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 perlhack and perlhacktut yet, you might want to do
       that first.

       Perl source plays by ANSI C89 rules: no C99 (or C++) extensions. In
       some cases we have to take pre-ANSI requirements into consideration.
       You don't care about some particular platform having broken Perl? I
       hear there is still a strong demand for J2EE programmers.

   Perl environment problems
       o   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_ ...);

           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 (or a dVAR) as the first thing in the function.

           See "How multiple interpreters and concurrency are supported" in
           perlguts for further discussion about context.

       o   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.

       o   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 intrpvar.h (at the very end for binary

           Introducing read-only (const) globals is okay, as long as you
           verify with e.g. "nm libperl.a|egrep -v ' [TURtr] '" (if your "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:

           PERL_IMPLICIT_CONTEXT" in perlguts.

       o   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
  in perlguts.

       o   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 in 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 in

   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 "-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 "-Dgccansipedantic" flag to enable the gcc "-ansi
       -pedantic" flags which enforce stricter ANSI rules.

       If using the "gcc -Wall" note that not all the possible warnings (like
       "-Wunitialized") are given unless you also compile with "-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 "-std=c89", "-ansi", "-pedantic", and a
       selection of "-W" flags (see cflags.SH).

       Also study perlport carefully to avoid any bad assumptions about the
       operating system, filesystems, and so forth.

       You may once in a while try a "make microperl" to see whether we can

               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().)

       o   Casting between data 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.

       o   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 not guaranteed to be exactly
           32 bits, they are at least 32 bits, nor are they guaranteed to be
           int or long. If you really explicitly need 64-bit variables, use
           I64 and U64, but only if guarded by HAS_QUAD.

       o   Assuming one can dereference any type of pointer for any type of

             char *p = ...;
             long pony = *p;    /* BAD */

           Many platforms, quite rightly so, will give you a core dump instead
           of a pony if the p happens not be correctly aligned.

       o   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.

       o   Assume anything about structs (especially the ones you don't
           control, like the ones coming from the system headers)

           o       That a certain field exists in a struct

           o       That no other fields exist besides the ones you know of

                           align the fields - the bytes can be anything

                   o       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

       o   Assuming the character set is ASCIIish

           Perl can compile and run under EBCDIC platforms. See 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 'A', but not
           0x41. You can safely say '\n', but not \012. If a character doesn't
           have a trivial input form, you can create a #define for it in both
           "utfebcdic.h" and "utf8.h", so that it resolves to different values
           depending on the character set being used. (There are three
           different EBCDIC character sets defined in "utfebcdic.h", so it
           might be best to insert the #define three times in that file.)

           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.

           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 "utf8.h" and "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 "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 "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, "hibit". The
           situation differs and is not so simple on EBCDIC machines, but as
           long as the code itself uses the "NATIVE_IS_INVARIANT()" macro
           appropriately, it works, even if the comments are wrong.

       o   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 is being changed starting in 5.12 so that these characters
           will be considered to be Latin-1 (ISO-8859-1).

       o   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 "-Wendif-labels" warns about the bad variant (by
           default on starting from Perl 5.9.4).

       o   Having a comma after the last element of an enum list

             enum color {
               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).

       o   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.

       o   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.

             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.

       o   Macros that have string constants and their arguments as substrings
           of the string constants

             #define FOO(n) printf("number = %d\n", n)    /* BAD */

           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.

       o   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 "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 config.h):

              Uid_t who = ...;
              printf("who = %d\n", who);    /* BAD */

           The problem here is that Uid_t might be not only not "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 */

              U8* p = ...;
              printf("p = %p\n", (void*)p);

           The gcc option "-Wformat" scans for such problems.

       o   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.

       o   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.

       o   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.

       o   Binding together several statements in a macro

           Use the macros STMT_START and STMT_END.

