97 Things Every Programmer Should Know

(Chris Devlin) #1

(^106) 97 Things Every Programmer Should Know

The Linker Is Not a Magical Program ..................

Walter Bright

DEPRESSiNGLY OFTEN (happened to me again just before I wrote this), the
view that many programmers have of the process of going from source code to
a statically linked executable in a compiled language is:

  1. Edit source code.

  2. Compile source code into object files.

  3. Something magical happens.

  4. Run executable.

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 con-
cerns again and again:

  1. The linker says def is defined more than once.

  2. The linker says abc is an unresolved symbol.

  3. 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 pro-
cess, 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 stupid, pedestrian, straightforward program. All it does is concate-
nate 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 typically all about decoding and generating the usually ridiculously over-
complicated file formats, but that doesn’t change the essential nature of a linker.

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