Help-Site Computer Manuals
  Algorithms & Data Structures   Programming Languages   Revision Control
  Cameras   Computers   Displays   Keyboards & Mice   Motherboards   Networking   Printers & Scanners   Storage
  Windows   Linux & Unix   Mac

Perl interface to C compiled code.

C::DynaLib - Perl interface to C compiled code.


C::DynaLib - Perl interface to C compiled code.


  use C::DynaLib;

  use sigtrap;  # recommended

  $lib = new C::DynaLib( $linker_arg );

  $func = $lib->DeclareSub( $symbol_name

                            [, $return_type [, @arg_types] ] );

  # or

  $func = $lib->DeclareSub( { "name"    => $symbol_name,

                              [param => $value,] ... } );

  # or

  use C::DynaLib qw(DeclareSub);

  $func = DeclareSub( $function_pointer,

                      [, $return_type [, @arg_types] ] );

  # or

  $func = DeclareSub( { "ptr" => $function_pointer,

                        [param => $value,] ... } );

  $result = $func->( @args );

  $callback = new C::DynaLib::Callback( \&my_sub,

                        $return_type, @arg_types );

  $callback_pointer = $callback->Ptr();


If you have a C compiler that Perl supports, you will get better results by writing XSubs than by using this module. I GUARANTEE IT! It may take you longer to do what you want, but your code will be much more solid and portable. See perlxs.

This module brings ``pointers'' to Perl. Perl's non-use of pointers is one of its great strengths. If you don't know what I mean, then maybe you ought to practice up a bit on C or C++ before using this module. If anything, pointers are more dangerous in Perl than in C, due to Perl's dynamic, interpretive nature.

The XSub interface and Perl objects provide a means of calling C and C++ code while preserving Perl's abstraction from pointers. Once again, I urge you to check out perlxs! It's really cool!!!


This module allows Perl programs to call C functions in dynamic libraries. It is useful for testing library functions, writing simple programs without the bother of XS, and generating C function pointers that call Perl code.

Your Perl must be of the dynamic variety and have a working DynaLoader to use the dynamic loading capabilities of this module. Be sure you answered ``y'' when Configure (from the Perl source kit) asked, ``Do you wish to use dynamic loading?''.

The mechanics of passing arguments to and returning values from C functions vary greatly among machines, operating systems, and compilers. Therefore, Makefile.PL checks the Perl configuration and may even compile and run a test program before the module is built.

This module is divided into two packages, C::DynaLib and C::DynaLib::Callback. Each makes use of Perl objects (see perlobj) and provides its own constructor.

A C::DynaLib object corresponds to a dynamic library whose functions are available to Perl. A C::DynaLib::Callback object corresponds to a Perl sub which may be accessed from C.

C::DynaLib public constructor

The argument to new may be the file name of a dynamic library. Alternatively, a linker command-line argument (e.g., "-lc") may be specified. See DynaLoader(3) for details on how such arguments are mapped to file names.

On failure, new returns undef. Error information might be obtainable by calling DynaLoader::dl_error().

Declaring a library routine

Before you can call a function in a dynamic library, you must specify its name, the return type, and the number and types of arguments it expects. This is handled by DeclareSub.

C::DynaLib::DeclareSub can be used as either an object method or an ordinary sub. You can pass its arguments either in a list (what we call positional parameters) or in a hash (named parameters).

The simplest way to use DeclareSub is as a method with positional parameters. This form is illustrated in the first example above and both examples below. When used in this way, the first argument is a library function name, the second is the function return type, and the rest are function argument types.

THIS IS VERY IMPORTANT. You must not forget to specify the return type as the second argument to DeclareSub. If the function returns void, you should use "" as the second argument.

C data types are specified using the codes used by Perl's pack and unpack operators. See perlfunc(1) for their description. As a convenience (and to hide system dependencies), PTR_TYPE is defined as a code suitable for pointer types (typically "i").

The possible arguments to DeclareSub are shown below. Each is listed under the name that is used when passing the arguments in a hash.

