/
perlhacktips.pod
1752 lines (1227 loc) · 57.2 KB
/
perlhacktips.pod
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
=encoding utf8
=for comment
Consistent formatting of this file is achieved with:
perl ./Porting/podtidy pod/perlhacktips.pod
=head1 NAME
perlhacktips - Tips for Perl core C code hacking
=head1 DESCRIPTION
This document will help you learn the best way to go about hacking on
the Perl core C code. It covers common problems, debugging, profiling,
and more.
If you haven't read L<perlhack> and L<perlhacktut> yet, you might want
to do that first.
=head1 COMMON PROBLEMS
Perl source plays by ANSI C89 rules: no C99 (or C++) extensions.
You don't care about some particular platform having broken Perl? I
hear there is still a strong demand for J2EE programmers.
=head2 Perl environment problems
=over 4
=item *
Not compiling with threading
Compiling with threading (-Duseithreads) completely rewrites the
function prototypes of Perl. You better try your changes with that.
Related to this is the difference between "Perl_-less" and "Perl_-ly"
APIs, for example:
Perl_sv_setiv(aTHX_ ...);
sv_setiv(...);
The first one explicitly passes in the context, which is needed for
e.g. threaded builds. The second one does that implicitly; do not get
them mixed. If you are not passing in a aTHX_, you will need to do a
dTHX as the first thing in the function.
See L<perlguts/"How multiple interpreters and concurrency are
supported"> for further discussion about context.
=item *
Not compiling with -DDEBUGGING
The DEBUGGING define exposes more code to the compiler, therefore more
ways for things to go wrong. You should try it.
=item *
Introducing (non-read-only) globals
Do not introduce any modifiable globals, truly global or file static.
They are bad form and complicate multithreading and other forms of
concurrency. The right way is to introduce them as new interpreter
variables, see F<intrpvar.h> (at the very end for binary
compatibility).
Introducing read-only (const) globals is okay, as long as you verify
with e.g. C<nm libperl.a|egrep -v ' [TURtr] '> (if your C<nm> has
BSD-style output) that the data you added really is read-only. (If it
is, it shouldn't show up in the output of that command.)
If you want to have static strings, make them constant:
static const char etc[] = "...";
If you want to have arrays of constant strings, note carefully the
right combination of C<const>s:
static const char * const yippee[] =
{"hi", "ho", "silver"};
=item *
Not exporting your new function
Some platforms (Win32, AIX, VMS, OS/2, to name a few) require any
function that is part of the public API (the shared Perl library) to be
explicitly marked as exported. See the discussion about F<embed.pl> in
L<perlguts>.
=item *
Exporting your new function
The new shiny result of either genuine new functionality or your
arduous refactoring is now ready and correctly exported. So what could
possibly go wrong?
Maybe simply that your function did not need to be exported in the
first place. Perl has a long and not so glorious history of exporting
functions that it should not have.
If the function is used only inside one source code file, make it
static. See the discussion about F<embed.pl> in L<perlguts>.
If the function is used across several files, but intended only for
Perl's internal use (and this should be the common case), do not export
it to the public API. See the discussion about F<embed.pl> in
L<perlguts>.
=back
=head2 Portability problems
The following are common causes of compilation and/or execution
failures, not common to Perl as such. The C FAQ is good bedtime
reading. Please test your changes with as many C compilers and
platforms as possible; we will, anyway, and it's nice to save oneself
from public embarrassment.
If using gcc, you can add the C<-std=c89> option which will hopefully
catch most of these unportabilities. (However it might also catch
incompatibilities in your system's header files.)
Use the Configure C<-Dgccansipedantic> flag to enable the gcc C<-ansi
-pedantic> flags which enforce stricter ANSI rules.
If using the C<gcc -Wall> note that not all the possible warnings (like
C<-Wuninitialized>) are given unless you also compile with C<-O>.
Note that if using gcc, starting from Perl 5.9.5 the Perl core source
code files (the ones at the top level of the source code distribution,
but not e.g. the extensions under ext/) are automatically compiled with
as many as possible of the C<-std=c89>, C<-ansi>, C<-pedantic>, and a
selection of C<-W> flags (see cflags.SH).
Also study L<perlport> carefully to avoid any bad assumptions about the
operating system, filesystems, character set, and so forth.