              STMT_START {
              } STMT_END

       o   Testing for operating systems or versions when should be testing
           for features

             #ifdef __FOONIX__    /* BAD */
             foo = quux();

           Unless you know with 100% certainty that quux() is only ever
           available for the "Foonix" operating system and that is available
           and correctly working for all past, present, 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();

           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

             #ifdef HAS_QUUX
             foo = quux();

           But in any case, try to keep the features and operating systems

   Problematic System Interfaces
       o   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

       o   snprintf() - the return type is unportable. Use my_snprintf()

   Security problems
       Last but not least, here are various tips for safer coding.

       o   Do not use gets()

           Or we will publicly ridicule you. Seriously.

       o   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).

       o   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 SVs and

       You can compile a special debugging version of Perl, which allows you
       to use the "-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.

   Poking at Perl
       To really poke around with Perl, you'll probably want to build Perl for
       debugging, like this:

           ./Configure -d -D optimize=-g

           l  Context (loop) stack processing
           t  Trace execution
           o  Method and overloading resolution
           c  String/numeric conversions

       Some of the functionality of the debugging code can be achieved using
       XS modules.

           -Dr => use re 'debug'
           -Dx => use O 'Debug'

   Using a source-level debugger
       If the debugging output of "-D" doesn't help you, it's time to step
       through perl's execution with a source-level debugger.

       o  We'll use "gdb" for our examples here; the principles will apply to
          any debugger (many vendors call their debugger "dbx"), but check the
          manual of the one you're using.

       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.


       "help" will get you into the documentation, but here are the most
       useful commands:

       o  run [args]

          Run the program with the given arguments.

       o  break function_name

       o  break source.c:xxx

          Tells the debugger that we'll want to pause execution when we reach
          either the named function (but see "Internal Functions" in
          perlguts!) or the given line in the named source file.

       o  step

          Steps through the program a line at a time.

       o  next

          Just pressing Enter will do the most recent operation again - it's a
          blessing when stepping through miles of source code.

       o  print

          Execute the given C code and print its results. WARNING: Perl makes
          heavy use of macros, and gdb does not necessarily support macros
          (see later "gdb macro support"). You'll have to substitute them
          yourself, or to invoke cpp on the source code files (see "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)

       You may find it helpful to have a "macro dictionary", which you can
       produce by saying "cpp -dM perl.c | sort". Even then, cpp won't
       recursively apply those macros for you.

   gdb macro support
       Recent versions of 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 gcc version 3.1, this means
       configuring with "-Doptimize=-g3". Other compilers might use a
       different switch (if they support debugging macros at all).

   Dumping Perl Data Structures
       One way to get around this macro hell is to use the dumping functions
       in dump.c; these work a little like an internal 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 "$a = $b + $c" we used before,
       but give it a bit of context: "$b = "6XXXX"; $c = 2.3;". Where's a good
       place to stop and poke around?

       What about "pp_add", the function we examined earlier to implement the
       "+" operator:

           (gdb) break Perl_pp_add
           Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.

       Notice we use "Perl_pp_add" and not "pp_add" - see "Internal Functions"
       in perlguts. 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
           309         dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
           (gdb) step

       directly (if "SvNOK" is set) or by calling the "sv_2nv" function.
       "TOPs" takes the next SV from the top of the stack - yes, "POPn" uses
       "TOPs" - but doesn't remove it. We then use "SvNV" to get the NV from
       "leftsv" in the same way as before - yes, "POPn" uses "SvNV".

       Since we don't have an NV for $b, we'll have to use "sv_2nv" to convert
       it. If we step again, we'll find ourselves there:

           Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
           1669        if (!sv)

       We can now use "Perl_sv_dump" to investigate the 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 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
       "PL_op", and we can dump it with "Perl_op_dump". This'll give us
       similar output to B::Debug.

           13  TYPE = add  ===> 14
               TARG = 1
               FLAGS = (SCALAR,KIDS)
                   TYPE = null  ===> (12)
                     (was rv2sv)
                   FLAGS = (SCALAR,KIDS)
           11          TYPE = gvsv  ===> 12
                       FLAGS = (SCALAR)
                       GV = main::b

       # finish this later #

       Various tools exist for analysing C source code statically, as opposed
       to dynamically, that is, without executing the code. It is possible to
       detect resource leaks, undefined behaviour, type mismatches,
       There is a lint variant called "splint" (Secure Programming Lint)
       available from that should compile on any Unix-
       like platform.

