A typemap translates between types in different programming languages. We typically need to do this if we want to call code written in one programming language (say C or C++) from code written in another programming language (say Perl or Python). Note that we have to transfer data in both directions: a function both receives and transmits data!

Tools such as Swig (for calling C and C++ from Perl, Python, TCL, Guile) and XS (for calling C and C++ from Perl) employ typemaps as a structured way of presenting these conversions.

The mapping is sometimes reasonably simple: a C int should map to an integer scalar in Perl (an IV, a type of SV). Usually it isn't. A 64 bit integer value in C is too large to fit into a double precision IEEE floating point field, so it cannot directly be converted to a Perl numeric scalar. But you'll still probably want to do just that in many cases.

Sometimes the mapping involves intent, not just type; you use a named typemap for these cases. C pointers provide the clearest instance. Say I have a parameter of type double *. It can be any of the following:

  • A pointer to the first element of a 3-element vector being passed into the function.
  • A pointer to the first element of an array being passed into the function, the size of which is in another variable.
  • Either of the above, but denoting memory locations in which a vector is returned.
  • The above, but used both for input and for output.
  • A double being passed as if it were a Pascal ref ("call by reference") parameter.
char * pointers are even worse -- they could also refer to NUL terminated strings!

Additionally, it's potentially unclear whether the C function expects to be able to free the pointer, or if it expects the caller to free it.

Perl supports multiple return values (by returning a list), while C doesn't (and uses pointers instead); how should the C function be called?

The solution is to provide separate typemaps for these cases; when "wrapping" the code the correct typemap must be specified. This complexity is inherent in the problem, not in the solution!

See an example of type mapping for a simple specific example.

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