perlhacktips - Tips for Perl core C code hacking


   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
   *   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.

   *   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.

   *   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:

         static const char etc[] = "...";

       If you want to have arrays of constant strings, note carefully the
       right combination of "const"s:

           static const char * const yippee[] =
               {"hi", "ho", "silver"};

       There is a way to completely hide any modifiable globals (they are
       all moved to heap), the compilation setting
       "-DPERL_GLOBAL_STRUCT_PRIVATE".  It is not normally used, but can
       be used for testing, read more about it in "Background and
       PERL_IMPLICIT_CONTEXT" in perlguts.

   *   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.

   *   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, 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

   Do not assume an operating system indicates a certain compiler.

   *   Casting pointers to integers or casting integers to pointers

           void castaway(U8* p)
             IV i = p;


           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().)

   *   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.

   *   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.

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

         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.

   *   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.

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

       *       That a certain field exists in a struct

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

       *       That a field is of certain signedness, sizeof, or type

       *       That the fields are in a certain order

               *       While C guarantees the ordering specified in the
                       struct definition, between different platforms the
                       definitions might differ

       *       That the sizeof(struct) or the alignments are the same

               *       There might be padding bytes between the fields to
                       align the fields - the bytes can be anything

               *       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

   *   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".  However, you can
       use macros defined in utf8.h to specify any code point portably.
       "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
       "LATIN1_TO_NATIVE" are from 0x00 through 0xFF.  If your input isn't
       guaranteed to be in that range, use "UNICODE_TO_NATIVE" instead.
       "NATIVE_TO_LATIN1" and "NATIVE_TO_UNICODE" translate the opposite

       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
       regen/, and have Perl create "#define"'s for
       you, based on the current platform.

       Note that the "isFOO" and "toFOO" macros in 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

       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 UTF-8 "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.

       As noted in "TESTING" in perlhack, when writing test scripts, the
       file t/ 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 "\0", "\r", and "\N{VT}" (also specifiable as
       "\cK", "\x0B", "\N{U+0B}", or "\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.

   *   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).

   *   Mixing #define and #ifdef

         #define BURGLE(x) ... \
         #ifdef BURGLE_OLD_STYLE        /* BAD */
         ... do it the old way ... \
         ... do it the new way ... \

       You cannot portably "stack" cpp directives.  For example in the
       above you need two separate BURGLE() #defines, one for each #ifdef

   *   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).

   *   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).

   *   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.

   *   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

       The gcc option "-Wdeclaration-after-statements" scans for such
       problems (by default on starting from Perl 5.9.4).

   *   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

   *   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.

   *   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.

   *   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 */

          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);

       Also remember that the %p format really does require a void

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

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

   *   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.

   *   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.

   *   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.

   *   Binding together several statements in a macro

       Use the macros STMT_START and STMT_END.

          STMT_START {
          } STMT_END

   *   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
       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


         #ifdef HAS_QUUX
         foo = quux();

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

       A good resource on the predefined macros for various operating
       systems, compilers, and so forth is

   *   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 "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 "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 "savepv" in
       perlapi 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 "s" is "NUL"-terminated; otherwise
       you have to pass its length to "newSVpv".

   Problematic System Interfaces
   *   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

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

   Security problems
   Last but not least, here are various tips for safer coding.  See also
   perlclib for libc/stdio replacements one should use.

   *   Do not use gets()

       Or we will publicly ridicule you.  Seriously.

   *   Do not use tmpfile()

       Use mkstemp() instead.

   *   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).

   *   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 "Perl_form"() or
       SVs and "Perl_sv_catpvf()".

       Note that glibc "printf()", "sprintf()", etc. are buggy before
       glibc version 2.17.  They won't allow a "%.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 %s and its operand are simply skipped without any notice.

   *   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.

   *   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.


   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

   "-g" is a flag to the C compiler 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).

   Configure will also turn on the "DEBUGGING" compilation symbol which
   enables all the internal debugging code in Perl.  There are a whole
   bunch of things you can debug with this: perlrun 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
       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.