The name of a function exported by $lib. This argument is ignored in the non-method forms of DeclareSub.

The address of the C function. This argument is required in the non-method forms of DeclareSub. Either it or the name must be specified in the method forms.

The return type of the function, encoded for use with the pack operator. Not all of the pack codes are supported, but the unsupported ones mostly don't make sense as C return types. Functions that return a struct are not supported. However, a pointer to struct is okay.

Many C functions return pointers to various things. If you have a function that returns char * and all you're interested in is the string (i.e., the char sequence pointed to, up to the first nul), then you may use "p" as the return type. The "P" code (followed by a number of bytes) is also permissible.

For the case where a returned pointer value must be remembered (for example, malloc()), use PTR_TYPE. The returned scalar will be the pointer itself. You can use unpack to find the thing pointed to.

A list of the types of arguments expected by the function, specified using the notation of Perl's pack operator. For example, "i" means an integer, "d" means a double, and "p" means a nul-terminated string pointer. If you need to handle pointers to things other than Perl scalars, use type PTR_TYPE.

Note: you probably don't want to use "c" or "s" here, since C normally converts the corresponding types (char and short) to int when passing them to a function. The C::DynaLib package may or may not perform such conversions. Use "i" instead. Likewise, use "I" in place of "C" or "S", and "d" in place of "f". Stick with "i", "I", "d", "p", "P", and PTR_TYPE if you want to be safe.

Passing structs by value is not generally supported, but you might find a way to do it with a given compiler by experimenting.

Allows you to specify a function's calling convention. This is possible only with a named-parameter form of DeclareSub. See below for information about the supported calling conventions.

A library reference obtained from either DynaLoader::dl_load_file or the C::DynaLib::LibRef method. You must use a named-parameter form of DeclareSub in order to specify this argument.

Calling a declared function

The returned value of DeclareSub is a code reference. Calling through it results in a call to the C function. See perlref(1) on how to use code references.

Using callback routines

Some C functions expect a pointer to another C function as an argument. The library code that receives the pointer may use it to call an application function at a later time. Such functions are called callbacks.

This module allows you to use a Perl sub as a C callback, subject to certain restrictions. There is a hard-coded maximum number of callbacks that can be active at any given time. The default (4) may be changed by specifying CALLBACKS=number on the Makefile.PL command line.

A callback's argument and return types are specified using pack codes, as described above for library functions. Currently, the return value must be interpretable as type int or void, so the only valid codes are "i", "I", and "". There are also restrictions on the permissible argument types, especially for the first argument position. These limitations are considered bugs to be fixed someday.

To enable a Perl sub to be used as a callback, you must construct an object of class C::DynaLib::Callback. The syntax is

  $cb_ref = new C::DynaLib::Callback( \&some_sub,

                    $ret_type, @arg_types );

where $ret_type and @arg_types are the pack-style types of the function return value and arguments, respectively. \&some_sub must be a code reference or sub name (see perlref).

$cb_ref->Ptr() then returns a function pointer. C code that calls it will end up calling &some_sub.


This code loads and calls the math library function sinh(). It assumes that you have a dynamic version of the math library which will be found by DynaLoader::dl_findfile("-lm"). If this doesn't work, replace "-lm" with the name of your dynamic math library.

  use C::DynaLib;

  $libm = new C::DynaLib("-lm");

  $sinh = $libm->DeclareSub("sinh", "d", "d");

  print "The hyperbolic sine of 3 is ", &{$sinh}(3), "\n";

  # The hyperbolic sine of 3 is 10.0178749274099

The following example uses the C library's strncmp() to compare the first n characters of two strings:

  use C::DynaLib;

  $libc = new C::DynaLib("-lc");

  $strncmp = $libc->DeclareSub("strncmp", "i", "p", "p", "I");

  $string1 = "foobar";

  $string2 = "foolish";

  $result = &{$strncmp}($string1, $string2, 3);  # $result is 0

  $result = &{$strncmp}($string1, $string2, 4);  # $result is -1

The files and README.win32 contain examples using callbacks.