       There are "lint" and <splint> targets in Makefile, but you may have to
       diddle with the flags (see above).

       Coverity ( is a product similar to lint and as
       a testbed for their product they periodically check several open source
       projects, and they give out accounts to open source developers to the
       defect databases.

   cpd (cut-and-paste detector)
       The cpd tool detects cut-and-paste coding. If one instance of the cut-
       and-pasted code changes, all the other spots should probably be
       changed, too. Therefore such code should probably be turned into a
       subroutine or a macro.

       cpd ( is part of the pmd project
       ( pmd was originally written for static
       analysis of Java code, but later the cpd part of it was extended to
       parse also C and C++.

       Download the () from the SourceForge site, extract the
       pmd-X.Y.jar from it, and then run that on source code thusly:

         java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD --minimum-tokens 100 --files /some/where/src --language c > cpd.txt

       You may run into memory limits, in which case you should use the -Xmx

         java -Xmx512M ...

   gcc warnings
       Though much can be written about the inconsistency and coverage
       problems of gcc warnings (like "-Wall" not meaning "all the warnings",
       or some common portability problems not being covered by "-Wall", or
       "-ansi" and "-pedantic" both being a poorly defined collection of
       warnings, and so forth), gcc is still a useful tool in keeping our
       coding nose clean.

       The "-Wall" is by default on.

       The "-ansi" (and its sidekick, "-pedantic") would be nice to be on
       always, but unfortunately they are not safe on all platforms, they can
       for example cause fatal conflicts with the system headers (Solaris
       being a prime example). If Configure "-Dgccansipedantic" is used, the
       "cflags" frontend selects "-ansi -pedantic" for the platforms where
       they are known to be safe.

       Starting from Perl 5.9.4 the following extra flags are added:

       o   "-Wendif-labels"
       o   "-Wstrict-prototypes"

       The "-Wtraditional" is another example of the annoying tendency of gcc
       to bundle a lot of warnings under one switch (it would be impossible to
       deploy in practice because it would complain a lot) but it does contain
       some warnings that would be beneficial to have available on their own,
       such as the warning about string constants inside macros containing the
       macro arguments: this behaved differently pre-ANSI than it does in
       ANSI, and some C compilers are still in transition, AIX being an

   Warnings of other C compilers
       Other C compilers (yes, there are other C compilers than gcc) often
       have their "strict ANSI" or "strict ANSI with some portability
       extensions" modes on, like for example the Sun Workshop has its "-Xa"
       mode on (though implicitly), or the DEC (these days, HP...) has its
       "-std1" mode on.

       NOTE 1: Running under memory debuggers such as Purify, valgrind, or
       Third Degree greatly slows down the execution: seconds become minutes,
       minutes become hours. For example as of Perl 5.8.1, the
       ext/Encode/t/Unicode.t takes extraordinarily long to complete under
       e.g. Purify, Third Degree, and valgrind. Under valgrind it takes more
       than six hours, even on a snappy computer. The said test must be doing
       something that is quite unfriendly for memory debuggers. If you don't
       feel like waiting, that you can simply kill away the perl process.

       NOTE 2: To minimize the number of memory leak false alarms (see
       "PERL_DESTRUCT_LEVEL" for more information), you have to set the
       environment variable PERL_DESTRUCT_LEVEL to 2.

       For csh-like shells:

           setenv PERL_DESTRUCT_LEVEL 2

       For Bourne-type shells:

           export PERL_DESTRUCT_LEVEL

       In Unixy environments you can also use the "env" command:

           env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...

       NOTE 3: There are known memory leaks when there are compile-time errors
       within eval or require, seeing "S_doeval" in the call stack is a good
       sign of these. Fixing these leaks is non-trivial, unfortunately, but
       they must be fixed eventually.

