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1 | =head1 NAME |
2 | |
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3 | perlfaq3 - Programming Tools ($Revision: 1.22 $, $Date: 1997/04/24 22:43:42 $) |
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4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | This section of the FAQ answers questions related to programmer tools |
8 | and programming support. |
9 | |
10 | =head2 How do I do (anything)? |
11 | |
12 | Have you looked at CPAN (see L<perlfaq2>)? The chances are that |
13 | someone has already written a module that can solve your problem. |
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14 | Have you read the appropriate man pages? Here's a brief index: |
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15 | |
16 | Objects perlref, perlmod, perlobj, perltie |
17 | Data Structures perlref, perllol, perldsc |
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18 | Modules perlmod, perlmodlib, perlsub |
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19 | Regexps perlre, perlfunc, perlop |
20 | Moving to perl5 perltrap, perl |
21 | Linking w/C perlxstut, perlxs, perlcall, perlguts, perlembed |
22 | Various http://www.perl.com/CPAN/doc/FMTEYEWTK/index.html |
23 | (not a man-page but still useful) |
24 | |
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25 | L<perltoc> provides a crude table of contents for the perl man page set. |
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26 | |
27 | =head2 How can I use Perl interactively? |
28 | |
29 | The typical approach uses the Perl debugger, described in the |
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30 | perldebug(1) man page, on an "empty" program, like this: |
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31 | |
32 | perl -de 42 |
33 | |
34 | Now just type in any legal Perl code, and it will be immediately |
35 | evaluated. You can also examine the symbol table, get stack |
36 | backtraces, check variable values, set breakpoints, and other |
37 | operations typically found in symbolic debuggers |
38 | |
39 | =head2 Is there a Perl shell? |
40 | |
41 | In general, no. The Shell.pm module (distributed with perl) makes |
42 | perl try commands which aren't part of the Perl language as shell |
43 | commands. perlsh from the source distribution is simplistic and |
44 | uninteresting, but may still be what you want. |
45 | |
46 | =head2 How do I debug my Perl programs? |
47 | |
48 | Have you used C<-w>? |
49 | |
50 | Have you tried C<use strict>? |
51 | |
52 | Did you check the returns of each and every system call? |
53 | |
54 | Did you read L<perltrap>? |
55 | |
56 | Have you tried the Perl debugger, described in L<perldebug>? |
57 | |
58 | =head2 How do I profile my Perl programs? |
59 | |
60 | You should get the Devel::DProf module from CPAN, and also use |
61 | Benchmark.pm from the standard distribution. Benchmark lets you time |
62 | specific portions of your code, while Devel::DProf gives detailed |
63 | breakdowns of where your code spends its time. |
64 | |
65 | =head2 How do I cross-reference my Perl programs? |
66 | |
67 | The B::Xref module, shipped with the new, alpha-release Perl compiler |
68 | (not the general distribution), can be used to generate |
69 | cross-reference reports for Perl programs. |
70 | |
71 | perl -MO=Xref[,OPTIONS] foo.pl |
72 | |
73 | =head2 Is there a pretty-printer (formatter) for Perl? |
74 | |
75 | There is no program that will reformat Perl as much as indent(1) will |
76 | do for C. The complex feedback between the scanner and the parser |
77 | (this feedback is what confuses the vgrind and emacs programs) makes it |
78 | challenging at best to write a stand-alone Perl parser. |
79 | |
80 | Of course, if you simply follow the guidelines in L<perlstyle>, you |
81 | shouldn't need to reformat. |
82 | |
83 | Your editor can and should help you with source formatting. The |
84 | perl-mode for emacs can provide a remarkable amount of help with most |
85 | (but not all) code, and even less programmable editors can provide |
86 | significant assistance. |
87 | |
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88 | If you are used to using vgrind program for printing out nice code to |
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89 | a laser printer, you can take a stab at this using |
90 | http://www.perl.com/CPAN/doc/misc/tips/working.vgrind.entry, but the |