   *  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:

   *  run [args]

      Run the program with the given arguments.

   *  break function_name

   *  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.

   *  step

      Steps through the program a line at a time.

   *  next

      Steps through the program a line at a time, without descending into

   *  continue

      Run until the next breakpoint.

   *  finish

      Run until the end of the current function, then stop again.

   *  'enter'

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

   *  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;
        } *

   *  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
       311           dPOPTOPnnrl_ul;

   We looked at this bit of code before, and we said that "dPOPTOPnnrl_ul"
   arranges for two "NV"s to be placed into "left" and "right" - let's
   slightly expand it:

    #define dPOPTOPnnrl_ul  NV right = POPn; \
                            SV *leftsv = TOPs; \
                            NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0

   "POPn" takes the SV from the top of the stack and obtains its NV either
   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:

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

   We can now use "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 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.

       (gdb) print Perl_op_dump(PL_op)
       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 #

   Using gdb to look at specific parts of a program
   With the example above, you knew to look for "Perl_pp_add", but what if
   there were multiple calls to it all over the place, or you didn't know
   what the op was you were looking for?

   One way to do this is to inject a rare call somewhere near what you're
   looking for.  For example, you could add "study" before your method:


   And in gdb do:

       (gdb) break Perl_pp_study

   And then step until you hit what you're looking for.  This works well
   in a loop if you want to only break at certain iterations:

       for my $c (1..100) {
           study if $c == 50;

   Using gdb to look at what the parser/lexer are doing
   If you want to see what perl is doing when parsing/lexing your code,
   you can use "BEGIN {}":

       print "Before\n";
       BEGIN { study; }
       print "After\n";

   And in gdb:

       (gdb) break Perl_pp_study

   If you want to see what the parser/lexer is doing inside of "if" blocks
   and the like you need to be a little trickier:

       if ($a && $b && do { BEGIN { study } 1 } && $c) { ... }


   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,
   portability problems, code paths that would cause illegal memory
   accesses, and other similar problems by just parsing the C code and
   looking at the resulting graph, what does it tell about the execution
   and data flows.  As a matter of fact, this is exactly how C compilers
   know to give warnings about dubious code.

   lint, splint
   The good old C code quality inspector, "lint", is available in several
   platforms, but please be aware that there are several different
   implementations of it by different vendors, which means that the flags
   are not identical across different platforms.

   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.

   There is Coverity setup for the perl5 project:

   HP-UX cadvise (Code Advisor)
   HP has a C/C++ static analyzer product for HP-UX caller Code Advisor.
   (Link not given here because the URL is horribly long and seems
   horribly unstable; use the search engine of your choice to find it.)
   The use of the "cadvise_cc" recipe with "Configure ...
   -Dcc=./cadvise_cc" (see cadvise "User Guide") is recommended; as is the
   use of "+wall".

   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:

   *   "-Wendif-labels"

   *   "-Wextra"

   *   "-Wdeclaration-after-statement"

   The following flags would be nice to have but they would first need
   their own Augean stablemaster:

   *   "-Wpointer-arith"

   *   "-Wshadow"

   *   "-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 older 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.
   Roughly valgrind slows down execution by factor 10, AddressSanitizer by
   factor 2.

   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 example, like this:

       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

   The valgrind tool can be used to find out both memory leaks and illegal
   heap memory accesses.  As of version 3.3.0, Valgrind only supports
   Linux on x86, x86-64 and PowerPC and Darwin (OS X) on x86 and x86-64.
   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 and by default output is displayed inline.

   Example usage:

       make test.valgrind

   Since valgrind adds significant overhead, tests will take much longer
   to run.  The valgrind tests support being run in parallel to help with

       TEST_JOBS=9 make test.valgrind

   Note that the above two invocations will be very verbose as reachable
   memory and leak-checking is enabled by default.  If you want to just
   see pure errors, try:

       VG_OPTS='-q --leak-check=no --show-reachable=no' TEST_JOBS=9 \
           make test.valgrind

   Valgrind also provides a cachegrind tool, invoked on perl as:

       VG_OPTS=--tool=cachegrind make test.valgrind

   As system libraries (most notably glibc) are also triggering errors,
   valgrind allows to suppress such errors using suppression files.  The
   default suppression file that comes with valgrind already catches a lot
   of them.  Some additional suppressions are defined in t/perl.supp.