You may once in a while try a "make microperl" to see whether we can
still compile Perl with just the bare minimum of interfaces. (See
README.micro.)
Do not assume an operating system indicates a certain compiler.
=over 4
=item *
Casting pointers to integers or casting integers to pointers
void castaway(U8* p)
{
IV i = p;
or
void castaway(U8* p)
{
IV i = (IV)p;
Both are bad, and broken, and unportable. Use the PTR2IV() macro that
does it right. (Likewise, there are PTR2UV(), PTR2NV(), INT2PTR(), and
NUM2PTR().)
=item *
Casting between function pointers and data pointers
Technically speaking casting between function pointers and data
pointers is unportable and undefined, but practically speaking it seems
to work, but you should use the FPTR2DPTR() and DPTR2FPTR() macros.
Sometimes you can also play games with unions.
=item *
Assuming sizeof(int) == sizeof(long)
There are platforms where longs are 64 bits, and platforms where ints
are 64 bits, and while we are out to shock you, even platforms where
shorts are 64 bits. This is all legal according to the C standard. (In
other words, "long long" is not a portable way to specify 64 bits, and
"long long" is not even guaranteed to be any wider than "long".)
Instead, use the definitions IV, UV, IVSIZE, I32SIZE, and so forth.
Avoid things like I32 because they are B<not> guaranteed to be
I<exactly> 32 bits, they are I<at least> 32 bits, nor are they
guaranteed to be B<int> or B<long>. If you really explicitly need
64-bit variables, use I64 and U64, but only if guarded by HAS_QUAD.
=item *
Assuming one can dereference any type of pointer for any type of data
char *p = ...;
long pony = *(long *)p; /* BAD */
Many platforms, quite rightly so, will give you a core dump instead of
a pony if the p happens not to be correctly aligned.
=item *
Lvalue casts
(int)*p = ...; /* BAD */
Simply not portable. Get your lvalue to be of the right type, or maybe
use temporary variables, or dirty tricks with unions.
=item *
Assume B<anything> about structs (especially the ones you don't
control, like the ones coming from the system headers)
=over 8
=item *
That a certain field exists in a struct
=item *
That no other fields exist besides the ones you know of
=item *
That a field is of certain signedness, sizeof, or type
=item *
That the fields are in a certain order
=over 8
=item *
While C guarantees the ordering specified in the struct definition,
between different platforms the definitions might differ
=back
=item *
That the sizeof(struct) or the alignments are the same everywhere
=over 8
=item *
There might be padding bytes between the fields to align the fields -
the bytes can be anything
=item *
Structs are required to be aligned to the maximum alignment required by
the fields - which for native types is for usually equivalent to
sizeof() of the field
=back
=back
=item *
Assuming the character set is ASCIIish
Perl can compile and run under EBCDIC platforms. See L<perlebcdic>.
This is transparent for the most part, but because the character sets
differ, you shouldn't use numeric (decimal, octal, nor hex) constants
to refer to characters. You can safely say C<'A'>, but not C<0x41>.
You can safely say C<'\n'>, but not C<\012>. However, you can use
macros defined in F<utf8.h> to specify any code point portably.
C<LATIN1_TO_NATIVE(0xDF)> is going to be the code point that means
LATIN SMALL LETTER SHARP S on whatever platform you are running on (on
ASCII platforms it compiles without adding any extra code, so there is
zero performance hit on those). The acceptable inputs to
C<LATIN1_TO_NATIVE> are from C<0x00> through C<0xFF>. If your input
isn't guaranteed to be in that range, use C<UNICODE_TO_NATIVE> instead.
C<NATIVE_TO_LATIN1> and C<NATIVE_TO_UNICODE> translate the opposite
direction.
If you need the string representation of a character that doesn't have a
mnemonic name in C, you should add it to the list in
F<regen/unicode_constants.pl>, and have Perl create C<#define>'s for you,
based on the current platform.
Note that the C<isI<FOO>> and C<toI<FOO>> macros in F<handy.h> work
properly on native code points and strings.
