» CS 330 Lecture 19 – Weak Typing in C

CS 330 Lecture 19 – Weak Typing in C

March 11, 2016 by Chris Johnson. Filed under cs330, lectures, spring 2016.

Agenda

what ?s
program this
weak typing examples
- array allocation
- big endian
- is negative check for floats
- unions
“polymorphism” in C

TODO

The midterm exam will be during class next Wednesday. General topics we’ve covered include the shell, regular expressions, grammars, abstract syntax trees, the mechanics of assembly, and type systems through the lens of C. Exam questions are designed to use knowledge gained from homework and synthesize ideas. I’m not so interested in you having memorized a bunch of facts or API. See the past semesters’ exams under the Teaching tab.

Note

Let’s start with a Program This:

In pseudo-C code, write down an implementation of itoa, which converts an int (assumed >= 0) into a C string.

From there we’ll look to some examples of weak typing in C, in which we undermine the type system to do some sometimes perfectly reasonable things. We’ll see why arrays traditionally couldn’t be allocated on the stack via consts, how we can check the negativity of a float, and how one might support a limited form of polymorphism for data structures.

Code

itoa.c

/usr/lib/ruby/2.7.0/rubygems/dependency.rb:311:in `to_specs': Could not find 'coderay' (>= 0) among 56 total gem(s) (Gem::MissingSpecError)
Checked in 'GEM_PATH=/.gem/ruby/2.7.0:/var/lib/gems/2.7.0:/usr/lib/ruby/gems/2.7.0:/usr/share/rubygems-integration/2.7.0:/usr/share/rubygems-integration/all:/usr/lib/x86_64-linux-gnu/rubygems-integration/2.7.0:/home/johnch/.gems', execute `gem env` for more information
	from /usr/lib/ruby/2.7.0/rubygems/dependency.rb:323:in `to_spec'
	from /usr/lib/ruby/2.7.0/rubygems/core_ext/kernel_gem.rb:62:in `gem'
	from ./coderay:24:in `'