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Compile:
$ gcc testld.c -o testld
$ gcc -c mymemcpy.c && ld -G mymemcpy.o -o mymemcpy.so -ldl

Run:
$ ./testld
$ cat output.txt
$ LD_PRELOAD=/home/bz/Dev/Src/bz/C/testld/mymemcpy.so /home/bz/Dev/Src/bz/C/testld/testld
$ cat output.txt

and feel the difference...
(c) 2010 baltazar