Also, the range 'A' - 'Z' in ASCII is an unbroken sequence of 26 upper
case alphabetic characters. That is not true in EBCDIC. Nor for 'a' to
'z'. But '0' - '9' is an unbroken range in both systems. Don't assume
anything about other ranges. (Note that special handling of ranges in
regular expression patterns and transliterations makes it appear to Perl
code that the aforementioned ranges are all unbroken.)
Many of the comments in the existing code ignore the possibility of
EBCDIC, and may be wrong therefore, even if the code works. This is
actually a tribute to the successful transparent insertion of being
able to handle EBCDIC without having to change pre-existing code.
UTF-8 and UTF-EBCDIC are two different encodings used to represent
Unicode code points as sequences of bytes. Macros with the same names
(but different definitions) in F<utf8.h> and F<utfebcdic.h> are used to
allow the calling code to think that there is only one such encoding.
This is almost always referred to as C<utf8>, but it means the EBCDIC
version as well. Again, comments in the code may well be wrong even if
the code itself is right. For example, the concept of UTF-8 C<invariant
characters> differs between ASCII and EBCDIC. On ASCII platforms, only
characters that do not have the high-order bit set (i.e. whose ordinals
are strict ASCII, 0 - 127) are invariant, and the documentation and
comments in the code may assume that, often referring to something
like, say, C<hibit>. The situation differs and is not so simple on
EBCDIC machines, but as long as the code itself uses the
C<NATIVE_IS_INVARIANT()> macro appropriately, it works, even if the
comments are wrong.
As noted in L<perlhack/TESTING>, when writing test scripts, the file
F<t/charset_tools.pl> contains some helpful functions for writing tests
valid on both ASCII and EBCDIC platforms. Sometimes, though, a test
can't use a function and it's inconvenient to have different test
versions depending on the platform. There are 20 code points that are
the same in all 4 character sets currently recognized by Perl (the 3
EBCDIC code pages plus ISO 8859-1 (ASCII/Latin1)). These can be used in
such tests, though there is a small possibility that Perl will become
available in yet another character set, breaking your test. All but one
of these code points are C0 control characters. The most significant
controls that are the same are C<\0>, C<\r>, and C<\N{VT}> (also
specifiable as C<\cK>, C<\x0B>, C<\N{U+0B}>, or C<\013>). The single
non-control is U+00B6 PILCROW SIGN. The controls that are the same have
the same bit pattern in all 4 character sets, regardless of the UTF8ness
of the string containing them. The bit pattern for U+B6 is the same in
all 4 for non-UTF8 strings, but differs in each when its containing
string is UTF-8 encoded. The only other code points that have some sort
of sameness across all 4 character sets are the pair 0xDC and 0xFC.
Together these represent upper- and lowercase LATIN LETTER U WITH
DIAERESIS, but which is upper and which is lower may be reversed: 0xDC
is the capital in Latin1 and 0xFC is the small letter, while 0xFC is the
capital in EBCDIC and 0xDC is the small one. This factoid may be
exploited in writing case insensitive tests that are the same across all
4 character sets.
=item *
Assuming the character set is just ASCII
ASCII is a 7 bit encoding, but bytes have 8 bits in them. The 128 extra
characters have different meanings depending on the locale. Absent a
locale, currently these extra characters are generally considered to be
unassigned, and this has presented some problems. This has being
changed starting in 5.12 so that these characters can be considered to
be Latin-1 (ISO-8859-1).
=item *
Mixing #define and #ifdef
#define BURGLE(x) ... \
#ifdef BURGLE_OLD_STYLE /* BAD */
... do it the old way ... \
#else
... do it the new way ... \
#endif
You cannot portably "stack" cpp directives. For example in the above
you need two separate BURGLE() #defines, one for each #ifdef branch.
=item *
Adding non-comment stuff after #endif or #else
#ifdef SNOSH
...
#else !SNOSH /* BAD */
...
#endif SNOSH /* BAD */
The #endif and #else cannot portably have anything non-comment after
them. If you want to document what is going (which is a good idea
especially if the branches are long), use (C) comments:
#ifdef SNOSH
...
#else /* !SNOSH */
...
#endif /* SNOSH */
The gcc option C<-Wendif-labels> warns about the bad variant (by
default on starting from Perl 5.9.4).
=item *
Having a comma after the last element of an enum list
enum color {
CERULEAN,
CHARTREUSE,
CINNABAR, /* BAD */
};
is not portable. Leave out the last comma.
