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68dc0745 | 1 | =head1 NAME |
2 | ||
b68463f7 | 3 | perlfaq5 - Files and Formats ($Revision: 1.37 $, $Date: 2005/08/10 15:55:23 $) |
68dc0745 | 4 | |
5 | =head1 DESCRIPTION | |
6 | ||
7 | This section deals with I/O and the "f" issues: filehandles, flushing, | |
8 | formats, and footers. | |
9 | ||
5a964f20 | 10 | =head2 How do I flush/unbuffer an output filehandle? Why must I do this? |
68dc0745 | 11 | |
c90536be JH |
12 | Perl does not support truly unbuffered output (except |
13 | insofar as you can C<syswrite(OUT, $char, 1)>), although it | |
14 | does support is "command buffering", in which a physical | |
15 | write is performed after every output command. | |
16 | ||
17 | The C standard I/O library (stdio) normally buffers | |
18 | characters sent to devices so that there isn't a system call | |
19 | for each byte. In most stdio implementations, the type of | |
20 | output buffering and the size of the buffer varies according | |
21 | to the type of device. Perl's print() and write() functions | |
22 | normally buffer output, while syswrite() bypasses buffering | |
23 | all together. | |
24 | ||
25 | If you want your output to be sent immediately when you | |
26 | execute print() or write() (for instance, for some network | |
27 | protocols), you must set the handle's autoflush flag. This | |
28 | flag is the Perl variable $| and when it is set to a true | |
29 | value, Perl will flush the handle's buffer after each | |
30 | print() or write(). Setting $| affects buffering only for | |
31 | the currently selected default file handle. You choose this | |
32 | handle with the one argument select() call (see | |
197aec24 | 33 | L<perlvar/$E<verbar>> and L<perlfunc/select>). |
c90536be JH |
34 | |
35 | Use select() to choose the desired handle, then set its | |
36 | per-filehandle variables. | |
5a964f20 TC |
37 | |
38 | $old_fh = select(OUTPUT_HANDLE); | |
39 | $| = 1; | |
40 | select($old_fh); | |
41 | ||
c90536be | 42 | Some idioms can handle this in a single statement: |
5a964f20 TC |
43 | |
44 | select((select(OUTPUT_HANDLE), $| = 1)[0]); | |
818c4caa | 45 | |
c90536be | 46 | $| = 1, select $_ for select OUTPUT_HANDLE; |
5a964f20 | 47 | |
c90536be JH |
48 | Some modules offer object-oriented access to handles and their |
49 | variables, although they may be overkill if this is the only | |
50 | thing you do with them. You can use IO::Handle: | |
68dc0745 | 51 | |
52 | use IO::Handle; | |
53 | open(DEV, ">/dev/printer"); # but is this? | |
54 | DEV->autoflush(1); | |
55 | ||
c90536be | 56 | or IO::Socket: |
68dc0745 | 57 | |
58 | use IO::Socket; # this one is kinda a pipe? | |
c90536be | 59 | my $sock = IO::Socket::INET->new( 'www.example.com:80' ) ; |
68dc0745 | 60 | |
61 | $sock->autoflush(); | |
68dc0745 | 62 | |
63 | =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? | |
64 | ||
1f089b22 JH |
65 | Use the Tie::File module, which is included in the standard |
66 | distribution since Perl 5.8.0. | |
68dc0745 | 67 | |
68 | =head2 How do I count the number of lines in a file? | |
69 | ||
70 | One fairly efficient way is to count newlines in the file. The | |
71 | following program uses a feature of tr///, as documented in L<perlop>. | |
72 | If your text file doesn't end with a newline, then it's not really a | |
73 | proper text file, so this may report one fewer line than you expect. | |
74 | ||
75 | $lines = 0; | |
76 | open(FILE, $filename) or die "Can't open `$filename': $!"; | |
77 | while (sysread FILE, $buffer, 4096) { | |
78 | $lines += ($buffer =~ tr/\n//); | |
79 | } | |
80 | close FILE; | |
81 | ||
5a964f20 TC |
82 | This assumes no funny games with newline translations. |
83 | ||
4750257b MJD |
84 | =head2 How can I use Perl's C<-i> option from within a program? |
85 | ||
86 | C<-i> sets the value of Perl's C<$^I> variable, which in turn affects | |
87 | the behavior of C<< <> >>; see L<perlrun> for more details. By | |
88 | modifying the appropriate variables directly, you can get the same | |
89 | behavior within a larger program. For example: | |
90 | ||
91 | # ... | |
92 | { | |
93 | local($^I, @ARGV) = ('.orig', glob("*.c")); | |
94 | while (<>) { | |
95 | if ($. == 1) { | |
96 | print "This line should appear at the top of each file\n"; | |
97 | } | |
98 | s/\b(p)earl\b/${1}erl/i; # Correct typos, preserving case | |
99 | print; | |
100 | close ARGV if eof; # Reset $. | |
101 | } | |
102 | } | |
103 | # $^I and @ARGV return to their old values here | |
104 | ||
105 | This block modifies all the C<.c> files in the current directory, | |
106 | leaving a backup of the original data from each file in a new | |
107 | C<.c.orig> file. | |
108 | ||
7678cced RGS |
109 | =head2 How can I copy a file? |
110 | ||
111 | (contributed by brian d foy) | |
112 | ||
113 | Use the File::Copy module. It comes with Perl and can do a | |
114 | true copy across file systems, and it does its magic in | |
115 | a portable fashion. | |
116 | ||
117 | use File::Copy; | |
118 | ||
119 | copy( $original, $new_copy ) or die "Copy failed: $!"; | |
120 | ||
121 | If you can't use File::Copy, you'll have to do the work yourself: | |
122 | open the original file, open the destination file, then print | |
123 | to the destination file as you read the original. | |
124 | ||
68dc0745 | 125 | =head2 How do I make a temporary file name? |
126 | ||
7678cced RGS |
127 | If you don't need to know the name of the file, you can use C<open()> |
128 | with C<undef> in place of the file name. The C<open()> function | |
129 | creates an anonymous temporary file. | |
130 | ||
131 | open my $tmp, '+>', undef or die $!; | |
6670e5e7 | 132 | |
7678cced | 133 | Otherwise, you can use the File::Temp module. |
68dc0745 | 134 | |
197aec24 | 135 | use File::Temp qw/ tempfile tempdir /; |
a6dd486b | 136 | |
16394a69 JH |
137 | $dir = tempdir( CLEANUP => 1 ); |
138 | ($fh, $filename) = tempfile( DIR => $dir ); | |
5a964f20 | 139 | |
16394a69 | 140 | # or if you don't need to know the filename |
5a964f20 | 141 | |
16394a69 | 142 | $fh = tempfile( DIR => $dir ); |
5a964f20 | 143 | |
16394a69 JH |
144 | The File::Temp has been a standard module since Perl 5.6.1. If you |
145 | don't have a modern enough Perl installed, use the C<new_tmpfile> | |