91 | results are not particularly satisfying for sophisticated code. |
92 | |
93 | =head2 Is there a ctags for Perl? |
94 | |
95 | There's a simple one at |
96 | http://www.perl.com/CPAN/authors/id/TOMC/scripts/ptags.gz which may do |
97 | the trick. |
98 | |
99 | =head2 Where can I get Perl macros for vi? |
100 | |
101 | For a complete version of Tom Christiansen's vi configuration file, |
102 | see ftp://ftp.perl.com/pub/vi/toms.exrc, the standard benchmark file |
103 | for vi emulators. This runs best with nvi, the current version of vi |
104 | out of Berkeley, which incidentally can be built with an embedded Perl |
105 | interpreter -- see http://www.perl.com/CPAN/src/misc . |
106 | |
107 | =head2 Where can I get perl-mode for emacs? |
108 | |
109 | Since Emacs version 19 patchlevel 22 or so, there have been both a |
110 | perl-mode.el and support for the perl debugger built in. These should |
111 | come with the standard Emacs 19 distribution. |
112 | |
113 | In the perl source directory, you'll find a directory called "emacs", |
114 | which contains a cperl-mode that color-codes keywords, provides |
115 | context-sensitive help, and other nifty things. |
116 | |
117 | Note that the perl-mode of emacs will have fits with "main'foo" |
118 | (single quote), and mess up the indentation and hilighting. You |
119 | should be using "main::foo", anyway. |
120 | |
121 | =head2 How can I use curses with Perl? |
122 | |
123 | The Curses module from CPAN provides a dynamically loadable object |
124 | module interface to a curses library. |
125 | |
126 | =head2 How can I use X or Tk with Perl? |
127 | |
128 | Tk is a completely Perl-based, object-oriented interface to the Tk |
129 | toolkit that doesn't force you to use Tcl just to get at Tk. Sx is an |
130 | interface to the Athena Widget set. Both are available from CPAN. |
131 | |
132 | =head2 How can I generate simple menus without using CGI or Tk? |
133 | |
134 | The http://www.perl.com/CPAN/authors/id/SKUNZ/perlmenu.v4.0.tar.gz |
135 | module, which is curses-based, can help with this. |
136 | |
137 | =head2 Can I dynamically load C routines into Perl? |
138 | |
139 | If your system architecture supports it, then the standard perl |
140 | on your system should also provide you with this via the |
141 | DynaLoader module. Read L<perlxstut> for details. |
142 | |
143 | =head2 What is undump? |
144 | |
145 | See the next questions. |
146 | |
147 | =head2 How can I make my Perl program run faster? |
148 | |
149 | The best way to do this is to come up with a better algorithm. |
150 | This can often make a dramatic difference. Chapter 8 in the Camel |
151 | has some efficiency tips in it you might want to look at. |
152 | |
153 | Other approaches include autoloading seldom-used Perl code. See the |
154 | AutoSplit and AutoLoader modules in the standard distribution for |
155 | that. Or you could locate the bottleneck and think about writing just |
156 | that part in C, the way we used to take bottlenecks in C code and |
157 | write them in assembler. Similar to rewriting in C is the use of |
158 | modules that have critical sections written in C (for instance, the |
159 | PDL module from CPAN). |
160 | |
161 | In some cases, it may be worth it to use the backend compiler to |
162 | produce byte code (saving compilation time) or compile into C, which |
163 | will certainly save compilation time and sometimes a small amount (but |
164 | not much) execution time. See the question about compiling your Perl |
165 | programs. |
166 | |
167 | If you're currently linking your perl executable to a shared libc.so, |
168 | you can often gain a 10-25% performance benefit by rebuilding it to |
169 | link with a static libc.a instead. This will make a bigger perl |
170 | executable, but your Perl programs (and programmers) may thank you for |
171 | it. See the F<INSTALL> file in the source distribution for more |
172 | information. |
173 | |
174 | Unsubstantiated reports allege that Perl interpreters that use sfio |
175 | outperform those that don't (for IO intensive applications). To try |
176 | this, see the F<INSTALL> file in the source distribution, especially |
177 | the "Selecting File IO mechanisms" section. |
178 | |
179 | The undump program was an old attempt to speed up your Perl program |
180 | by storing the already-compiled form to disk. This is no longer |
181 | a viable option, as it only worked on a few architectures, and |
182 | wasn't a good solution anyway. |
183 | |
