+Or, just use the fdopen(3S) feature of open():
+
+ {
+ local *F;
+ open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
+ close F;
+ }
+
+=head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
Whoops! You just put a tab and a formfeed into that filename!
Remember that within double quoted strings ("like\this"), the
backslash is an escape character. The full list of these is in
L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't
have a file called "c:(tab)emp(formfeed)oo" or
-"c:(tab)emp(formfeed)oo.exe" on your DOS filesystem.
+"c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem.
Either single-quote your strings, or (preferably) use forward slashes.
Since all DOS and Windows versions since something like MS-DOS 2.0 or so
have treated C</> and C<\> the same in a path, you might as well use the
-one that doesn't clash with Perl -- or the POSIX shell, ANSI C and C++,
-awk, Tcl, Java, or Python, just to mention a few.
+one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++,
+awk, Tcl, Java, or Python, just to mention a few. POSIX paths
+are more portable, too.
=head2 Why doesn't glob("*.*") get all the files?
Because even on non-Unix ports, Perl's glob function follows standard
Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden)
-files.
+files. This makes glob() portable even to legacy systems. Your
+port may include proprietary globbing functions as well. Check its
+documentation for details.
=head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
-This is elaborately and painstakingly described in the "Far More Than
-You Every Wanted To Know" in
-http://www.perl.com/CPAN/doc/FMTEYEWTK/file-dir-perms .
+This is elaborately and painstakingly described in the
+F<file-dir-perms> article in the "Far More Than You Ever Wanted To
+Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .
The executive summary: learn how your filesystem works. The
permissions on a file say what can happen to the data in that file.
srand;
rand($.) < 1 && ($line = $_) while <>;
-This has a significant advantage in space over reading the whole
-file in.
+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
+Programming>, Volume 2, Section 3.4.2, by Donald E. Knuth.
+
+You can use the File::Random module which provides a function
+for that algorithm:
+
+ use File::Random qw/random_line/;
+ my $line = random_line($filename);
+
+Another way is to use the Tie::File module, which treats the entire
+file as an array. Simply access a random array element.
+
+=head2 Why do I get weird spaces when I print an array of lines?
+
+Saying
+
+ 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
+
+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
+
+If your array contains lines, just print them:
+
+ print @lines;
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
-All rights reserved. See L<perlfaq> for distribution information.
+Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
+All rights reserved.
+
+This documentation is free; you can redistribute it and/or modify it
+under the same terms as Perl itself.
+
+Irrespective of its distribution, all code examples here are in the public
+domain. You are permitted and encouraged to use this code and any
+derivatives thereof in your own programs for fun or for profit as you
+see fit. A simple comment in the code giving credit to the FAQ would
+be courteous but is not required.
projects / p5sagit/p5-mst-13.2.git / blobdiff