Linking and Loading

1
Linking and Loading

• Linker collects procedures and links them
  together object modules into one executable
  program.
• Why isn't everything written as just one big
  program, saving the necessity of linking?
• Efficiency if just one function is changed in a
  100K line program, why recompile the whole
  program? Just recompile the one function and
  relink.
• Multiple-language programs
• Other reasons?

2
Relocation

• Before widespread support for virtual memory,
  code had to be relocatable (could not contain
  fixed memory addresses)
• With VM, when a page is swapped out, bringing it
  back in makes the references change (update PT)
• With shared libraries, each library decides on
  its own (fixed) addresses ? collision of
  addresses
• Relocation problem same as before virtual memory
• What is the problem? No fixed addresses (calls,
  vars, ifs)
• What is the solution? Shared libraries!

3
Object Modules linking and loading

• Structure of an object module
• Entry points
• External symbols
• Binding time and dynamic relocation possible
  binding times
• Virtual addresses
• Relocation register
• Use addresses relative to program counter

4
Dynamic linking

• Unix systems Code is typically compiled as a
  dynamic shared object (DSO)
• Dynamic vs. static linking gcc -static hello.c
  -o hello-static gcc hello.c -o hello-dynamic
  ls -l hello 80 hello.c 13724
  hello-dynamic 383 hello.s1688756
  hello-static
• if you are the sys admin, which do you prefer?

5
Advantages of dynamic linking

• The executable is smaller (it not include the
  library information explicitly),
• When the library is changed, the code that
  references it does not usually need to be
  recompiled.
• The executable accesses the .DLL at run time
  therefore, multiple codes can access the same
  .DLL at the same time (saves memory)

6
Shortcomings of dynamic linking

• Performance hit 10
• Need to load shared objects (once)
• Need to resolve addresses (once or every time)
• What if the necessary dynamic library is
  missing?
• Could have the library, but wrong version

7
Unix Dynamic Objects (.so)

• Host library
• Target library
• Compiler Options (cont)
• -nostartfiles skip linking of standard start
  files, like /usr/lib/crt0,1.o, /usr/lib/crti.o,
  etc.
• -static link only to static (.aarchive)
  libraries
• -shared if possible, prefer shared libraries over
  static
• Linker Options (gcc gives unknown options to
  linker)
• -l lib (default naming convention liblib.a)
• -L lib path (in addition to default /usr/lib and
  /lib)
• -s strip final executable code of symbol and
  relocation tables

8
DLL dynamic link libraries

• Static Windows libraries are .LIB
• Implicit linking
• Statically linked to import library
• Checks which DLLs in memory loads missing ones
• Have to map DLLs into program address space
• Explicit linking
• prog makes explicit runtime to bind to DLL, then
• makes a call per procedure to determine address
• then calls procedure.
• What are the pros/cons of the two approaches?