18.10 Building mixed C/Caml libraries: ocamlmklib

The ocamlmklib command facilitates the construction of libraries containing both Caml code and C code, and usable both in static linking and dynamic linking modes.


This command is available only under Cygwin, but not for the native Win32 port.

The ocamlmklib command takes three kinds of arguments:

It generates the following outputs:

C object files.

In addition, the following options are recognized:


Consider a Caml interface to the standard libz C library for reading and writing compressed files. Assume this library resides in /usr/local/zlib. This interface is composed of a Caml part zip.cmo/zip.cmx and a C part zipstubs.o containing the stub code around the libz entry points. The following command builds the Caml libraries zip.cma and zip.cmxa, as well as the companion C libraries dllzip.so and libzip.a:

ocamlmklib -o zip zip.cmo zip.cmx zipstubs.o -lz -L/usr/local/zlib

If shared libraries are supported, this performs the following commands:

ocamlc -a -o zip.cma zip.cmo -dllib -lzip \
        -cclib -lzip -cclib -lz -ccopt -L/usr/local/zlib
ocamlopt -a -o zip.cmxa zip.cmx -cclib -lzip \
        -cclib -lzip -cclib -lz -ccopt -L/usr/local/zlib
gcc -shared -o dllzip.so zipstubs.o -lz -L/usr/local/zlib
ar rc libzip.a zipstubs.o

If shared libraries are not supported, the following commands are performed instead:

ocamlc -a -custom -o zip.cma zip.cmo -cclib -lzip \
        -cclib -lz -ccopt -L/usr/local/zlib
ocamlopt -a -o zip.cmxa zip.cmx -lzip \
        -cclib -lz -ccopt -L/usr/local/zlib
ar rc libzip.a zipstubs.o

Instead of building simultaneously the bytecode library, the native-code library and the C libraries, ocamlmklib can be called three times to build each separately. Thus,

ocamlmklib -o zip zip.cmo -lz -L/usr/local/zlib

builds the bytecode library zip.cma, and

ocamlmklib -o zip zip.cmx -lz -L/usr/local/zlib

builds the native-code library zip.cmxa, and

ocamlmklib -o zip zipstubs.o -lz -L/usr/local/zlib

builds the C libraries dllzip.so and libzip.a. Notice that the support libraries (-lz) and the corresponding options (-L/usr/local/zlib) must be given on all three invocations of ocamlmklib, because they are needed at different times depending on whether shared libraries are supported.