146 | class method from the IO::File module to get a filehandle opened for | |
147 | reading and writing. Use it if you don't need to know the file's name: | |
5a964f20 | 148 | |
16394a69 JH |
149 | use IO::File; |
150 | $fh = IO::File->new_tmpfile() | |
151 | or die "Unable to make new temporary file: $!"; | |
5a964f20 | 152 | |
a6dd486b JB |
153 | If you're committed to creating a temporary file by hand, use the |
154 | process ID and/or the current time-value. If you need to have many | |
155 | temporary files in one process, use a counter: | |
5a964f20 TC |
156 | |
157 | BEGIN { | |
68dc0745 | 158 | use Fcntl; |
16394a69 | 159 | my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR} || $ENV{TEMP}; |
68dc0745 | 160 | my $base_name = sprintf("%s/%d-%d-0000", $temp_dir, $$, time()); |
161 | sub temp_file { | |
5a964f20 | 162 | local *FH; |
68dc0745 | 163 | my $count = 0; |
5a964f20 | 164 | until (defined(fileno(FH)) || $count++ > 100) { |
68dc0745 | 165 | $base_name =~ s/-(\d+)$/"-" . (1 + $1)/e; |
2359510d | 166 | # O_EXCL is required for security reasons. |
5a964f20 | 167 | sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT); |
68dc0745 | 168 | } |
5a964f20 TC |
169 | if (defined(fileno(FH)) |
170 | return (*FH, $base_name); | |
68dc0745 | 171 | } else { |
172 | return (); | |
173 | } | |
174 | } | |
175 | } | |
176 | ||
68dc0745 | 177 | =head2 How can I manipulate fixed-record-length files? |
178 | ||
793f5136 RGS |
179 | The most efficient way is using L<pack()|perlfunc/"pack"> and |
180 | L<unpack()|perlfunc/"unpack">. This is faster than using | |
181 | L<substr()|perlfunc/"substr"> when taking many, many strings. It is | |
182 | slower for just a few. | |
5a964f20 TC |
183 | |
184 | Here is a sample chunk of code to break up and put back together again | |
185 | some fixed-format input lines, in this case from the output of a normal, | |
186 | Berkeley-style ps: | |
68dc0745 | 187 | |
188 | # sample input line: | |
189 | # 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what | |
793f5136 RGS |
190 | my $PS_T = 'A6 A4 A7 A5 A*'; |
191 | open my $ps, '-|', 'ps'; | |
192 | print scalar <$ps>; | |
193 | my @fields = qw( pid tt stat time command ); | |
194 | while (<$ps>) { | |
195 | my %process; | |
196 | @process{@fields} = unpack($PS_T, $_); | |
197 | for my $field ( @fields ) { | |
198 | print "$field: <$process{$field}>\n"; | |
68dc0745 | 199 | } |
793f5136 | 200 | print 'line=', pack($PS_T, @process{@fields} ), "\n"; |
68dc0745 | 201 | } |
202 | ||
793f5136 RGS |
203 | We've used a hash slice in order to easily handle the fields of each row. |
204 | Storing the keys in an array means it's easy to operate on them as a | |
205 | group or loop over them with for. It also avoids polluting the program | |
206 | with global variables and using symbolic references. | |
5a964f20 | 207 | |
68dc0745 | 208 | =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? |
209 | ||
c90536be JH |
210 | As of perl5.6, open() autovivifies file and directory handles |
211 | as references if you pass it an uninitialized scalar variable. | |
212 | You can then pass these references just like any other scalar, | |
213 | and use them in the place of named handles. | |
68dc0745 | 214 | |
c90536be | 215 | open my $fh, $file_name; |
818c4caa | 216 | |
c90536be | 217 | open local $fh, $file_name; |
818c4caa | 218 | |
c90536be | 219 | print $fh "Hello World!\n"; |
818c4caa | 220 | |
c90536be | 221 | process_file( $fh ); |
68dc0745 | 222 | |
c90536be JH |
223 | Before perl5.6, you had to deal with various typeglob idioms |
224 | which you may see in older code. | |
68dc0745 | 225 | |
c90536be JH |
226 | open FILE, "> $filename"; |
227 | process_typeglob( *FILE ); | |
228 | process_reference( \*FILE ); | |
818c4caa | 229 | |
c90536be JH |
230 | sub process_typeglob { local *FH = shift; print FH "Typeglob!" } |
231 | sub process_reference { local $fh = shift; print $fh "Reference!" } | |
5a964f20 | 232 | |
c90536be JH |
233 | If you want to create many anonymous handles, you should |
234 | check out the Symbol or IO::Handle modules. | |
5a964f20 TC |
235 | |
236 | =head2 How can I use a filehandle indirectly? | |
237 | ||
238 | An indirect filehandle is using something other than a symbol | |
239 | in a place that a filehandle is expected. Here are ways | |
a6dd486b | 240 | to get indirect filehandles: |
5a964f20 TC |
241 | |
242 | $fh = SOME_FH; # bareword is strict-subs hostile | |
243 | $fh = "SOME_FH"; # strict-refs hostile; same package only | |
244 | $fh = *SOME_FH; # typeglob | |
245 | $fh = \*SOME_FH; # ref to typeglob (bless-able) | |
246 | $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob | |
247 | ||
c90536be | 248 | Or, you can use the C<new> method from one of the IO::* modules to |
5a964f20 TC |
249 | create an anonymous filehandle, store that in a scalar variable, |
250 | and use it as though it were a normal filehandle. | |
251 | ||
5a964f20 TC |
252 | use IO::Handle; # 5.004 or higher |
253 | $fh = IO::Handle->new(); | |
254 | ||
255 | Then use any of those as you would a normal filehandle. Anywhere that | |
256 | Perl is expecting a filehandle, an indirect filehandle may be used | |
257 | instead. An indirect filehandle is just a scalar variable that contains | |
368c9434 | 258 | a filehandle. Functions like C<print>, C<open>, C<seek>, or |
c90536be | 259 | the C<< <FH> >> diamond operator will accept either a named filehandle |
5a964f20 TC |
260 | or a scalar variable containing one: |
261 | ||
262 | ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR); | |
263 | print $ofh "Type it: "; | |
264 | $got = <$ifh> | |
265 | print $efh "What was that: $got"; | |
266 | ||
368c9434 | 267 | If you're passing a filehandle to a function, you can write |
5a964f20 TC |
268 | the function in two ways: |
269 | ||
270 | sub accept_fh { | |
271 | my $fh = shift; | |
272 | print $fh "Sending to indirect filehandle\n"; | |
46fc3d4c | 273 | } |
274 | ||
5a964f20 | 275 | Or it can localize a typeglob and use the filehandle directly: |
46fc3d4c | 276 | |
5a964f20 TC |
277 | sub accept_fh { |
278 | local *FH = shift; | |
279 | print FH "Sending to localized filehandle\n"; | |
46fc3d4c | 280 | } |
281 | ||
