The next major directory is labelled mysys, which stands for MySQL System Library. This is the toolbox directory, for example it has low level routines for file access. The .c files in mysys have procedures and functions that are handy for calling by main programs, for example by the programs in the myisam directory. There are 115 .c files in mysys, so we only can note a sampling.
Sampling of programs on mysql-5.0/mysys
size name comment ---- ---- ------- 17684 charset.c character sets 6165 mf_qsort.c quicksort 5609 mf__tempfile.c temporary files + 112 more *.c programs
Example one: with charset.c routines, you can change the character set.
Example two: mf_qsort.c contains our quicksort package.
Example three: mf_tempfile.c has what's needed for maintaining MySQL's temporary files.
You can see from these examples that mysys is a hodgepodge. That's why we went to the trouble of producing extra documentation in this document to help you analyze mysys's contents.