       NOTE 4: DynaLoader will not clean up after itself completely unless
       Perl is built with the Configure option

            -Uusemymalloc -Dusemultiplicity

       where these arguments mean:

       o   -Accflags=-DPURIFY

           Disables Perl's arena memory allocation functions, as well as
           forcing use of memory allocation functions derived from the system

       o   -Doptimize='-g'

           Adds debugging information so that you see the exact source
           statements where the problem occurs. Without this flag, all you
           will see is the source filename of where the error occurred.

       o   -Uusemymalloc

           Disable Perl's malloc so that Purify can more closely monitor
           allocations and leaks. Using Perl's malloc will make Purify report
           most leaks in the "potential" leaks category.

       o   -Dusemultiplicity

           Enabling the multiplicity option allows perl to clean up thoroughly
           when the interpreter shuts down, which reduces the number of bogus
           leak reports from Purify.

       Once you've compiled a perl suitable for Purify'ing, then you can just:

           make pureperl

       which creates a binary named 'pureperl' that has been Purify'ed. This
       binary is used in place of the standard 'perl' binary when you want to
       debug Perl memory problems.

       As an example, to show any memory leaks produced during the standard
       Perl testset you would create and run the Purify'ed perl as:

           make pureperl
           cd t
           ../pureperl -I../lib harness

       which would run Perl on and report any memory problems.

       Purify outputs messages in "Viewer" windows by default. If you don't
       have a windowing environment or if you simply want the Purify output to
       unobtrusively go to a log file instead of to the interactive window,
       use these following options to output to the log file "perl.log":

           setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
            -log-file=perl.log -append-logfile=yes"

       If you plan to use the "Viewer" windows, then you only need this

       Purify on NT

       Purify on Windows NT instruments the Perl binary 'perl.exe' on the fly.
        There are several options in the makefile you should change to get the
       most use out of Purify:

       o   DEFINES

           You should add -DPURIFY to the DEFINES line so the DEFINES line
           looks something like:


           to disable Perl's arena memory allocation functions, as well as to
           force use of memory allocation functions derived from the system

       o   USE_MULTI = define

           Enabling the multiplicity option allows perl to clean up thoroughly
           when the interpreter shuts down, which reduces the number of bogus
           leak reports from Purify.

       o   #PERL_MALLOC = define

           Disable Perl's malloc so that Purify can more closely monitor
           allocations and leaks. Using Perl's malloc will make Purify report
           most leaks in the "potential" leaks category.

       o   CFG = Debug

           Adds debugging information so that you see the exact source
           statements where the problem occurs. Without this flag, all you
           will see is the source filename of where the error occurred.

       As an example, to show any memory leaks produced during the standard
       Perl testset you would create and run Purify as:

           cd win32
           cd ../t
           purify ../perl -I../lib harness

       which would instrument Perl in memory, run Perl on, then
       finally report any memory problems.

       The excellent valgrind tool can be used to find out both memory leaks
       and illegal memory accesses. As of version 3.3.0, Valgrind only
       supports Linux on x86, x86-64 and PowerPC. The special "test.valgrind"
       target can be used to run the tests under valgrind. Found errors  and
       memory leaks are logged in files named testfile.valgrind.

       Depending on your platform there are various ways of profiling Perl.

       There are two commonly used techniques of profiling executables:
       statistical time-sampling and basic-block counting.

       The first method takes periodically samples of the CPU program counter,
       and since the program counter can be correlated with the code generated
       for functions, we get a statistical view of in which functions the
       program is spending its time. The caveats are that very small/fast
       functions have lower probability of showing up in the profile, and that
       periodically interrupting the program (this is usually done rather
       frequently, in the scale of milliseconds) imposes an additional
       overhead that may skew the results. The first problem can be alleviated
       by running the code for longer (in general this is a good idea for
       profiling), the second problem is usually kept in guard by the
       profiling tools themselves.