184 | =head2 How can I make my Perl program take less memory? |
185 | |
186 | When it comes to time-space tradeoffs, Perl nearly always prefers to |
187 | throw memory at a problem. Scalars in Perl use more memory than |
188 | strings in C, arrays take more that, and hashes use even more. While |
189 | there's still a lot to be done, recent releases have been addressing |
190 | these issues. For example, as of 5.004, duplicate hash keys are |
191 | shared amongst all hashes using them, so require no reallocation. |
192 | |
193 | In some cases, using substr() or vec() to simulate arrays can be |
194 | highly beneficial. For example, an array of a thousand booleans will |
195 | take at least 20,000 bytes of space, but it can be turned into one |
196 | 125-byte bit vector for a considerable memory savings. The standard |
197 | Tie::SubstrHash module can also help for certain types of data |
198 | structure. If you're working with specialist data structures |
199 | (matrices, for instance) modules that implement these in C may use |
200 | less memory than equivalent Perl modules. |
201 | |
202 | Another thing to try is learning whether your Perl was compiled with |
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203 | the system malloc or with Perl's builtin malloc. Whichever one it |
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204 | is, try using the other one and see whether this makes a difference. |
205 | Information about malloc is in the F<INSTALL> file in the source |
206 | distribution. You can find out whether you are using perl's malloc by |
207 | typing C<perl -V:usemymalloc>. |
208 | |
209 | =head2 Is it unsafe to return a pointer to local data? |
210 | |
211 | No, Perl's garbage collection system takes care of this. |
212 | |
213 | sub makeone { |
214 | my @a = ( 1 .. 10 ); |
215 | return \@a; |
216 | } |
217 | |
218 | for $i ( 1 .. 10 ) { |
219 | push @many, makeone(); |
220 | } |
221 | |
222 | print $many[4][5], "\n"; |
223 | |
224 | print "@many\n"; |
225 | |
226 | =head2 How can I free an array or hash so my program shrinks? |
227 | |
228 | You can't. Memory the system allocates to a program will never be |
229 | returned to the system. That's why long-running programs sometimes |
230 | re-exec themselves. |
231 | |
232 | However, judicious use of my() on your variables will help make sure |
233 | that they go out of scope so that Perl can free up their storage for |
234 | use in other parts of your program. (NB: my() variables also execute |
235 | about 10% faster than globals.) A global variable, of course, never |
236 | goes out of scope, so you can't get its space automatically reclaimed, |
237 | although undef()ing and/or delete()ing it will achieve the same effect. |
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238 | In general, memory allocation and de-allocation isn't something you can |
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239 | or should be worrying about much in Perl, but even this capability |
240 | (preallocation of data types) is in the works. |
241 | |
242 | =head2 How can I make my CGI script more efficient? |
243 | |
244 | Beyond the normal measures described to make general Perl programs |
245 | faster or smaller, a CGI program has additional issues. It may be run |
246 | several times per second. Given that each time it runs it will need |
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247 | to be re-compiled and will often allocate a megabyte or more of system |
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248 | memory, this can be a killer. Compiling into C B<isn't going to help |
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249 | you> because the process start-up overhead is where the bottleneck is. |
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250 | |
251 | There are at least two popular ways to avoid this overhead. One |
252 | solution involves running the Apache HTTP server (available from |
253 | http://www.apache.org/) with either of the mod_perl or mod_fastcgi |
254 | plugin modules. With mod_perl and the Apache::* modules (from CPAN), |
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255 | httpd will run with an embedded Perl interpreter which pre-compiles |
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256 | your script and then executes it within the same address space without |
257 | forking. The Apache extension also gives Perl access to the internal |