   To get valgrind and for more information see

   AddressSanitizer is a clang and gcc extension, included in clang since
   v3.1 and gcc since v4.8.  It checks illegal heap pointers, global
   pointers, stack pointers and use after free errors, and is fast enough
   that you can easily compile your debugging or optimized perl with it.
   It does not check memory leaks though.  AddressSanitizer is available
   for Linux, Mac OS X and soon on Windows.

   To build perl with AddressSanitizer, your Configure invocation should
   look like:

       sh Configure -des -Dcc=clang \
          -Accflags=-faddress-sanitizer -Aldflags=-faddress-sanitizer \
          -Alddlflags=-shared\ -faddress-sanitizer

   where these arguments mean:

   *   -Dcc=clang

       This should be replaced by the full path to your clang executable
       if it is not in your path.

   *   -Accflags=-faddress-sanitizer

       Compile perl and extensions sources with AddressSanitizer.

   *   -Aldflags=-faddress-sanitizer

       Link the perl executable with AddressSanitizer.

   *   -Alddlflags=-shared\ -faddress-sanitizer

       Link dynamic extensions with AddressSanitizer.  You must manually
       specify "-shared" because using "-Alddlflags=-shared" will prevent
       Configure from setting a default value for "lddlflags", which
       usually contains "-shared" (at least on Linux).

   See also


   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 which uses
   statistical time-sampling.  You can build a profiled version of perl by
   compiling using gcc with the flag "-pg".  Either edit or re-
   run Configure.  Running the profiled version of Perl will create an
   output file called gmon.out which contains the profiling data collected
   during the execution.

   quick hint:

       $ sh Configure -des -Dusedevel -Accflags='-pg' \
           -Aldflags='-pg' -Alddlflags='-pg -shared' \
           && make perl
       $ ./perl ... # creates gmon.out in current directory
       $ gprof ./perl > out
       $ less out

   (you probably need to add "-shared" to the <-Alddlflags> line until RT
   #118199 is resolved)

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

   *   -a

       Suppress statically defined functions from the profile.

   *   -b

       Suppress the verbose descriptions in the profile.

   *   -e routine

       Exclude the given routine and its descendants from the profile.

   *   -f routine

       Display only the given routine and its descendants in the profile.

   *   -s

       Generate a summary file called gmon.sum which then may be given to
       subsequent gprof runs to accumulate data over several runs.

   *   -z

       Display routines that have zero usage.

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

   GCC gcov Profiling
   basic block profiling is officially available in gcc 3.0 and later.
   You can build a profiled version of perl by compiling using gcc with
   the flags "-fprofile-arcs -ftest-coverage".  Either edit or
   re-run Configure.

   quick hint:

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

   (you probably need to add "-shared" to the <-Alddlflags> line until RT
   #118199 is resolved)

   Running the profiled version of Perl will cause profile output to be
   generated.  For each source file an accompanying .gcda 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.  If you want to generate .gcov files for all profiled
   object files, you can run something like this:

       for file in `find . -name \*.gcno`
       do sh -c "cd `dirname $file` && gcov `basename $file .gcno`"

   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 4.8, this is at


   If you want to run any of the tests yourself manually using e.g.
   valgrind, please note that by default perl does not explicitly cleanup
   all the memory it has allocated (such as global memory arenas) but
   instead lets the exit() of the whole program "take care" of such
   allocations, also known as "global destruction of objects".

   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 running under valgrind

       env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -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" ("-Accflags=-DPERL_MEM_LOG"), both
   memory and SV allocations go through logging functions, which is handy
   for breakpoint setting.

