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MAL modules

The  MAL module signatures are collected in individual files named .../lib(64)/<modulename>.mal.  The server is bootstrapped by processing a MAL script, called .../lib/MonetDB5/mal_init.mal, with the names of the modules to be loaded. For each module file encountered, the object library lib_<modulename>.so is searched for in the location identified by monet_mod_path=exec_prefix/lib/MonetDB5:exec_prefix/lib/MonetDB5/lib:exec_prefix/lib/MonetDB5/bin. Loading further relies on the Linux policy to search for the module location in the following order: 1) the colon-separated list of directories in the user's LD_LIBRARY_PATH, 2) the libraries specified in /etc/ld.so.cache and 3) /usr/lib followed by /lib.

The MAL module should be compiled with -rdynamic and -ldl (Linux). This enables loading the routines and finding out the address of a particular routine.  Since all modules libraries are loaded completely with GLOBAL visibility, it suffices to provide the internal function name. In case an attempt to link to an address fails, a final attempt is made to locate the *.o file in the current directory.

A module loading conflict emerges if a function is redefined. A duplicate load is simply ignored by keeping track of modules already loaded.