Also note that whether enums are implicitly morphable to ints varies
between compilers, you might need to (int).
=item *
Using //-comments
// This function bamfoodles the zorklator. /* BAD */
That is C99 or C++. Perl is C89. Using the //-comments is silently
allowed by many C compilers but cranking up the ANSI C89 strictness
(which we like to do) causes the compilation to fail.
=item *
Mixing declarations and code
void zorklator()
{
int n = 3;
set_zorkmids(n); /* BAD */
int q = 4;
That is C99 or C++. Some C compilers allow that, but you shouldn't.
The gcc option C<-Wdeclaration-after-statement> scans for such
problems (by default on starting from Perl 5.9.4).
=item *
Introducing variables inside for()
for(int i = ...; ...; ...) { /* BAD */
That is C99 or C++. While it would indeed be awfully nice to have that
also in C89, to limit the scope of the loop variable, alas, we cannot.
=item *
Mixing signed char pointers with unsigned char pointers
int foo(char *s) { ... }
...
unsigned char *t = ...; /* Or U8* t = ... */
foo(t); /* BAD */
While this is legal practice, it is certainly dubious, and downright
fatal in at least one platform: for example VMS cc considers this a
fatal error. One cause for people often making this mistake is that a
"naked char" and therefore dereferencing a "naked char pointer" have an
undefined signedness: it depends on the compiler and the flags of the
compiler and the underlying platform whether the result is signed or
unsigned. For this very same reason using a 'char' as an array index is
bad.
=item *
Macros that have string constants and their arguments as substrings of
the string constants
#define FOO(n) printf("number = %d\n", n) /* BAD */
FOO(10);
Pre-ANSI semantics for that was equivalent to
printf("10umber = %d\10");
which is probably not what you were expecting. Unfortunately at least
one reasonably common and modern C compiler does "real backward
compatibility" here, in AIX that is what still happens even though the
rest of the AIX compiler is very happily C89.
=item *
Using printf formats for non-basic C types
IV i = ...;
printf("i = %d\n", i); /* BAD */
While this might by accident work in some platform (where IV happens to
be an C<int>), in general it cannot. IV might be something larger. Even
worse the situation is with more specific types (defined by Perl's
configuration step in F<config.h>):
Uid_t who = ...;
printf("who = %d\n", who); /* BAD */
The problem here is that Uid_t might be not only not C<int>-wide but it
might also be unsigned, in which case large uids would be printed as
negative values.
There is no simple solution to this because of printf()'s limited
intelligence, but for many types the right format is available as with
either 'f' or '_f' suffix, for example:
IVdf /* IV in decimal */
UVxf /* UV is hexadecimal */
printf("i = %"IVdf"\n", i); /* The IVdf is a string constant. */
Uid_t_f /* Uid_t in decimal */
printf("who = %"Uid_t_f"\n", who);
Or you can try casting to a "wide enough" type:
printf("i = %"IVdf"\n", (IV)something_very_small_and_signed);
See L<perlguts/Formatted Printing of Size_t and SSize_t> for how to
print those.
Also remember that the C<%p> format really does require a void pointer:
U8* p = ...;
printf("p = %p\n", (void*)p);
The gcc option C<-Wformat> scans for such problems.
=item *
Blindly using variadic macros
gcc has had them for a while with its own syntax, and C99 brought them
with a standardized syntax. Don't use the former, and use the latter
only if the HAS_C99_VARIADIC_MACROS is defined.
=item *
Blindly passing va_list
Not all platforms support passing va_list to further varargs (stdarg)
functions. The right thing to do is to copy the va_list using the
Perl_va_copy() if the NEED_VA_COPY is defined.
=item *
Using gcc statement expressions
val = ({...;...;...}); /* BAD */
While a nice extension, it's not portable. The Perl code does
admittedly use them if available to gain some extra speed (essentially
as a funky form of inlining), but you shouldn't.
=item *
Binding together several statements in a macro
Use the macros STMT_START and STMT_END.
STMT_START {
...