5a964f20 TC |
282 | Both styles work with either objects or typeglobs of real filehandles. |
283 | (They might also work with strings under some circumstances, but this | |
284 | is risky.) | |
285 | ||
286 | accept_fh(*STDOUT); | |
287 | accept_fh($handle); | |
288 | ||
289 | In the examples above, we assigned the filehandle to a scalar variable | |
a6dd486b JB |
290 | before using it. That is because only simple scalar variables, not |
291 | expressions or subscripts of hashes or arrays, can be used with | |
292 | built-ins like C<print>, C<printf>, or the diamond operator. Using | |
8305e449 | 293 | something other than a simple scalar variable as a filehandle is |
5a964f20 TC |
294 | illegal and won't even compile: |
295 | ||
296 | @fd = (*STDIN, *STDOUT, *STDERR); | |
297 | print $fd[1] "Type it: "; # WRONG | |
298 | $got = <$fd[0]> # WRONG | |
299 | print $fd[2] "What was that: $got"; # WRONG | |
300 | ||
301 | With C<print> and C<printf>, you get around this by using a block and | |
302 | an expression where you would place the filehandle: | |
303 | ||
304 | print { $fd[1] } "funny stuff\n"; | |
305 | printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559; | |
306 | # Pity the poor deadbeef. | |
307 | ||
308 | That block is a proper block like any other, so you can put more | |
309 | complicated code there. This sends the message out to one of two places: | |
310 | ||
197aec24 | 311 | $ok = -x "/bin/cat"; |
5a964f20 | 312 | print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n"; |
197aec24 | 313 | print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok\n"; |
5a964f20 TC |
314 | |
315 | This approach of treating C<print> and C<printf> like object methods | |
316 | calls doesn't work for the diamond operator. That's because it's a | |
317 | real operator, not just a function with a comma-less argument. Assuming | |
318 | you've been storing typeglobs in your structure as we did above, you | |
c90536be | 319 | can use the built-in function named C<readline> to read a record just |
c47ff5f1 | 320 | as C<< <> >> does. Given the initialization shown above for @fd, this |
c90536be | 321 | would work, but only because readline() requires a typeglob. It doesn't |
5a964f20 TC |
322 | work with objects or strings, which might be a bug we haven't fixed yet. |
323 | ||
324 | $got = readline($fd[0]); | |
325 | ||
326 | Let it be noted that the flakiness of indirect filehandles is not | |
327 | related to whether they're strings, typeglobs, objects, or anything else. | |
328 | It's the syntax of the fundamental operators. Playing the object | |
329 | game doesn't help you at all here. | |
46fc3d4c | 330 | |
68dc0745 | 331 | =head2 How can I set up a footer format to be used with write()? |
332 | ||
54310121 | 333 | There's no builtin way to do this, but L<perlform> has a couple of |
68dc0745 | 334 | techniques to make it possible for the intrepid hacker. |
335 | ||
336 | =head2 How can I write() into a string? | |
337 | ||
65acb1b1 | 338 | See L<perlform/"Accessing Formatting Internals"> for an swrite() function. |
68dc0745 | 339 | |
340 | =head2 How can I output my numbers with commas added? | |
341 | ||
b68463f7 RGS |
342 | (contributed by brian d foy and Benjamin Goldberg) |
343 | ||
344 | You can use L<Number::Format> to separate places in a number. | |
345 | It handles locale information for those of you who want to insert | |
346 | full stops instead (or anything else that they want to use, | |
347 | really). | |
348 | ||
49d635f9 RGS |
349 | This subroutine will add commas to your number: |
350 | ||
351 | sub commify { | |
352 | local $_ = shift; | |
353 | 1 while s/^([-+]?\d+)(\d{3})/$1,$2/; | |
354 | return $_; | |
355 | } | |
356 | ||
357 | This regex from Benjamin Goldberg will add commas to numbers: | |
68dc0745 | 358 | |
881bdbd4 | 359 | s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g; |
68dc0745 | 360 | |
49d635f9 | 361 | It is easier to see with comments: |
68dc0745 | 362 | |
881bdbd4 JH |
363 | s/( |
364 | ^[-+]? # beginning of number. | |
7678cced | 365 | \d+? # first digits before first comma |
881bdbd4 JH |
366 | (?= # followed by, (but not included in the match) : |
367 | (?>(?:\d{3})+) # some positive multiple of three digits. | |
368 | (?!\d) # an *exact* multiple, not x * 3 + 1 or whatever. | |
369 | ) | |
370 | | # or: | |
371 | \G\d{3} # after the last group, get three digits | |
372 | (?=\d) # but they have to have more digits after them. | |
373 | )/$1,/xg; | |
46fc3d4c | 374 | |
68dc0745 | 375 | =head2 How can I translate tildes (~) in a filename? |
376 | ||
575cc754 JH |
377 | Use the <> (glob()) operator, documented in L<perlfunc>. Older |
378 | versions of Perl require that you have a shell installed that groks | |
379 | tildes. Recent perl versions have this feature built in. The | |
d6260402 | 380 | File::KGlob module (available from CPAN) gives more portable glob |
575cc754 | 381 | functionality. |
68dc0745 | 382 | |
383 | Within Perl, you may use this directly: | |
384 | ||
385 | $filename =~ s{ | |
386 | ^ ~ # find a leading tilde | |
387 | ( # save this in $1 | |
388 | [^/] # a non-slash character | |
389 | * # repeated 0 or more times (0 means me) | |
390 | ) | |
391 | }{ | |
392 | $1 | |
393 | ? (getpwnam($1))[7] | |
394 | : ( $ENV{HOME} || $ENV{LOGDIR} ) | |
395 | }ex; | |
396 | ||
5a964f20 | 397 | =head2 How come when I open a file read-write it wipes it out? |
68dc0745 | 398 | |
399 | Because you're using something like this, which truncates the file and | |
400 | I<then> gives you read-write access: | |
401 | ||
5a964f20 | 402 | open(FH, "+> /path/name"); # WRONG (almost always) |
68dc0745 | 403 | |
404 | Whoops. You should instead use this, which will fail if the file | |
197aec24 | 405 | doesn't exist. |
d92eb7b0 GS |
406 | |
407 | open(FH, "+< /path/name"); # open for update | |
408 | ||
c47ff5f1 | 409 | Using ">" always clobbers or creates. Using "<" never does |
d92eb7b0 | 410 | either. The "+" doesn't change this. |
68dc0745 | 411 | |
5a964f20 TC |
412 | Here are examples of many kinds of file opens. Those using sysopen() |
413 | all assume | |
68dc0745 | 414 | |
5a964f20 | 415 | use Fcntl; |
68dc0745 | 416 | |
5a964f20 | 417 | To open file for reading: |
68dc0745 | 418 | |