258 | server API, so modules written in Perl can do just about anything a |
259 | module written in C can. With the FCGI module (from CPAN), a Perl |
260 | executable compiled with sfio (see the F<INSTALL> file in the |
261 | distribution) and the mod_fastcgi module (available from |
262 | http://www.fastcgi.com/) each of your perl scripts becomes a permanent |
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263 | CGI daemon process. |
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264 | |
265 | Both of these solutions can have far-reaching effects on your system |
266 | and on the way you write your CGI scripts, so investigate them with |
267 | care. |
268 | |
269 | =head2 How can I hide the source for my Perl program? |
270 | |
271 | Delete it. :-) Seriously, there are a number of (mostly |
272 | unsatisfactory) solutions with varying levels of "security". |
273 | |
274 | First of all, however, you I<can't> take away read permission, because |
275 | the source code has to be readable in order to be compiled and |
276 | interpreted. (That doesn't mean that a CGI script's source is |
277 | readable by people on the web, though.) So you have to leave the |
278 | permissions at the socially friendly 0755 level. |
279 | |
280 | Some people regard this as a security problem. If your program does |
281 | insecure things, and relies on people not knowing how to exploit those |
282 | insecurities, it is not secure. It is often possible for someone to |
283 | determine the insecure things and exploit them without viewing the |
284 | source. Security through obscurity, the name for hiding your bugs |
285 | instead of fixing them, is little security indeed. |
286 | |
287 | You can try using encryption via source filters (Filter::* from CPAN). |
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288 | But crackers might be able to decrypt it. You can try using the byte |
289 | code compiler and interpreter described below, but crackers might be |
290 | able to de-compile it. You can try using the native-code compiler |
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291 | described below, but crackers might be able to disassemble it. These |
292 | pose varying degrees of difficulty to people wanting to get at your |
293 | code, but none can definitively conceal it (this is true of every |
294 | language, not just Perl). |
295 | |
296 | If you're concerned about people profiting from your code, then the |
297 | bottom line is that nothing but a restrictive licence will give you |
298 | legal security. License your software and pepper it with threatening |
299 | statements like "This is unpublished proprietary software of XYZ Corp. |
300 | Your access to it does not give you permission to use it blah blah |
301 | blah." We are not lawyers, of course, so you should see a lawyer if |
302 | you want to be sure your licence's wording will stand up in court. |
303 | |
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304 | =head2 How can I compile my Perl program into byte code or C? |
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305 | |
306 | Malcolm Beattie has written a multifunction backend compiler, |
307 | available from CPAN, that can do both these things. It is as of |
308 | Feb-1997 in late alpha release, which means it's fun to play with if |
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309 | you're a programmer but not really for people looking for turn-key |
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310 | solutions. |
311 | |
312 | I<Please> understand that merely compiling into C does not in and of |
313 | itself guarantee that your code will run very much faster. That's |
314 | because except for lucky cases where a lot of native type inferencing |
315 | is possible, the normal Perl run time system is still present and thus |
316 | will still take just as long to run and be just as big. Most programs |
317 | save little more than compilation time, leaving execution no more than |
318 | 10-30% faster. A few rare programs actually benefit significantly |
319 | (like several times faster), but this takes some tweaking of your |
320 | code. |
321 | |
322 | Malcolm will be in charge of the 5.005 release of Perl itself |
323 | to try to unify and merge his compiler and multithreading work into |
324 | the main release. |
325 | |
326 | You'll probably be astonished to learn that the current version of the |
327 | compiler generates a compiled form of your script whose executable is |
