rtapi has a big data structure called rtapi_data, which lives in shared 
memory and is used by all the rtapi modules.  This struct keeps track of 
the list of realtime modules loaded, the amount of shared memory in use, 
etc etc.  It's in src/rtapi/rtapi_common.h [0].

0: 
http://git.linuxcnc.org/gitweb?p=emc2.git;a=blob;f=src/rtapi/rtapi_common.h;h=480cb26dd08cf5456f44b77ccd703489704728f4;hb=HEAD#l171

rtapi_data has a cool feature: it has a "revision code" integer inside 
it, and all users of this shared memory verify that they are using the 
same revision of the struct as everyone else before they'll touch 
it[1].  I guess this is handy when you have out-of-tree rtapi modules 
and forget to recompile them when someone changes rtapi_data_t.  I can't 
think of another use for it...

1: 
http://git.linuxcnc.org/gitweb?p=emc2.git;a=blob;f=src/rtapi/rtai_rtapi.c;h=cf728292d0c168536baab96173bbbaecec5abe63;hb=HEAD#l164

Unfortunately the system is totally broken, and has been since we 
switched to git.  It computes the rtapi_data revision code from the 
$Revision$ CVS Keyword in rtapi_common.h [2], but of course git doesn't 
do any keyword substitution.  We've been using revision code of zero 
since switching to git [3].

2: 
http://git.linuxcnc.org/gitweb?p=emc2.git;a=blob;f=src/rtapi/rtapi_common.h;h=480cb26dd08cf5456f44b77ccd703489704728f4;hb=HEAD#l93
3: 
http://git.linuxcnc.org/gitweb?p=emc2.git;a=blob;f=src/rtapi/rtapi_common.h;h=480cb26dd08cf5456f44b77ccd703489704728f4;hb=HEAD#l223


I want to just rip it out.  But if anyone thinks it's useful and wants 
to keep it, i could try to fix it maybe using the git sha1 of the blob 
of rtapi_common.h and some build-system obscenity.

What does the executive decision from the board say?


-- 
Sebastian Kuzminsky


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