PERLHACKTIPS(1) Perl Programmers Reference Guide PERLHACKTIPS(1)
NAME
perlhacktips - Tips for Perl core C code hacking
DESCRIPTION
This document will help you learn the best way to go about hacking on the Perl core C code. It covers common problems, debugging,
profiling, and more.
If you haven't read perlhack and perlhacktut yet, you might want to do that first.
COMMON PROBLEMS
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_ ...);
sv_setiv(...);
The first one explicitly passes in the context, which is needed for e.g. threaded builds. The second one does that implicitly; do not
get them mixed. If you are not passing in a aTHX_, you will need to do a dTHX (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
compatibility).
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.
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 embed.pl 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 embed.pl 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 embed.pl in perlguts.
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 still compile Perl with just the bare minimum of interfaces. (See
README.micro.)
Do not assume an operating system indicates a certain compiler.
o Casting pointers to integers or casting integers to pointers
void castaway(U8* p)
{
IV i = p;
or
void castaway(U8* p)
{
IV i = (IV)p;
Both are bad, and broken, and unportable. Use the PTR2IV() macro that does it right. (Likewise, there are PTR2UV(), PTR2NV(),
INT2PTR(), and NUM2PTR().)
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 data
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 to 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
o That a field is of certain signedness, sizeof, or type
o That the fields are in a certain order
o While C guarantees the ordering specified in the struct definition, between different platforms the definitions might
differ
o That the sizeof(struct) or the alignments are the same everywhere
o There might be padding bytes between the fields to 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 field
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 '
', but not 12. 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 Mixing #define and #ifdef
#define BURGLE(x) ...
#ifdef BURGLE_OLD_STYLE /* BAD */
... do it the old way ...
#else
... do it the new way ...
#endif
You cannot portably "stack" cpp directives. For example in the above you need two separate BURGLE() #defines, one for each #ifdef
branch.
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 {
CERULEAN,
CHARTREUSE,
CINNABAR, /* BAD */
};
is not portable. Leave out the last comma.
Also note that whether enums are implicitly morphable to ints varies between compilers, you might need to (int).
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.
The gcc option "-Wdeclaration-after-statements" scans for such problems (by default on starting from Perl 5.9.4).
o Introducing variables inside for()
for(int i = ...; ...; ...) { /* BAD */
That is C99 or C++. While it would indeed be awfully nice to have that also in C89, to limit the scope of the loop variable, alas, we
cannot.
o 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.
o Macros that have string constants and their arguments as substrings of the string constants
#define FOO(n) printf("number = %d
", n) /* BAD */
FOO(10);
Pre-ANSI semantics for that was equivalent to
printf("10umber = %d10");
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
", 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
", 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"
", i); /* The IVdf is a string constant. */
Uid_t_f /* Uid_t in decimal */
printf("who = %"Uid_t_f"
", who);
Or you can try casting to a "wide enough" type:
printf("i = %"IVdf"
", (IV)something_very_small_and_signed);
Also remember that the %p format really does require a void pointer:
U8* p = ...;
printf("p = %p
", (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();
#endif
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();
#endif
How does the HAS_QUUX become defined where it needs to be? Well, if Foonix happens to be Unixy enough to be able to run the Configure
script, and Configure has been taught about detecting and testing quux(), the HAS_QUUX will be correctly defined. In other platforms,
the corresponding configuration step will hopefully do the same.
In a pinch, if you cannot wait for Configure to be educated, or if you have a good hunch of where quux() might be available, you can
temporarily try the following:
#if (defined(__FOONIX__) || defined(__BARNIX__))
# define HAS_QUUX
#endif
...
#ifdef HAS_QUUX
foo = quux();
#endif
But in any case, try to keep the features and operating systems separate.
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 wrappers.)
o snprintf() - the return type is unportable. Use my_snprintf() instead.
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 Perl_sv_catpvf().
DEBUGGING
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
make
"-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.
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.
(gdb)
"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
Steps through the program a line at a time, without descending into functions.
o continue
Run until the next breakpoint.
o finish
Run until the end of the current function, then stop again.
o 'enter'
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
311 dPOPTOPnnrl_ul;
(gdb)
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:
Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
1669 if (!sv)
(gdb)
We can now use "Perl_sv_dump" to investigate the SV:
SV = PV(0xa057cc0) at 0xa0675d0
REFCNT = 1
FLAGS = (POK,pPOK)
PV = 0xa06a510 "6XXXX"