This section is intended for anyone who is interested in debugging or extending this module. You probably don't need to read it just to use the module.

The problem

The hardest thing about writing this module is to accommodate the different calling conventions used by different compilers, operating systems, and CPU types.

``What's a calling convention?'' you may be wondering. It is how compiler-generated functions receive their arguments from and make their return values known to the code that calls them, at the level of machine instructions and registers. Each machine has a set of rules for this. Compilers and operating systems may use variations even on the same machine type. In some cases, it is necessary to support more than one calling convention on the same system.

``But that's all handled by the compiler!'' you might object. True enough, if the calling code knows the signature of the called function at compile time. For example, consider this C code:

  int foo(double bar, const char *baz);


  int res;

  res = foo(sqrt(2.0), "hi");

A compiler will generate specific instruction sequences to load the return value from sqrt() and a pointer to the string "hi" into whatever registers or memory locations foo() expects to receive them in, based on its calling convention and the types double and char *. Another specific instruction sequence stores the return value in the variable res.

But when you compile the C code in this module, it must be general enough to handle all sorts of function argument and return types.

``Why not use varargs/stdarg?'' Most C compilers support a special set of macros that allow a function to receive a variable number of arguments of variable type. When the function receiving the arguments is compiled, it does not know with what argument types it will be called.

But the code that calls such a function does know at compile time how many and what type of arguments it is passing to the varargs function. There is no ``reverse stdarg'' standard for passing types to be determined at run time. You can't simply pass a va_list to a function unless that function is defined to receive a va_list. This module uses varargs/stdarg where appropriate, but the only appropriate place is in the callback support.

The solution (well, half-solution)

Having failed to find a magic bullet to spare us from the whims of system designers and compiler writers, we are forced to examine the calling conventions in common use and try to put together some ``glue'' code that stands a chance of being portable.

In writing glue code (that which allows code written in one language to call code in another), an important issue is reliability. If we don't get the convention just right, chances are we will get a core dump (protection fault or illegal instruction).

To write really solid Perl-to-C glue, we would have to use assembly language and have detailed knowledge of each calling convention. Compiler source code can be helpful in this regard, and if your compiler can output assembly code, that helps, too.

However, this is Perl, Perl is meant to be ported, and assembly language is generally not portable. This module typically uses C constructs that happen to work most of the time, as opposed to assembly code that follows the conventions faithfully.

By avoiding the use of assembly, we lose some reliability and flexibility. By loss of reliability, I mean we can expect crashes, especially on untested platforms. Lost flexibility means having restrictions on what parameter types and return types are allowed.

The code for all conventions other than hack30 (described below) relies on C's alloca() function. Unfortunately, alloca() itself is not standard, so its use introduces new portability concerns. For cdecl (the most general convention) Makefile.PL creates and runs a test program to try to ferret out any compiler peculiarities regarding alloca(). If the test program fails, the default choice becomes hack30.

Supported conventions

C::DynaLib currently supports the parameter-passing conventions listed below. The module can be compiled with support for one or more of them by specifying (for example) DECL=cdecl on Makefile.PL's command-line. If none are given, Makefile.PL will try to choose based on your perl configuration and/or the results of running a test program.

At run time, a calling convention may be specified using a named-parameter form of DeclareSub (described above), or a default may be used. The first DECL=... supplied to Makefile.PL will be the default convention.

Note that the convention must match that of the function in the dynamic library, otherwise crashes or incorrect results are likely to occur.

All arguments are placed on the stack in reverse order from how the function is invoked. This seems to be the default for Intel-based machines and some others.

The first 24 bytes of arguments are cast to an array of six ints. The remaining args (and possibly piece of an arg) are placed on the stack. Then the C function is called as if it expected six integer arguments. On a Sparc, the six ``pseudo-arguments'' are passed in special registers.

