Object files

• An object file (.o) contains a table of symbols used in the corresponding source file (.cxx)
• The symbols whose definition was given in the corresponding source file are resolved
• The symbols whose definition is found in another source file are unresolved
• Unresolved symbols in an object file can be resolved by linking the object with another object file containing the missing definitions
• An executable cannot contain any unresolved symbol
• A group of object files file1.o, ..., fileN.o can be linked together as a single executable file by the command c++ -o file file1.o ... fileN.o only if:
  1. the symbol main is resolved exactly once in exactly one object file in the group
  2. for each object file in the group and for each unresolved symbol in the object file, the symbol must be resolved in exactly one other file of the group