ld a.o b.o, it complain that :
ld: warning: cannot find entry symbol _start; defaulting to 08048074
a.o(.text+0xf): In function `my_main()':
/tmp/a.cxx:8: undefined reference to `printf'
b.o(.text+0x11): In function `main':
/tmp/b.c:6: undefined reference to `my_main'