path: root/runtime-ld.txt

.:: Runtime ld ::.
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# toc
------
  |ld_so|
    |load_init_order|

# ld.so(8)                                                               *ld_so*
===========
  env:
    LD_PRELOAD=<l_so>       colon separated list of libso's to be pre loaded
    LD_DEBUG=<opts>         comman separated list of debug options
            =help           list available options
            =libs           show library search path
            =files          processing of input files
            =symbols        show search path for symbol lookup
            =bindings       show against which definition a symbol is bound


  ## LD_PRELOAD load & init order                              *load_init_order*
    > ldd ./main
      >> libc.so.6 => /usr/lib/libc.so.6

    > LD_PRELOAD=liba.so:libb.so ./main
               -->
        preloaded in this order
               <--
        initialized in this order

    - preload order determines the order libs are inserted into the link map

    - resulting link map:
        +------+    +------+    +------+    +------+
        | main | -> | liba | -> | libb | -> | libc |
        +------+    +------+    +------+    +------+

    - see preload and init order in action
      > LD_DEBUG=files LD_PRELOAD=liba.so:libb.so ./main
        # load order (-> determines link map)
        >> file=liba.so [0];  generating link map
        >> file=libb.so [0];  generating link map
        >> file=libc.so.6 [0];  generating link map

        # init order
        >> calling init: /usr/lib/libc.so.6
        >> calling init: <path>/libb.so
        >> calling init: <path>/liba.so
        >> initialize program: ./main

    - see the symbol lookup in action and therefore the link map order
      > LD_DEBUG=symbols,bindings LD_PRELOAD=liba.so:libb.so ./main
        >> symbol=memcpy;  lookup in file=./main [0]
        >> symbol=memcpy;  lookup in file=<path>/liba.so [0]
        >> symbol=memcpy;  lookup in file=<path>/libb.so [0]
        >> symbol=memcpy;  lookup in file=/usr/lib/libc.so.6 [0]
        >> binding file ./main [0] to /usr/lib/libc.so.6 [0]: normal symbol
           `memcpy' [GLIBC_2.14]

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