5a964f20 TC |
419 | open(FH, "< $path") || die $!; |
420 | sysopen(FH, $path, O_RDONLY) || die $!; | |
421 | ||
422 | To open file for writing, create new file if needed or else truncate old file: | |
423 | ||
424 | open(FH, "> $path") || die $!; | |
425 | sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) || die $!; | |
426 | sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) || die $!; | |
427 | ||
428 | To open file for writing, create new file, file must not exist: | |
429 | ||
430 | sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) || die $!; | |
431 | sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) || die $!; | |
432 | ||
433 | To open file for appending, create if necessary: | |
434 | ||
435 | open(FH, ">> $path") || die $!; | |
436 | sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) || die $!; | |
437 | sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!; | |
438 | ||
439 | To open file for appending, file must exist: | |
440 | ||
441 | sysopen(FH, $path, O_WRONLY|O_APPEND) || die $!; | |
442 | ||
443 | To open file for update, file must exist: | |
444 | ||
445 | open(FH, "+< $path") || die $!; | |
446 | sysopen(FH, $path, O_RDWR) || die $!; | |
447 | ||
448 | To open file for update, create file if necessary: | |
449 | ||
450 | sysopen(FH, $path, O_RDWR|O_CREAT) || die $!; | |
451 | sysopen(FH, $path, O_RDWR|O_CREAT, 0666) || die $!; | |
452 | ||
453 | To open file for update, file must not exist: | |
454 | ||
455 | sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) || die $!; | |
456 | sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) || die $!; | |
457 | ||
458 | To open a file without blocking, creating if necessary: | |
459 | ||
2359510d SD |
460 | sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT) |
461 | or die "can't open /foo/somefile: $!": | |
5a964f20 TC |
462 | |
463 | Be warned that neither creation nor deletion of files is guaranteed to | |
464 | be an atomic operation over NFS. That is, two processes might both | |
a6dd486b JB |
465 | successfully create or unlink the same file! Therefore O_EXCL |
466 | isn't as exclusive as you might wish. | |
68dc0745 | 467 | |
87275199 | 468 | See also the new L<perlopentut> if you have it (new for 5.6). |
65acb1b1 | 469 | |
04d666b1 | 470 | =head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>? |
68dc0745 | 471 | |
c47ff5f1 | 472 | The C<< <> >> operator performs a globbing operation (see above). |
3a4b19e4 GS |
473 | In Perl versions earlier than v5.6.0, the internal glob() operator forks |
474 | csh(1) to do the actual glob expansion, but | |
68dc0745 | 475 | csh can't handle more than 127 items and so gives the error message |
476 | C<Argument list too long>. People who installed tcsh as csh won't | |
477 | have this problem, but their users may be surprised by it. | |
478 | ||
3a4b19e4 | 479 | To get around this, either upgrade to Perl v5.6.0 or later, do the glob |
d6260402 | 480 | yourself with readdir() and patterns, or use a module like File::KGlob, |
3a4b19e4 | 481 | one that doesn't use the shell to do globbing. |
68dc0745 | 482 | |
483 | =head2 Is there a leak/bug in glob()? | |
484 | ||
485 | Due to the current implementation on some operating systems, when you | |
486 | use the glob() function or its angle-bracket alias in a scalar | |
a6dd486b | 487 | context, you may cause a memory leak and/or unpredictable behavior. It's |
68dc0745 | 488 | best therefore to use glob() only in list context. |
489 | ||
c47ff5f1 | 490 | =head2 How can I open a file with a leading ">" or trailing blanks? |
68dc0745 | 491 | |
b68463f7 | 492 | (contributed by Brian McCauley) |
68dc0745 | 493 | |
b68463f7 RGS |
494 | The special two argument form of Perl's open() function ignores |
495 | trailing blanks in filenames and infers the mode from certain leading | |
496 | characters (or a trailing "|"). In older versions of Perl this was the | |
497 | only version of open() and so it is prevalent in old code and books. | |
65acb1b1 | 498 | |
b68463f7 RGS |
499 | Unless you have a particular reason to use the two argument form you |
500 | should use the three argument form of open() which does not treat any | |
501 | charcters in the filename as special. | |
502 | ||
881bdbd4 JH |
503 | open FILE, "<", " file "; # filename is " file " |
504 | open FILE, ">", ">file"; # filename is ">file" | |
65acb1b1 | 505 | |
68dc0745 | 506 | =head2 How can I reliably rename a file? |
507 | ||
49d635f9 RGS |
508 | If your operating system supports a proper mv(1) utility or its |
509 | functional equivalent, this works: | |
68dc0745 | 510 | |
511 | rename($old, $new) or system("mv", $old, $new); | |
512 | ||
d2321c93 JH |
513 | It may be more portable to use the File::Copy module instead. |
514 | You just copy to the new file to the new name (checking return | |
515 | values), then delete the old one. This isn't really the same | |
516 | semantically as a rename(), which preserves meta-information like | |
68dc0745 | 517 | permissions, timestamps, inode info, etc. |
518 | ||
d2321c93 | 519 | Newer versions of File::Copy export a move() function. |
5a964f20 | 520 | |
68dc0745 | 521 | =head2 How can I lock a file? |
522 | ||
54310121 | 523 | Perl's builtin flock() function (see L<perlfunc> for details) will call |
68dc0745 | 524 | flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and |
525 | later), and lockf(3) if neither of the two previous system calls exists. | |
526 | On some systems, it may even use a different form of native locking. | |
527 | Here are some gotchas with Perl's flock(): | |
528 | ||
529 | =over 4 | |
530 | ||
531 | =item 1 | |
532 | ||
533 | Produces a fatal error if none of the three system calls (or their | |
534 | close equivalent) exists. | |
535 | ||
536 | =item 2 | |
537 | ||
538 | lockf(3) does not provide shared locking, and requires that the | |
539 | filehandle be open for writing (or appending, or read/writing). | |
540 | ||
541 | =item 3 | |
542 | ||
d92eb7b0 GS |
543 | Some versions of flock() can't lock files over a network (e.g. on NFS file |
544 | systems), so you'd need to force the use of fcntl(2) when you build Perl. | |
a6dd486b | 545 | But even this is dubious at best. See the flock entry of L<perlfunc> |