       The second method divides up the generated code into basic blocks.
       Basic blocks are sections of code that are entered only in the
       beginning and exited only at the end. For example, a conditional jump
       starts a basic block. Basic block profiling usually works by
       instrumenting the code by adding enter basic block #nnnn book-keeping
       code to the generated code. During the execution of the code the basic
       block counters are then updated appropriately. The caveat is that the
       added extra code can skew the results: again, the profiling tools
       usually try to factor their own effects out of the results.

   Gprof Profiling
       gprof is a profiling tool available in many Unix platforms, it uses
       statistical time-sampling.

       You can build a profiled version of perl called "perl.gprof" by
       invoking the make target "perl.gprof"  (What is required is that Perl
       must be compiled using the "-pg" flag, you may need to re-Configure).
       Running the profiled version of Perl will create an output file called
       gmon.out is created which contains the profiling data collected during
       the execution.

       The gprof tool can then display the collected data in various ways.
       Usually gprof understands the following options:

       o   -a

           Suppress statically defined functions from the profile.

       o   -b

           Suppress the verbose descriptions in the profile.

       o   -e routine

           Exclude the given routine and its descendants from the profile.
           Display routines that have zero usage.

       For more detailed explanation of the available commands and output
       formats, see your own local documentation of gprof.

       quick hint:

           $ sh Configure -des -Dusedevel -Doptimize='-pg' && make perl.gprof
           $ ./perl.gprof someprog # creates gmon.out in current directory
           $ gprof ./perl.gprof > out
           $ view out

   GCC gcov Profiling
       Starting from GCC 3.0 basic block profiling is officially available for
       the GNU CC.

       You can build a profiled version of perl called perl.gcov by invoking
       the make target "perl.gcov" (what is required that Perl must be
       compiled using gcc with the flags "-fprofile-arcs -ftest-coverage", you
       may need to re-Configure).

       Running the profiled version of Perl will cause profile output to be
       generated. For each source file an accompanying ".da" file will be

       To display the results you use the "gcov" utility (which should be
       installed if you have gcc 3.0 or newer installed). gcov is run on
       source code files, like this

           gcov sv.c

       which will cause sv.c.gcov to be created. The .gcov files contain the
       source code annotated with relative frequencies of execution indicated
       by "#" markers.

       Useful options of gcov include "-b" which will summarise the basic
       block, branch, and function call coverage, and "-c" which instead of
       relative frequencies will use the actual counts. For more information
       on the use of gcov and basic block profiling with gcc, see the latest
       GNU CC manual, as of GCC 3.0 see


       and its section titled "8. gcov: a Test Coverage Program"


       quick hint:

           $ sh Configure -des -Dusedevel -Doptimize='-g' \
               -Accflags='-fprofile-arcs -ftest-coverage' \
               -Aldflags='-fprofile-arcs -ftest-coverage' && make perl.gcov
           $ rm -f regexec.c.gcov regexec.gcda
           $ ./perl.gcov

       There is a way to tell perl to do complete cleanup: set the environment
       variable PERL_DESTRUCT_LEVEL to a non-zero value. The t/TEST wrapper
       does set this to 2, and this is what you need to do too, if you don't
       want to see the "global leaks": For example, for "third-degreed" Perl:

               env PERL_DESTRUCT_LEVEL=2 ./perl.third -Ilib t/foo/bar.t

       (Note: the mod_perl apache module uses also this environment variable
       for its own purposes and extended its semantics. Refer to the mod_perl
       documentation for more information. Also, spawned threads do the
       equivalent of setting this variable to the value 1.)

       If, at the end of a run you get the message N scalars leaked, you can
       recompile with "-DDEBUG_LEAKING_SCALARS", which will cause the
       addresses of all those leaked SVs to be dumped along with details as to
       where each SV was originally allocated. This information is also
       displayed by Devel::Peek. Note that the extra details recorded with
       each SV increases memory usage, so it shouldn't be used in production
       environments. It also converts "new_SV()" from a macro into a real
       function, so you can use your favourite debugger to discover where
       those pesky SVs were allocated.