   Unless "-DPERL_MEM_LOG_NOIMPL" ("-Accflags=-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
       $ENV{PERL_MEM_LOG} =~ /^(\d+)/      write to FD given (default is 2)

   Memory logging is somewhat similar to "-Dm" but is independent of
   "-DDEBUGGING", and at a higher level; all uses of Newx(), Renew(), and
   Safefree() are logged with the caller's source code file and line
   number (and C function name, if supported by the C compiler).  In
   contrast, "-Dm" is directly at the point of "malloc()".  SV logging is

   Since the logging doesn't use PerlIO, all SV allocations are logged and
   no extra SV allocations are introduced by enabling the logging.  If
   compiled with "-DDEBUG_LEAKING_SCALARS", the serial number for each SV
   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.

   C backtrace
   On some platforms Perl supports retrieving the C level backtrace
   (similar to what symbolic debuggers like gdb do).

   The backtrace returns the stack trace of the C call frames, with the
   symbol names (function names), the object names (like "perl"), and if
   it can, also the source code locations (file:line).

   The supported platforms are Linux, and OS X (some *BSD might work at
   least partly, but they have not yet been tested).

   This feature hasn't been tested with multiple threads, but it will only
   show the backtrace of the thread doing the backtracing.

   The feature needs to be enabled with "Configure -Dusecbacktrace".

   The "-Dusecbacktrace" also enables keeping the debug information when
   compiling/linking (often: "-g").  Many compilers/linkers do support
   having both optimization and keeping the debug information.  The debug
   information is needed for the symbol names and the source locations.

   Static functions might not be visible for the backtrace.

   Source code locations, even if available, can often be missing or
   misleading if the compiler has e.g. inlined code.  Optimizer can make
   matching the source code and the object code quite challenging.

       You must have the BFD (-lbfd) library installed, otherwise "perl"
       will fail to link.  The BFD is usually distributed as part of the
       GNU binutils.

       Summary: "Configure ... -Dusecbacktrace" and you need "-lbfd".

   OS X
       The source code locations are supported only if you have the
       Developer Tools installed.  (BFD is not needed.)

       Summary: "Configure ... -Dusecbacktrace" and installing the
       Developer Tools would be good.

   Optionally, for trying out the feature, you may want to enable
   automatic dumping of the backtrace just before a warning or croak (die)
   message is emitted, by adding "-Accflags=-DUSE_C_BACKTRACE_ON_ERROR"
   for Configure.

   Unless the above additional feature is enabled, nothing about the
   backtrace functionality is visible, except for the Perl/XS level.

   Furthermore, even if you have enabled this feature to be compiled, you
   need to enable it in runtime with an environment variable:
   "PERL_C_BACKTRACE_ON_ERROR=10".  It must be an integer higher than
   zero, telling the desired frame count.

   Retrieving the backtrace from Perl level (using for example an XS
   extension) would be much less exciting than one would hope: normally
   you would see "runops", "entersub", and not much else.  This API is
   intended to be called from within the Perl implementation, not from
   Perl level execution.

   The C API for the backtrace is as follows:


   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
   "-Accflags=-DPERL_DEBUG_READONLY_OPS" to enable code that allocates op
   memory via "mmap", and sets it read-only when it is attached to a
   subroutine.  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 does not apply
   to op slabs belonging to "BEGIN" blocks.

   However, as an 80% solution it is still effective, as it has caught
   bugs in the past.

   When is a bool not a bool?
   On pre-C99 compilers, "bool" is defined as equivalent to "char".
   Consequently assignment of any larger type to a "bool" is unsafe and
   may be truncated.  The "cBOOL" macro exists to cast it correctly.

   On those platforms and compilers where "bool" really is a boolean (C++,
   C99), it is easy to forget the cast.  You can force "bool" to be a
   "char" by compiling with "-Accflags=-DPERL_BOOL_AS_CHAR".  You may also
   wish to run "Configure" with something like

       -Accflags='-Wconversion -Wno-sign-conversion -Wno-shorten-64-to-32'

   or your compiler's equivalent to make it easier to spot any unsafe
   truncations that show up.

   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.


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.

Free Software

Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.

Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.

Free Books

The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.

Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.

Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.