} STMT_END
=item *
Testing for operating systems or versions when should be testing for
features
#ifdef __FOONIX__ /* BAD */
foo = quux();
#endif
Unless you know with 100% certainty that quux() is only ever available
for the "Foonix" operating system B<and> that is available B<and>
correctly working for B<all> past, present, B<and> future versions of
"Foonix", the above is very wrong. This is more correct (though still
not perfect, because the below is a compile-time check):
#ifdef HAS_QUUX
foo = quux();
#endif
How does the HAS_QUUX become defined where it needs to be? Well, if
Foonix happens to be Unixy enough to be able to run the Configure
script, and Configure has been taught about detecting and testing
quux(), the HAS_QUUX will be correctly defined. In other platforms, the
corresponding configuration step will hopefully do the same.
In a pinch, if you cannot wait for Configure to be educated, or if you
have a good hunch of where quux() might be available, you can
temporarily try the following:
#if (defined(__FOONIX__) || defined(__BARNIX__))
# define HAS_QUUX
#endif
...
#ifdef HAS_QUUX
foo = quux();
#endif
But in any case, try to keep the features and operating systems
separate.
A good resource on the predefined macros for various operating
systems, compilers, and so forth is
L<http://sourceforge.net/p/predef/wiki/Home/>
=item *
Assuming the contents of static memory pointed to by the return values
of Perl wrappers for C library functions doesn't change. Many C library
functions return pointers to static storage that can be overwritten by
subsequent calls to the same or related functions. Perl has
light-weight wrappers for some of these functions, and which don't make
copies of the static memory. A good example is the interface to the
environment variables that are in effect for the program. Perl has
C<PerlEnv_getenv> to get values from the environment. But the return is
a pointer to static memory in the C library. If you are using the value
to immediately test for something, that's fine, but if you save the
value and expect it to be unchanged by later processing, you would be
wrong, but perhaps you wouldn't know it because different C library
implementations behave differently, and the one on the platform you're
testing on might work for your situation. But on some platforms, a
subsequent call to C<PerlEnv_getenv> or related function WILL overwrite
the memory that your first call points to. This has led to some
hard-to-debug problems. Do a L<perlapi/savepv> to make a copy, thus
avoiding these problems. You will have to free the copy when you're
done to avoid memory leaks. If you don't have control over when it gets
freed, you'll need to make the copy in a mortal scalar, like so:
if ((s = PerlEnv_getenv("foo") == NULL) {
... /* handle NULL case */
}
else {
s = SvPVX(sv_2mortal(newSVpv(s, 0)));
}
The above example works only if C<"s"> is C<NUL>-terminated; otherwise
you have to pass its length to C<newSVpv>.
=back
=head2 Problematic System Interfaces
=over 4
=item *
Perl strings are NOT the same as C strings: They may contain C<NUL>
characters, whereas a C string is terminated by the first C<NUL>.
That is why Perl API functions that deal with strings generally take a
pointer to the first byte and either a length or a pointer to the byte
just beyond the final one.
And this is the reason that many of the C library string handling
functions should not be used. They don't cope with the full generality
of Perl strings. It may be that your test cases don't have embedded
C<NUL>s, and so the tests pass, whereas there may well eventually arise
real-world cases where they fail. A lesson here is to include C<NUL>s
in your tests. Now it's fairly rare in most real world cases to get
C<NUL>s, so your code may seem to work, until one day a C<NUL> comes
along.
Here's an example. It used to be a common paradigm, for decades, in the
perl core to use S<C<strchr("list", c)>> to see if the character C<c> is
any of the ones given in C<"list">, a double-quote-enclosed string of
the set of characters that we are seeing if C<c> is one of. As long as
C<c> isn't a C<NUL>, it works. But when C<c> is a C<NUL>, C<strchr>
returns a pointer to the terminating C<NUL> in C<"list">. This likely
will result in a segfault or a security issue when the caller uses that
end pointer as the starting point to read from.
A solution to this and many similar issues is to use the C<mem>I<-foo> C
library functions instead. In this case C<memchr> can be used to see if
C<c> is in C<"list"> and works even if C<c> is C<NUL>. These functions
need an additional parameter to give the string length.
In the case of literal string parameters, perl has defined macros that
calculate the length for you. See L<perlapi/String Handling>.
=item *
malloc(0), realloc(0), calloc(0, 0) are non-portable. To be portable
allocate at least one byte. (In general you should rarely need to work
at this low level, but instead use the various malloc wrappers.)