d92eb7b0 GS |
546 | and the F<INSTALL> file in the source distribution for information on |
547 | building Perl to do this. | |
548 | ||
549 | Two potentially non-obvious but traditional flock semantics are that | |
a6dd486b | 550 | it waits indefinitely until the lock is granted, and that its locks are |
d92eb7b0 GS |
551 | I<merely advisory>. Such discretionary locks are more flexible, but |
552 | offer fewer guarantees. This means that files locked with flock() may | |
553 | be modified by programs that do not also use flock(). Cars that stop | |
554 | for red lights get on well with each other, but not with cars that don't | |
555 | stop for red lights. See the perlport manpage, your port's specific | |
556 | documentation, or your system-specific local manpages for details. It's | |
557 | best to assume traditional behavior if you're writing portable programs. | |
a6dd486b | 558 | (If you're not, you should as always feel perfectly free to write |
d92eb7b0 GS |
559 | for your own system's idiosyncrasies (sometimes called "features"). |
560 | Slavish adherence to portability concerns shouldn't get in the way of | |
561 | your getting your job done.) | |
68dc0745 | 562 | |
197aec24 | 563 | For more information on file locking, see also |
13a2d996 | 564 | L<perlopentut/"File Locking"> if you have it (new for 5.6). |
65acb1b1 | 565 | |
68dc0745 | 566 | =back |
567 | ||
04d666b1 | 568 | =head2 Why can't I just open(FH, "E<gt>file.lock")? |
68dc0745 | 569 | |
570 | A common bit of code B<NOT TO USE> is this: | |
571 | ||
572 | sleep(3) while -e "file.lock"; # PLEASE DO NOT USE | |
573 | open(LCK, "> file.lock"); # THIS BROKEN CODE | |
574 | ||
575 | This is a classic race condition: you take two steps to do something | |
576 | which must be done in one. That's why computer hardware provides an | |
577 | atomic test-and-set instruction. In theory, this "ought" to work: | |
578 | ||
5a964f20 | 579 | sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT) |
9b55d3ab | 580 | or die "can't open file.lock: $!"; |
68dc0745 | 581 | |
582 | except that lamentably, file creation (and deletion) is not atomic | |
583 | over NFS, so this won't work (at least, not every time) over the net. | |
65acb1b1 | 584 | Various schemes involving link() have been suggested, but |
46fc3d4c | 585 | these tend to involve busy-wait, which is also subdesirable. |
68dc0745 | 586 | |
fc36a67e | 587 | =head2 I still don't get locking. I just want to increment the number in the file. How can I do this? |
68dc0745 | 588 | |
46fc3d4c | 589 | Didn't anyone ever tell you web-page hit counters were useless? |
5a964f20 | 590 | They don't count number of hits, they're a waste of time, and they serve |
a6dd486b JB |
591 | only to stroke the writer's vanity. It's better to pick a random number; |
592 | they're more realistic. | |
68dc0745 | 593 | |
5a964f20 | 594 | Anyway, this is what you can do if you can't help yourself. |
68dc0745 | 595 | |
e2c57c3e | 596 | use Fcntl qw(:DEFAULT :flock); |
5a964f20 | 597 | sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!"; |
65acb1b1 | 598 | flock(FH, LOCK_EX) or die "can't flock numfile: $!"; |
68dc0745 | 599 | $num = <FH> || 0; |
600 | seek(FH, 0, 0) or die "can't rewind numfile: $!"; | |
601 | truncate(FH, 0) or die "can't truncate numfile: $!"; | |
602 | (print FH $num+1, "\n") or die "can't write numfile: $!"; | |
68dc0745 | 603 | close FH or die "can't close numfile: $!"; |
604 | ||
46fc3d4c | 605 | Here's a much better web-page hit counter: |
68dc0745 | 606 | |
607 | $hits = int( (time() - 850_000_000) / rand(1_000) ); | |
608 | ||
609 | If the count doesn't impress your friends, then the code might. :-) | |
610 | ||
f52f3be2 | 611 | =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? |
05caf3a7 GJ |
612 | |
613 | If you are on a system that correctly implements flock() and you use the | |
614 | example appending code from "perldoc -f flock" everything will be OK | |
615 | even if the OS you are on doesn't implement append mode correctly (if | |
616 | such a system exists.) So if you are happy to restrict yourself to OSs | |
617 | that implement flock() (and that's not really much of a restriction) | |
618 | then that is what you should do. | |
619 | ||
620 | If you know you are only going to use a system that does correctly | |
621 | implement appending (i.e. not Win32) then you can omit the seek() from | |
622 | the above code. | |
623 | ||
624 | If you know you are only writing code to run on an OS and filesystem that | |
625 | does implement append mode correctly (a local filesystem on a modern | |
626 | Unix for example), and you keep the file in block-buffered mode and you | |
627 | write less than one buffer-full of output between each manual flushing | |
8305e449 | 628 | of the buffer then each bufferload is almost guaranteed to be written to |
05caf3a7 GJ |
629 | the end of the file in one chunk without getting intermingled with |
630 | anyone else's output. You can also use the syswrite() function which is | |
631 | simply a wrapper around your systems write(2) system call. | |
632 | ||
633 | There is still a small theoretical chance that a signal will interrupt | |
634 | the system level write() operation before completion. There is also a | |
635 | possibility that some STDIO implementations may call multiple system | |
636 | level write()s even if the buffer was empty to start. There may be some | |
637 | systems where this probability is reduced to zero. | |
638 | ||
68dc0745 | 639 | =head2 How do I randomly update a binary file? |
640 | ||
641 | If you're just trying to patch a binary, in many cases something as | |
642 | simple as this works: | |
643 | ||
644 | perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs | |
645 | ||
646 | However, if you have fixed sized records, then you might do something more | |
647 | like this: | |
648 | ||
649 | $RECSIZE = 220; # size of record, in bytes | |
650 | $recno = 37; # which record to update | |
651 | open(FH, "+<somewhere") || die "can't update somewhere: $!"; | |
652 | seek(FH, $recno * $RECSIZE, 0); | |
653 | read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!"; | |
654 | # munge the record | |
65acb1b1 | 655 | seek(FH, -$RECSIZE, 1); |
