Install expat (probably it is already installed. at least on Mac is installed by the OS)

mac: gcc -dynamiclib -undefined dynamic_lookup -m32 -lexpat -o expat.so expat.c
linux: gcc -shared -m32 -lexpat -o expat.so expat.c

for 64bit use -m64.

You will need k.h to compile as above. See http://code.kx.com/wiki/Cookbook/ExtendingWithC for details and download link.

• copy expat.so in the right location for your kdb+ setup ($QHOME/<l32|l64|m32|m64>/)
• copy expat.q in a directory searchable by q (i use $QHOME/c)

\l <install dir for expat.q>/expat.q  
xmlparse "<tag/>"  
xmlparse ("<tag attr1=\"1234\">";"text";"</tag>")  
xmlparse read0 `:a_file.xml