328 | just as big as the original perl executable, and then some. That's |
329 | because as currently written, all programs are prepared for a full |
330 | eval() statement. You can tremendously reduce this cost by building a |
331 | shared libperl.so library and linking against that. See the |
332 | F<INSTALL> podfile in the perl source distribution for details. If |
333 | you link your main perl binary with this, it will make it miniscule. |
334 | For example, on one author's system, /usr/bin/perl is only 11k in |
335 | size! |
336 | |
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337 | =head2 How can I get '#!perl' to work on [MS-DOS,NT,...]? |
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338 | |
339 | For OS/2 just use |
340 | |
341 | extproc perl -S -your_switches |
342 | |
343 | as the first line in C<*.cmd> file (C<-S> due to a bug in cmd.exe's |
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344 | `extproc' handling). For DOS one should first invent a corresponding |
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345 | batch file, and codify it in C<ALTERNATIVE_SHEBANG> (see the |
346 | F<INSTALL> file in the source distribution for more information). |
347 | |
348 | The Win95/NT installation, when using the Activeware port of Perl, |
349 | will modify the Registry to associate the .pl extension with the perl |
350 | interpreter. If you install another port, or (eventually) build your |
351 | own Win95/NT Perl using WinGCC, then you'll have to modify the |
352 | Registry yourself. |
353 | |
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354 | Macintosh perl scripts will have the the appropriate Creator and |
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355 | Type, so that double-clicking them will invoke the perl application. |
356 | |
357 | I<IMPORTANT!>: Whatever you do, PLEASE don't get frustrated, and just |
358 | throw the perl interpreter into your cgi-bin directory, in order to |
359 | get your scripts working for a web server. This is an EXTREMELY big |
360 | security risk. Take the time to figure out how to do it correctly. |
361 | |
362 | =head2 Can I write useful perl programs on the command line? |
363 | |
364 | Yes. Read L<perlrun> for more information. Some examples follow. |
365 | (These assume standard Unix shell quoting rules.) |
366 | |
367 | # sum first and last fields |
368 | perl -lane 'print $F[0] + $F[-1]' |
369 | |
370 | # identify text files |
371 | perl -le 'for(@ARGV) {print if -f && -T _}' * |
372 | |
373 | # remove comments from C program |
374 | perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c |
375 | |
376 | # make file a month younger than today, defeating reaper daemons |
377 | perl -e '$X=24*60*60; utime(time(),time() + 30 * $X,@ARGV)' * |
378 | |
379 | # find first unused uid |
380 | perl -le '$i++ while getpwuid($i); print $i' |
381 | |
382 | # display reasonable manpath |
383 | echo $PATH | perl -nl -072 -e ' |
384 | s![^/+]*$!man!&&-d&&!$s{$_}++&&push@m,$_;END{print"@m"}' |
385 | |
386 | Ok, the last one was actually an obfuscated perl entry. :-) |
387 | |
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388 | =head2 Why don't perl one-liners work on my DOS/Mac/VMS system? |
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389 | |
390 | The problem is usually that the command interpreters on those systems |
391 | have rather different ideas about quoting than the Unix shells under |
392 | which the one-liners were created. On some systems, you may have to |
393 | change single-quotes to double ones, which you must I<NOT> do on Unix |
394 | or Plan9 systems. You might also have to change a single % to a %%. |
395 | |
396 | For example: |
397 | |
398 | # Unix |
399 | perl -e 'print "Hello world\n"' |
400 | |
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401 | # DOS, etc. |
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402 | perl -e "print \"Hello world\n\"" |
403 | |
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404 | # Mac |
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405 | print "Hello world\n" |
406 | (then Run "Myscript" or Shift-Command-R) |
407 | |
408 | # VMS |
409 | perl -e "print ""Hello world\n""" |
410 | |
411 | The problem is that none of this is reliable: it depends on the command |
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412 | interpreter. Under Unix, the first two often work. Under DOS, it's |