68dc0745 | 656 | print FH $record; |
657 | close FH; | |
658 | ||
659 | Locking and error checking are left as an exercise for the reader. | |
a6dd486b | 660 | Don't forget them or you'll be quite sorry. |
68dc0745 | 661 | |
68dc0745 | 662 | =head2 How do I get a file's timestamp in perl? |
663 | ||
881bdbd4 JH |
664 | If you want to retrieve the time at which the file was last |
665 | read, written, or had its meta-data (owner, etc) changed, | |
666 | you use the B<-M>, B<-A>, or B<-C> file test operations as | |
667 | documented in L<perlfunc>. These retrieve the age of the | |
668 | file (measured against the start-time of your program) in | |
669 | days as a floating point number. Some platforms may not have | |
670 | all of these times. See L<perlport> for details. To | |
671 | retrieve the "raw" time in seconds since the epoch, you | |
672 | would call the stat function, then use localtime(), | |
673 | gmtime(), or POSIX::strftime() to convert this into | |
674 | human-readable form. | |
68dc0745 | 675 | |
676 | Here's an example: | |
677 | ||
678 | $write_secs = (stat($file))[9]; | |
c8db1d39 TC |
679 | printf "file %s updated at %s\n", $file, |
680 | scalar localtime($write_secs); | |
68dc0745 | 681 | |
682 | If you prefer something more legible, use the File::stat module | |
683 | (part of the standard distribution in version 5.004 and later): | |
684 | ||
65acb1b1 | 685 | # error checking left as an exercise for reader. |
68dc0745 | 686 | use File::stat; |
687 | use Time::localtime; | |
688 | $date_string = ctime(stat($file)->mtime); | |
689 | print "file $file updated at $date_string\n"; | |
690 | ||
65acb1b1 TC |
691 | The POSIX::strftime() approach has the benefit of being, |
692 | in theory, independent of the current locale. See L<perllocale> | |
693 | for details. | |
68dc0745 | 694 | |
695 | =head2 How do I set a file's timestamp in perl? | |
696 | ||
697 | You use the utime() function documented in L<perlfunc/utime>. | |
698 | By way of example, here's a little program that copies the | |
699 | read and write times from its first argument to all the rest | |
700 | of them. | |
701 | ||
702 | if (@ARGV < 2) { | |
703 | die "usage: cptimes timestamp_file other_files ...\n"; | |
704 | } | |
705 | $timestamp = shift; | |
706 | ($atime, $mtime) = (stat($timestamp))[8,9]; | |
707 | utime $atime, $mtime, @ARGV; | |
708 | ||
65acb1b1 | 709 | Error checking is, as usual, left as an exercise for the reader. |
68dc0745 | 710 | |
19a1cd16 SP |
711 | The perldoc for utime also has an example that has the same |
712 | effect as touch(1) on files that I<already exist>. | |
713 | ||
714 | Certain file systems have a limited ability to store the times | |
715 | on a file at the expected level of precision. For example, the | |
716 | FAT and HPFS filesystem are unable to create dates on files with | |
717 | a finer granularity than two seconds. This is a limitation of | |
718 | the filesystems, not of utime(). | |
68dc0745 | 719 | |
720 | =head2 How do I print to more than one file at once? | |
721 | ||
49d635f9 RGS |
722 | To connect one filehandle to several output filehandles, |
723 | you can use the IO::Tee or Tie::FileHandle::Multiplex modules. | |
68dc0745 | 724 | |
49d635f9 RGS |
725 | If you only have to do this once, you can print individually |
726 | to each filehandle. | |
68dc0745 | 727 | |
49d635f9 | 728 | for $fh (FH1, FH2, FH3) { print $fh "whatever\n" } |
5a964f20 | 729 | |
49d635f9 | 730 | =head2 How can I read in an entire file all at once? |
68dc0745 | 731 | |
49d635f9 | 732 | You can use the File::Slurp module to do it in one step. |
68dc0745 | 733 | |
49d635f9 | 734 | use File::Slurp; |
197aec24 | 735 | |
49d635f9 RGS |
736 | $all_of_it = read_file($filename); # entire file in scalar |
737 | @all_lines = read_file($filename); # one line perl element | |
d92eb7b0 GS |
738 | |
739 | The customary Perl approach for processing all the lines in a file is to | |
740 | do so one line at a time: | |
741 | ||
742 | open (INPUT, $file) || die "can't open $file: $!"; | |
743 | while (<INPUT>) { | |
744 | chomp; | |
745 | # do something with $_ | |
197aec24 | 746 | } |
d92eb7b0 GS |
747 | close(INPUT) || die "can't close $file: $!"; |
748 | ||
749 | This is tremendously more efficient than reading the entire file into | |
750 | memory as an array of lines and then processing it one element at a time, | |
a6dd486b | 751 | which is often--if not almost always--the wrong approach. Whenever |
d92eb7b0 GS |
752 | you see someone do this: |
753 | ||
754 | @lines = <INPUT>; | |
755 | ||
30852c57 JH |
756 | you should think long and hard about why you need everything loaded at |
757 | once. It's just not a scalable solution. You might also find it more | |
758 | fun to use the standard Tie::File module, or the DB_File module's | |
759 | $DB_RECNO bindings, which allow you to tie an array to a file so that | |
760 | accessing an element the array actually accesses the corresponding | |
761 | line in the file. | |
d92eb7b0 | 762 | |
f05bbc40 | 763 | You can read the entire filehandle contents into a scalar. |
d92eb7b0 GS |
764 | |
765 | { | |
766 | local(*INPUT, $/); | |
767 | open (INPUT, $file) || die "can't open $file: $!"; | |
768 | $var = <INPUT>; | |
769 | } | |
770 | ||
197aec24 | 771 | That temporarily undefs your record separator, and will automatically |
d92eb7b0 GS |
772 | close the file at block exit. If the file is already open, just use this: |
773 | ||
774 | $var = do { local $/; <INPUT> }; | |
775 | ||
f05bbc40 JH |
776 | For ordinary files you can also use the read function. |
777 | ||
778 | read( INPUT, $var, -s INPUT ); | |
779 | ||
780 | The third argument tests the byte size of the data on the INPUT filehandle | |
781 | and reads that many bytes into the buffer $var. | |
782 | ||
68dc0745 | 783 | =head2 How can I read in a file by paragraphs? |
784 | ||
65acb1b1 | 785 | Use the C<$/> variable (see L<perlvar> for details). You can either |
68dc0745 | 786 | set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">, |
787 | for instance, gets treated as two paragraphs and not three), or | |
788 | C<"\n\n"> to accept empty paragraphs. | |
789 | ||
197aec24 | 790 | Note that a blank line must have no blanks in it. Thus |