=item *
snprintf() - the return type is unportable. Use my_snprintf() instead.
=back
=head2 Security problems
Last but not least, here are various tips for safer coding.
See also L<perlclib> for libc/stdio replacements one should use.
=over 4
=item *
Do not use gets()
Or we will publicly ridicule you. Seriously.
=item *
Do not use tmpfile()
Use mkstemp() instead.
=item *
Do not use strcpy() or strcat() or strncpy() or strncat()
Use my_strlcpy() and my_strlcat() instead: they either use the native
implementation, or Perl's own implementation (borrowed from the public
domain implementation of INN).
=item *
Do not use sprintf() or vsprintf()
If you really want just plain byte strings, use my_snprintf() and
my_vsnprintf() instead, which will try to use snprintf() and
vsnprintf() if those safer APIs are available. If you want something
fancier than a plain byte string, use
L<C<Perl_form>()|perlapi/form> or SVs and
L<C<Perl_sv_catpvf()>|perlapi/sv_catpvf>.
Note that glibc C<printf()>, C<sprintf()>, etc. are buggy before glibc
version 2.17. They won't allow a C<%.s> format with a precision to
create a string that isn't valid UTF-8 if the current underlying locale
of the program is UTF-8. What happens is that the C<%s> and its operand are
simply skipped without any notice.
L<https://sourceware.org/bugzilla/show_bug.cgi?id=6530>.
=item *
Do not use atoi()
Use grok_atoUV() instead. atoi() has ill-defined behavior on overflows,
and cannot be used for incremental parsing. It is also affected by locale,
which is bad.
=item *
Do not use strtol() or strtoul()
Use grok_atoUV() instead. strtol() or strtoul() (or their IV/UV-friendly
macro disguises, Strtol() and Strtoul(), or Atol() and Atoul() are
affected by locale, which is bad.
=back
=head1 DEBUGGING
You can compile a special debugging version of Perl, which allows you
to use the C<-D> option of Perl to tell more about what Perl is doing.
But sometimes there is no alternative than to dive in with a debugger,
either to see the stack trace of a core dump (very useful in a bug
report), or trying to figure out what went wrong before the core dump
happened, or how did we end up having wrong or unexpected results.
=head2 Poking at Perl
To really poke around with Perl, you'll probably want to build Perl for
debugging, like this:
./Configure -d -DDEBUGGING
make
C<-DDEBUGGING> turns on the C compiler's C<-g> flag to have it produce
debugging information which will allow us to step through a running
program, and to see in which C function we are at (without the debugging
information we might see only the numerical addresses of the functions,
which is not very helpful). It will also turn on the C<DEBUGGING>
compilation symbol which enables all the internal debugging code in Perl.
There are a whole bunch of things you can debug with this:
L<perlrun|perlrun/-Dletters> lists them all, and the best way to find out
about them is to play about with them. The most useful options are
probably
l Context (loop) stack processing
s Stack snapshots (with v, displays all stacks)
t Trace execution
o Method and overloading resolution
c String/numeric conversions
For example
$ perl -Dst -e '$a + 1'
....
(-e:1) gvsv(main::a)
=> UNDEF
(-e:1) const(IV(1))
=> UNDEF IV(1)
(-e:1) add
=> NV(1)
Some of the functionality of the debugging code can be achieved with a
non-debugging perl by using XS modules:
-Dr => use re 'debug'
-Dx => use O 'Debug'
=head2 Using a source-level debugger
If the debugging output of C<-D> doesn't help you, it's time to step
through perl's execution with a source-level debugger.
=over 3
=item *
We'll use C<gdb> for our examples here; the principles will apply to
any debugger (many vendors call their debugger C<dbx>), but check the
manual of the one you're using.
=back
To fire up the debugger, type
gdb ./perl
Or if you have a core dump:
gdb ./perl core
You'll want to do that in your Perl source tree so the debugger can
read the source code. You should see the copyright message, followed by
the prompt.
(gdb)
C<help> will get you into the documentation, but here are the most
useful commands:
=over 3
=item * run [args]
Run the program with the given arguments.
=item * break function_name
=item * break source.c:xxx
Tells the debugger that we'll want to pause execution when we reach
either the named function (but see L<perlguts/Internal Functions>!) or
the given line in the named source file.
