xemacs-beta / dynodump / README

		DYNODUMP
		--------

Dynodump, not to be confused with DinoTurd (as seen in Jurassic Park),
is a shared object that provides one function:

	int dynodump(char *new_file);

dynodump(), called from a running program will write a new executable
in new_file a la unexec() in GNU Emacs.  The difference lies in the
relocations.

dynodump() will create an image with any relocations (which were
performed by the run-time dynamic linker) undone.  This allows the new
image to be run in a different environment.  There is, however, one
potentially major caveat.  If a symbol reference gets updated during
the running of the calling program, its updated value will be lost.
An example (with additional blank lines for legibility):

	$ cat lib.c
	char _foo[] = "hello";
	char _bar[] = "world";

	$ cc -G -o lib.so lib.c

	$ cat prog.c
	extern char _foo, _bar;
 
	int beenhere = 0;
	char * foo = &_foo;
	char * bar = &_bar;
 
	int
	main(void)
	{
	    (void) printf("%d: foo = %x\n", beenhere, foo);
	    (void) printf("%d: bar = %x, ", beenhere, bar);
 
	    if (!beenhere) {
	        beenhere = 1;
	        bar++;
	        dynodump("newfile");
	    }
	    (void) printf("%x\n", bar);
	}

	$ cc -o prog prog.c -R. lib.so dynodump.so

	$ ./prog
	0: foo = ef7503cc
	0: bar = ef7503d2, ef7503d3

	$ ./newfile
	1: foo = ef7503cc
	1: bar = ef7503d2, ef7503d2

Notice that in the run of newfile, bar points at "world" instead of
the perhaps expected "orld".

Dynodump supports sparc, intel, and power pc architectures.

Dynodump is buildable with GNU make and gcc.  If it works for you
with these tools, let me know.

unexec()
--------

Also supplied is an unexsol2.c which belongs in the XEmacs src
directory with the other unex*.c files.  The src/s/sol2.h should be
have the following added to it:

	#ifdef UNEXEC
	#undef UNEXEC
	#endif
	#define UNEXEC unexsol2.o

This unexec() will attempt to dlopen() the dynodump.so to find the
dynodump function.  The default is "../dynodump/dynodump.so" which
should be appropriate for the typical XEmacs build (unless you used
configure to set up separate build and source trees).  You may change
it by setting the DYNODUMP environment variable to the full path of
the dynodump shared object.

Other notes:

If you're using the 4.0 C compiler from Sun, you should arrange to
shut off the incremental linker.  This is done with the -xildoff
option to the compiler.  The common.mk and testsuite/Makefile files
have commentary and ready made lines for uncommenting for this
purpose.

If you're interested in playing with the UltraSPARC specific options
to the aforementioned compiler, then the same makefiles have some
commentary and flags settings to point you in that direction.

Questions:

dynodump() was developed by Rod.Evans@Eng.Sun.COM and
Georg.Nikodym@Canada.Sun.COM.  If you have questions, feel free to ask
them but be aware that Rod, "don't know jack about emacs."
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