c4db748a | 791 | S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two. |
65acb1b1 | 792 | |
68dc0745 | 793 | =head2 How can I read a single character from a file? From the keyboard? |
794 | ||
795 | You can use the builtin C<getc()> function for most filehandles, but | |
796 | it won't (easily) work on a terminal device. For STDIN, either use | |
a6dd486b | 797 | the Term::ReadKey module from CPAN or use the sample code in |
68dc0745 | 798 | L<perlfunc/getc>. |
799 | ||
65acb1b1 TC |
800 | If your system supports the portable operating system programming |
801 | interface (POSIX), you can use the following code, which you'll note | |
802 | turns off echo processing as well. | |
68dc0745 | 803 | |
804 | #!/usr/bin/perl -w | |
805 | use strict; | |
806 | $| = 1; | |
807 | for (1..4) { | |
808 | my $got; | |
809 | print "gimme: "; | |
810 | $got = getone(); | |
811 | print "--> $got\n"; | |
812 | } | |
813 | exit; | |
814 | ||
815 | BEGIN { | |
816 | use POSIX qw(:termios_h); | |
817 | ||
818 | my ($term, $oterm, $echo, $noecho, $fd_stdin); | |
819 | ||
820 | $fd_stdin = fileno(STDIN); | |
821 | ||
822 | $term = POSIX::Termios->new(); | |
823 | $term->getattr($fd_stdin); | |
824 | $oterm = $term->getlflag(); | |
825 | ||
826 | $echo = ECHO | ECHOK | ICANON; | |
827 | $noecho = $oterm & ~$echo; | |
828 | ||
829 | sub cbreak { | |
830 | $term->setlflag($noecho); | |
831 | $term->setcc(VTIME, 1); | |
832 | $term->setattr($fd_stdin, TCSANOW); | |
833 | } | |
834 | ||
835 | sub cooked { | |
836 | $term->setlflag($oterm); | |
837 | $term->setcc(VTIME, 0); | |
838 | $term->setattr($fd_stdin, TCSANOW); | |
839 | } | |
840 | ||
841 | sub getone { | |
842 | my $key = ''; | |
843 | cbreak(); | |
844 | sysread(STDIN, $key, 1); | |
845 | cooked(); | |
846 | return $key; | |
847 | } | |
848 | ||
849 | } | |
850 | ||
851 | END { cooked() } | |
852 | ||
a6dd486b | 853 | The Term::ReadKey module from CPAN may be easier to use. Recent versions |
65acb1b1 | 854 | include also support for non-portable systems as well. |
68dc0745 | 855 | |
856 | use Term::ReadKey; | |
857 | open(TTY, "</dev/tty"); | |
858 | print "Gimme a char: "; | |
859 | ReadMode "raw"; | |
860 | $key = ReadKey 0, *TTY; | |
861 | ReadMode "normal"; | |
862 | printf "\nYou said %s, char number %03d\n", | |
863 | $key, ord $key; | |
864 | ||
65acb1b1 | 865 | =head2 How can I tell whether there's a character waiting on a filehandle? |
68dc0745 | 866 | |
5a964f20 | 867 | The very first thing you should do is look into getting the Term::ReadKey |
65acb1b1 TC |
868 | extension from CPAN. As we mentioned earlier, it now even has limited |
869 | support for non-portable (read: not open systems, closed, proprietary, | |
870 | not POSIX, not Unix, etc) systems. | |
5a964f20 TC |
871 | |
872 | You should also check out the Frequently Asked Questions list in | |
68dc0745 | 873 | comp.unix.* for things like this: the answer is essentially the same. |
874 | It's very system dependent. Here's one solution that works on BSD | |
875 | systems: | |
876 | ||
877 | sub key_ready { | |
878 | my($rin, $nfd); | |
879 | vec($rin, fileno(STDIN), 1) = 1; | |
880 | return $nfd = select($rin,undef,undef,0); | |
881 | } | |
882 | ||
65acb1b1 TC |
883 | If you want to find out how many characters are waiting, there's |
884 | also the FIONREAD ioctl call to be looked at. The I<h2ph> tool that | |
885 | comes with Perl tries to convert C include files to Perl code, which | |
886 | can be C<require>d. FIONREAD ends up defined as a function in the | |
887 | I<sys/ioctl.ph> file: | |
68dc0745 | 888 | |
5a964f20 | 889 | require 'sys/ioctl.ph'; |
68dc0745 | 890 | |
5a964f20 TC |
891 | $size = pack("L", 0); |
892 | ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n"; | |
893 | $size = unpack("L", $size); | |
68dc0745 | 894 | |
5a964f20 TC |
895 | If I<h2ph> wasn't installed or doesn't work for you, you can |
896 | I<grep> the include files by hand: | |
68dc0745 | 897 | |
5a964f20 TC |
898 | % grep FIONREAD /usr/include/*/* |
899 | /usr/include/asm/ioctls.h:#define FIONREAD 0x541B | |
68dc0745 | 900 | |
5a964f20 | 901 | Or write a small C program using the editor of champions: |
68dc0745 | 902 | |
5a964f20 TC |
903 | % cat > fionread.c |
904 | #include <sys/ioctl.h> | |
905 | main() { | |
906 | printf("%#08x\n", FIONREAD); | |
907 | } | |
908 | ^D | |
65acb1b1 | 909 | % cc -o fionread fionread.c |
5a964f20 TC |
910 | % ./fionread |
911 | 0x4004667f | |
912 | ||
8305e449 | 913 | And then hard code it, leaving porting as an exercise to your successor. |
5a964f20 TC |
914 | |
915 | $FIONREAD = 0x4004667f; # XXX: opsys dependent | |
916 | ||
917 | $size = pack("L", 0); | |
918 | ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n"; | |
919 | $size = unpack("L", $size); | |
920 | ||
a6dd486b | 921 | FIONREAD requires a filehandle connected to a stream, meaning that sockets, |
5a964f20 | 922 | pipes, and tty devices work, but I<not> files. |
68dc0745 | 923 | |
924 | =head2 How do I do a C<tail -f> in perl? | |
925 | ||
926 | First try | |
927 | ||
928 | seek(GWFILE, 0, 1); | |
929 | ||
930 | The statement C<seek(GWFILE, 0, 1)> doesn't change the current position, | |
931 | but it does clear the end-of-file condition on the handle, so that the | |
932 | next <GWFILE> makes Perl try again to read something. | |
933 | ||
934 | If that doesn't work (it relies on features of your stdio implementation), | |
935 | then you need something more like this: | |
936 | ||
937 | for (;;) { | |
938 | for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) { | |
939 | # search for some stuff and put it into files | |
940 | } | |
941 | # sleep for a while | |
942 | seek(GWFILE, $curpos, 0); # seek to where we had been | |
943 | } | |
944 | ||
945 | If this still doesn't work, look into the POSIX module. POSIX defines | |
946 | the clearerr() method, which can remove the end of file condition on a | |
947 | filehandle. The method: read until end of file, clearerr(), read some | |
948 | more. Lather, rinse, repeat. | |
949 | ||
65acb1b1 TC |
950 | There's also a File::Tail module from CPAN. |
951 | ||
68dc0745 | 952 | =head2 How do I dup() a filehandle in Perl? |
953 | ||
954 | If you check L<perlfunc/open>, you'll see that several of the ways | |