=item * step
Steps through the program a line at a time.
=item * next
Steps through the program a line at a time, without descending into
functions.
=item * continue
Run until the next breakpoint.
=item * finish
Run until the end of the current function, then stop again.
=item * 'enter'
Just pressing Enter will do the most recent operation again - it's a
blessing when stepping through miles of source code.
=item * ptype
Prints the C definition of the argument given.
(gdb) ptype PL_op
type = struct op {
OP *op_next;
OP *op_sibparent;
OP *(*op_ppaddr)(void);
PADOFFSET op_targ;
unsigned int op_type : 9;
unsigned int op_opt : 1;
unsigned int op_slabbed : 1;
unsigned int op_savefree : 1;
unsigned int op_static : 1;
unsigned int op_folded : 1;
unsigned int op_spare : 2;
U8 op_flags;
U8 op_private;
} *
=item * print
Execute the given C code and print its results. B<WARNING>: Perl makes
heavy use of macros, and F<gdb> does not necessarily support macros
(see later L</"gdb macro support">). You'll have to substitute them
yourself, or to invoke cpp on the source code files (see L</"The .i
Targets">) So, for instance, you can't say
print SvPV_nolen(sv)
but you have to say
print Perl_sv_2pv_nolen(sv)
=back
You may find it helpful to have a "macro dictionary", which you can
produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't
recursively apply those macros for you.
=head2 gdb macro support
Recent versions of F<gdb> have fairly good macro support, but in order
to use it you'll need to compile perl with macro definitions included
in the debugging information. Using F<gcc> version 3.1, this means
configuring with C<-Doptimize=-g3>. Other compilers might use a
different switch (if they support debugging macros at all).
=head2 Dumping Perl Data Structures
One way to get around this macro hell is to use the dumping functions
in F<dump.c>; these work a little like an internal
L<Devel::Peek|Devel::Peek>, but they also cover OPs and other
structures that you can't get at from Perl. Let's take an example.
We'll use the C<$a = $b + $c> we used before, but give it a bit of
context: C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and
poke around?
What about C<pp_add>, the function we examined earlier to implement the
C<+> operator:
(gdb) break Perl_pp_add
Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
Notice we use C<Perl_pp_add> and not C<pp_add> - see
L<perlguts/Internal Functions>. With the breakpoint in place, we can
run our program:
(gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
Lots of junk will go past as gdb reads in the relevant source files and
libraries, and then:
Breakpoint 1, Perl_pp_add () at pp_hot.c:309
1396 dSP; dATARGET; bool useleft; SV *svl, *svr;
(gdb) step
311 dPOPTOPnnrl_ul;
(gdb)
We looked at this bit of code before, and we said that
C<dPOPTOPnnrl_ul> arranges for two C<NV>s to be placed into C<left> and
C<right> - let's slightly expand it:
#define dPOPTOPnnrl_ul NV right = POPn; \
SV *leftsv = TOPs; \
NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
C<POPn> takes the SV from the top of the stack and obtains its NV
either directly (if C<SvNOK> is set) or by calling the C<sv_2nv>
function. C<TOPs> takes the next SV from the top of the stack - yes,
C<POPn> uses C<TOPs> - but doesn't remove it. We then use C<SvNV> to
get the NV from C<leftsv> in the same way as before - yes, C<POPn> uses
C<SvNV>.
Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to
convert it. If we step again, we'll find ourselves there:
(gdb) step
Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
1669 if (!sv)
(gdb)
We can now use C<Perl_sv_dump> to investigate the SV:
(gdb) print Perl_sv_dump(sv)
SV = PV(0xa057cc0) at 0xa0675d0
REFCNT = 1
FLAGS = (POK,pPOK)
PV = 0xa06a510 "6XXXX"\0
CUR = 5
LEN = 6
$1 = void
We know we're going to get C<6> from this, so let's finish the
subroutine:
(gdb) finish
Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
0x462669 in Perl_pp_add () at pp_hot.c:311
311 dPOPTOPnnrl_ul;
We can also dump out this op: the current op is always stored in
C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us
similar output to CPAN module B::Debug.
(gdb) print Perl_op_dump(PL_op)
{
13 TYPE = add ===> 14
TARG = 1