For and overview and related topics, see Introduction to PEXlib.
The PEXlib specification currently defines include files for the C language only. The following include files contain constant definitions, macros, and data structures used by PEXlib: <X11/PEX5/PEX.h> and <X11/PEX5/PEXlib.h>. <X11/PEX5/PEXlib.h> automatically includes <X11/PEX5/PEX.h>.
Some data structures have fields named "reserved." These fields are necessary to align the data correctly for efficient transfer to the protocol transport buffer. PEXlib ignores any values assigned to these fields.