955 | to call open() should do the trick. For example: | |
956 | ||
2359510d | 957 | open(LOG, ">>/foo/logfile"); |
68dc0745 | 958 | open(STDERR, ">&LOG"); |
959 | ||
960 | Or even with a literal numeric descriptor: | |
961 | ||
962 | $fd = $ENV{MHCONTEXTFD}; | |
963 | open(MHCONTEXT, "<&=$fd"); # like fdopen(3S) | |
964 | ||
c47ff5f1 | 965 | Note that "<&STDIN" makes a copy, but "<&=STDIN" make |
5a964f20 | 966 | an alias. That means if you close an aliased handle, all |
197aec24 | 967 | aliases become inaccessible. This is not true with |
5a964f20 TC |
968 | a copied one. |
969 | ||
970 | Error checking, as always, has been left as an exercise for the reader. | |
68dc0745 | 971 | |
972 | =head2 How do I close a file descriptor by number? | |
973 | ||
974 | This should rarely be necessary, as the Perl close() function is to be | |
975 | used for things that Perl opened itself, even if it was a dup of a | |
a6dd486b | 976 | numeric descriptor as with MHCONTEXT above. But if you really have |
68dc0745 | 977 | to, you may be able to do this: |
978 | ||
979 | require 'sys/syscall.ph'; | |
980 | $rc = syscall(&SYS_close, $fd + 0); # must force numeric | |
981 | die "can't sysclose $fd: $!" unless $rc == -1; | |
982 | ||
a6dd486b | 983 | Or, just use the fdopen(3S) feature of open(): |
d92eb7b0 | 984 | |
197aec24 RGS |
985 | { |
986 | local *F; | |
d92eb7b0 GS |
987 | open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!"; |
988 | close F; | |
989 | } | |
990 | ||
883f1635 | 991 | =head2 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work? |
68dc0745 | 992 | |
993 | Whoops! You just put a tab and a formfeed into that filename! | |
994 | Remember that within double quoted strings ("like\this"), the | |
995 | backslash is an escape character. The full list of these is in | |
996 | L<perlop/Quote and Quote-like Operators>. Unsurprisingly, you don't | |
997 | have a file called "c:(tab)emp(formfeed)oo" or | |
65acb1b1 | 998 | "c:(tab)emp(formfeed)oo.exe" on your legacy DOS filesystem. |
68dc0745 | 999 | |
1000 | Either single-quote your strings, or (preferably) use forward slashes. | |
46fc3d4c | 1001 | Since all DOS and Windows versions since something like MS-DOS 2.0 or so |
68dc0745 | 1002 | have treated C</> and C<\> the same in a path, you might as well use the |
a6dd486b | 1003 | one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++, |
65acb1b1 TC |
1004 | awk, Tcl, Java, or Python, just to mention a few. POSIX paths |
1005 | are more portable, too. | |
68dc0745 | 1006 | |
1007 | =head2 Why doesn't glob("*.*") get all the files? | |
1008 | ||
1009 | Because even on non-Unix ports, Perl's glob function follows standard | |
46fc3d4c | 1010 | Unix globbing semantics. You'll need C<glob("*")> to get all (non-hidden) |
65acb1b1 TC |
1011 | files. This makes glob() portable even to legacy systems. Your |
1012 | port may include proprietary globbing functions as well. Check its | |
1013 | documentation for details. | |
68dc0745 | 1014 | |
1015 | =head2 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl? | |
1016 | ||
06a5f41f JH |
1017 | This is elaborately and painstakingly described in the |
1018 | F<file-dir-perms> article in the "Far More Than You Ever Wanted To | |
49d635f9 | 1019 | Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz . |
68dc0745 | 1020 | |
1021 | The executive summary: learn how your filesystem works. The | |
1022 | permissions on a file say what can happen to the data in that file. | |
1023 | The permissions on a directory say what can happen to the list of | |
1024 | files in that directory. If you delete a file, you're removing its | |
1025 | name from the directory (so the operation depends on the permissions | |
1026 | of the directory, not of the file). If you try to write to the file, | |
1027 | the permissions of the file govern whether you're allowed to. | |
1028 | ||
1029 | =head2 How do I select a random line from a file? | |
1030 | ||
1031 | Here's an algorithm from the Camel Book: | |
1032 | ||
1033 | srand; | |
1034 | rand($.) < 1 && ($line = $_) while <>; | |
1035 | ||
49d635f9 RGS |
1036 | This has a significant advantage in space over reading the whole file |
1037 | in. You can find a proof of this method in I<The Art of Computer | |
1038 | Programming>, Volume 2, Section 3.4.2, by Donald E. Knuth. | |
1039 | ||
1040 | You can use the File::Random module which provides a function | |
1041 | for that algorithm: | |
1042 | ||
1043 | use File::Random qw/random_line/; | |
1044 | my $line = random_line($filename); | |
1045 | ||
1046 | Another way is to use the Tie::File module, which treats the entire | |
1047 | file as an array. Simply access a random array element. | |
68dc0745 | 1048 | |
65acb1b1 TC |
1049 | =head2 Why do I get weird spaces when I print an array of lines? |
1050 | ||
1051 | Saying | |
1052 | ||
1053 | print "@lines\n"; | |
1054 | ||
1055 | joins together the elements of C<@lines> with a space between them. | |
1056 | If C<@lines> were C<("little", "fluffy", "clouds")> then the above | |
a6dd486b | 1057 | statement would print |
65acb1b1 TC |
1058 | |
1059 | little fluffy clouds | |
1060 | ||
1061 | but if each element of C<@lines> was a line of text, ending a newline | |
1062 | character C<("little\n", "fluffy\n", "clouds\n")> then it would print: | |
1063 | ||
1064 | little | |
1065 | fluffy | |
1066 | clouds | |
1067 | ||
1068 | If your array contains lines, just print them: | |
1069 | ||
1070 | print @lines; | |
1071 | ||
68dc0745 | 1072 | =head1 AUTHOR AND COPYRIGHT |
1073 | ||
7678cced RGS |
1074 | Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and |
1075 | other authors as noted. All rights reserved. | |
5a964f20 | 1076 | |
5a7beb56 JH |
1077 | This documentation is free; you can redistribute it and/or modify it |
1078 | under the same terms as Perl itself. | |
c8db1d39 | 1079 | |
87275199 | 1080 | Irrespective of its distribution, all code examples here are in the public |
c8db1d39 TC |
1081 | domain. You are permitted and encouraged to use this code and any |
1082 | derivatives thereof in your own programs for fun or for profit as you | |
1083 | see fit. A simple comment in the code giving credit to the FAQ would | |
1084 | be courteous but is not required. |