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413 | entirely possible neither works. If 4DOS was the command shell, I'd |
414 | probably have better luck like this: |
415 | |
416 | perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>"" |
417 | |
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418 | Under the Mac, it depends which environment you are using. The MacPerl |
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419 | shell, or MPW, is much like Unix shells in its support for several |
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420 | quoting variants, except that it makes free use of the Mac's non-ASCII |
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421 | characters as control characters. |
422 | |
423 | I'm afraid that there is no general solution to all of this. It is a |
424 | mess, pure and simple. |
425 | |
426 | [Some of this answer was contributed by Kenneth Albanowski.] |
427 | |
428 | =head2 Where can I learn about CGI or Web programming in Perl? |
429 | |
430 | For modules, get the CGI or LWP modules from CPAN. For textbooks, |
431 | see the two especially dedicated to web stuff in the question on |
432 | books. For problems and questions related to the web, like "Why |
433 | do I get 500 Errors" or "Why doesn't it run from the browser right |
434 | when it runs fine on the command line", see these sources: |
435 | |
436 | The Idiot's Guide to Solving Perl/CGI Problems, by Tom Christiansen |
437 | http://www.perl.com/perl/faq/idiots-guide.html |
438 | |
439 | Frequently Asked Questions about CGI Programming, by Nick Kew |
440 | ftp://rtfm.mit.edu/pub/usenet/news.answers/www/cgi-faq |
441 | http://www3.pair.com/webthing/docs/cgi/faqs/cgifaq.shtml |
442 | |
443 | Perl/CGI programming FAQ, by Shishir Gundavaram and Tom Christiansen |
444 | http://www.perl.com/perl/faq/perl-cgi-faq.html |
445 | |
446 | The WWW Security FAQ, by Lincoln Stein |
447 | http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html |
448 | |
449 | World Wide Web FAQ, by Thomas Boutell |
450 | http://www.boutell.com/faq/ |
451 | |
452 | =head2 Where can I learn about object-oriented Perl programming? |
453 | |
454 | L<perltoot> is a good place to start, and you can use L<perlobj> and |
455 | L<perlbot> for reference. Perltoot didn't come out until the 5.004 |
456 | release, but you can get a copy (in pod, html, or postscript) from |
457 | http://www.perl.com/CPAN/doc/FMTEYEWTK/ . |
458 | |
459 | =head2 Where can I learn about linking C with Perl? [h2xs, xsubpp] |
460 | |
461 | If you want to call C from Perl, start with L<perlxstut>, |
462 | moving on to L<perlxs>, L<xsubpp>, and L<perlguts>. If you want to |
463 | call Perl from C, then read L<perlembed>, L<perlcall>, and |
464 | L<perlguts>. Don't forget that you can learn a lot from looking at |
465 | how the authors of existing extension modules wrote their code and |
466 | solved their problems. |
467 | |
468 | =head2 I've read perlembed, perlguts, etc., but I can't embed perl in |
469 | my C program, what am I doing wrong? |
470 | |
471 | Download the ExtUtils::Embed kit from CPAN and run `make test'. If |
472 | the tests pass, read the pods again and again and again. If they |
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473 | fail, see L<perlbug> and send a bugreport with the output of |
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474 | C<make test TEST_VERBOSE=1> along with C<perl -V>. |
475 | |
476 | =head2 When I tried to run my script, I got this message. What does it |
477 | mean? |
478 | |
479 | L<perldiag> has a complete list of perl's error messages and warnings, |
480 | with explanatory text. You can also use the splain program (distributed |
481 | with perl) to explain the error messages: |
482 | |
483 | perl program 2>diag.out |
484 | splain [-v] [-p] diag.out |
485 | |
486 | or change your program to explain the messages for you: |
487 | |
488 | use diagnostics; |
489 | |
490 | or |
491 | |
492 | use diagnostics -verbose; |
493 | |
494 | =head2 What's MakeMaker? |
495 | |
496 | This module (part of the standard perl distribution) is designed to |
497 | write a Makefile for an extension module from a Makefile.PL. For more |
498 | information, see L<ExtUtils::MakeMaker>. |
499 | |
500 | =head1 AUTHOR AND COPYRIGHT |
501 | |
502 | Copyright (c) 1997 Tom Christiansen and Nathan Torkington. |
503 | All rights reserved. See L<perlfaq> for distribution information. |
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504 | |