       If you see that you're leaking memory at runtime, but neither valgrind
       nor "-DDEBUG_LEAKING_SCALARS" will find anything, you're probably
       leaking SVs that are still reachable and will be properly cleaned up
       during destruction of the interpreter. In such cases, using the "-Dm"
       switch can point you to the source of the leak. If the executable was
       built with "-DDEBUG_LEAKING_SCALARS", "-Dm" will output SV allocations
       in addition to memory allocations. Each SV allocation has a distinct
       serial number that will be written on creation and destruction of the
       SV. So if you're executing the leaking code in a loop, you need to look
       for SVs that are created, but never destroyed between each cycle. If
       such an SV is found, set a conditional breakpoint within "new_SV()" and
       make it break only when "PL_sv_serial" is equal to the serial number of
       the leaking SV. Then you will catch the interpreter in exactly the
       state where the leaking SV is allocated, which is sufficient in many
       cases to find the source of the leak.

       As "-Dm" is using the PerlIO layer for output, it will by itself
       allocate quite a bunch of SVs, which are hidden to avoid recursion. You
       can bypass the PerlIO layer if you use the SV logging provided by
       "-DPERL_MEM_LOG" instead.

       If compiled with "-DPERL_MEM_LOG", both memory and SV allocations go
       through logging functions, which is handy for breakpoint setting.

       Unless "-DPERL_MEM_LOG_NOIMPL" is also compiled, the logging functions
       read $ENV{PERL_MEM_LOG} to determine whether to log the event, and if
       so how:

           $ENV{PERL_MEM_LOG} =~ /m/           Log all memory ops
           $ENV{PERL_MEM_LOG} =~ /s/           Log all SV ops
           $ENV{PERL_MEM_LOG} =~ /t/           include timestamp in Log

       allocation is also logged.

   DDD over gdb
       Those debugging perl with the DDD frontend over gdb may find the
       following useful:

       You can extend the data conversion shortcuts menu, so for example you
       can display an SV's IV value with one click, without doing any typing.
       To do that simply edit ~/.ddd/init file and add after:

         ! Display shortcuts.
         Ddd*gdbDisplayShortcuts: \
         /t ()   // Convert to Bin\n\
         /d ()   // Convert to Dec\n\
         /x ()   // Convert to Hex\n\
         /o ()   // Convert to Oct(\n\

       the following two lines:

         ((XPV*) (())->sv_any )->xpv_pv  // 2pvx\n\
         ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx

       so now you can do ivx and pvx lookups or you can plug there the sv_peek

         Perl_sv_peek(my_perl, (SV*)()) // sv_peek

       (The my_perl is for threaded builds.) Just remember that every line,
       but the last one, should end with \n\

       Alternatively edit the init file interactively via: 3rd mouse button ->
       New Display -> Edit Menu

       Note: you can define up to 20 conversion shortcuts in the gdb section.

       If you see in a debugger a memory area mysteriously full of 0xABABABAB
       or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros, see

   Read-only optrees
       Under ithreads the optree is read only. If you want to enforce this, to
       check for write accesses from buggy code, compile with
       "-DPL_OP_SLAB_ALLOC" to enable the OP slab allocator and
       "-DPERL_DEBUG_READONLY_OPS" to enable code that allocates op memory via
       "mmap", and sets it read-only at run time. Any write access to an op
       results in a "SIGBUS" and abort.

       This code is intended for development only, and may not be portable
       even to all Unix variants. Also, it is an 80% solution, in that it
       isn't able to make all ops read only. Specifically it

       o   1

       It's not possible to turn the slabs to read-only after an action
       requiring read-write access, as either can happen during op tree
       building time, so there may still be legitimate write access.

       However, as an 80% solution it is still effective, as currently it
       catches a write access during the generation of, which means
       that we can't yet build perl with this enabled.

   The .i Targets
       You can expand the macros in a foo.c file by saying

           make foo.i

       which will expand the macros using cpp.  Don't be scared by the

       This document was originally written by Nathan Torkington, and is
       maintained by the perl5-porters mailing list.

perl v5.14.2                      2011-09-19                   PERLHACKTIPS(1)
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