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移除书签The Linker Is not a Magical Program
- Edit source code
- Something magical happens
Step 3 is, of course, the linking step. Why would I say such an outrageous thing? I’ve been doing tech support for decades, and I get the following questions again and again:
- The linker says def is defined more than once.
- Why is my executable so large?
Followed by “What do I do now?” usually with the phrases “seems to” and “somehow” mixed in, and an aura of utter bafflement. It’s the “seems to” and “somehow” that indicate that the linking process is viewed as a magical process, presumably understandable only by wizards and warlocks. The process of compiling does not elicit these kinds of phrases, implying that programmers generally understand how compilers work, or at least what they do.
A linker is a very stupid, pedestrian, straightforward program. All it does is concatenate together the code and data sections of the object files, connect the references to symbols with their definitions, pull unresolved symbols out of the library, and write out an executable. That’s it. No spells! No magic! The tedium in writing a linker is usually all about decoding and generating the usually ridiculously overcomplicated file formats, but that doesn’t change the essential nature of a linker.
which generates an external reference to the symbol . A definition, on the other hand, actually sets aside storage for the symbol, usually appears in the implementation file, and looks like this:
How many definitions can there be for each symbol? As in the film Highlander, there can be only one. So, what if a definition of iii appears in more than one implementation file?
The linker will complain about iii being multiply defined.
To determine why an executable is the size it is, take a look at the map file that linkers optionally generate. A map file is nothing more than a list of all the symbols in the executable along with their addresses. This tells you what modules were linked in from the library, and the sizes of each module. Now you can see where the bloat is coming from. Often there will be library modules that you have no idea why were linked in. To figure it out, temporarily remove the suspicious module from the library, and relink. The undefined symbol error then generated will indicate who is referencing that module.
Although it is not always immediately obvious why you get a particular linker message, there is nothing magical about linkers. The mechanics are straightforward; it’s the details you have to figure out in each case.