This is similar to the sparc convention, but the pseudo-arguments have type long instead of int, and all arguments are extended to eight bytes before being placed in the array. On the AXP, a special sequence of assembly instructions is used to ensure that any function parameters of type double are passed correctly.

This is not really a calling convention, it's just some C code that will successfully call a function most of the time on a variety of systems. All arguments are copied into an array of 6 long integers (or 30 if 6 is not enough). The function is called as if it expected 6 (or 30) long arguments.

You will run into problems if the C function either (1) takes more arguments than can fit in the array, (2) takes some non-long arguments on a system that passes them differently from longs (but cdecl currently has the same flaw), or (3) cares if it is passed extra arguments (Win32 API functions crash because of this).

Because of these problems, the use of hack30 is recommended only as a quick fix until your system's calling convention is supported.


Several unresolved issues surround this module.


The ``glue'' code that allows Perl values to be passed as arguments to C functions is architecture-dependent. This is because the author knows of no standard means of determining a system's parameter-passing conventions or passing arguments to a C function whose signature is not known at compile time.

Although some effort is made in Makefile.PL to find out how parameters are passed in C, this applies only to the integer type (Perl's I32, to be precise). Functions that recieve or return type double, for example, may not work on systems that use floating-point registers for this purpose. Specialized code may be required to support such systems.


Usually, Perl programs run under the control of the Perl interpreter. Perl is extremely stable and can almost guarantee an environment free of the problems of C, such as bad pointers causing memory access violations. Some modules use a Perl feature called ``XSubs'' to call C code directly from a Perl program. In such cases, a crash may occur if the C or XS code is faulty. However, once the XS module has been sufficiently debugged, one can be reasonably sure that it will work right.

Code called through this module lacks such protection. Since the association between Perl and C is made at run time, errors due to incompatible library interfaces or incorrect assumptions have a much greater chance of causing a crash than with either straight Perl or XS code.


This module does not require special privileges to run. I have no reason to think it contains any security bugs (except to the extent that the known bugs impact security). However, when this module is installed, Perl programs gain great power to exploit C code which could potentially have such bugs. I'm not really sure whether this is a major issue or not.

I haven't gotten around to understanding Perl's internal tainting interface, so taint-checking may not accomplish what you expect. (See perlsec)

Deallocation of Resources

To maximize portability, this module uses the DynaLoader interface to dynamic library linking. DynaLoader's main purpose is to support XS modules, which are loaded once by a program and not (to my knowledge) unloaded. It would be nice to be able to free the libraries loaded by this module when they are no longer needed. This will be impossible, as long as DynaLoader provides no means to do so.

Literal and temporary strings

Before Perl 5.00402, it was impossible to pass a string literal as a pointer-to-nul-terminated-string argument of a C function. For example, the following statement (incorrectly) produced the error Modification of a read-only value attempted:

  &$strncmp("foo", "bar", 3);

To work around this problem, one must assign the value to a variable and pass the variable in its place, as in

  &$strncmp($dummy1 = "foo", $dummy2 = "bar", 3);


Only a certain number of callbacks can exist at a time. Callbacks can mess up the message produced by die in the presence of nested evals. The Callback code uses global data, and is consequently not thread-safe.

Miscellaneous Bugs

There are restrictions on what C data types may be used. Using argument types of unusual size may have nasty results. The techniques used to pass values to and from C functions are generally hackish and nonstandard. Assembly code would be more complete. Makefile.PL does too much. I haven't yet checked for memory leaks.


Rewrite using GNU ffcall. Fiddle with autoloading so we don't have to call DeclareSub all the time. Mangle C++ symbol names. Get Perl to understand C header files (macros and function declarations) with enough confidence to make them useful here.


Copyright (c) 1997, 2000 by John Tobey. This package is distributed under the same license as Perl itself. There is no expressed or implied warranty, since it is free software. See the file README in the top level Perl source directory for details. The Perl source may be found at:


John Tobey,


perl(1), perlfunc(1) (for pack), perlref(1), sigtrap(3), DynaLoader(3), perlxs(1), perlcall(1).