What Every C Programmer Should Know About Undefined Behavior

https://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html

by mv9 • 5 years ago

59 12 5 years ago

blog.llvm.org

deaddabe3• 5 years ago

Nice read, I recommend the 3 articles of the series.

One thing that I find artificial in the examples is to use signed integers for loops. I always have seen unsigned integers in practice. In general I try to avoid signed integers like plague for big values storage and only use them for error propagation, with negative values for errors like the libc does.

junon2• 5 years ago

The benefit of using signed integers these days is that compilers can utilize signed integer overflow to trap, which is helpful in debugging, since it's undefined behavior whereas unsigned overflow is well defined.

flqn• 5 years ago

Signed integral operations are often faster too, since the well-definedness of unsigned overflow can require additional instructions esp. if the width of the unsigned integral is not the same as the architecture word width.

Someone• 5 years ago

I don't know whether the standard guarantees you can index into arrays with large unsigned ints (it says "integer type”. I don’t know whether that means it will support both without silently converting them, or whether that means an implementation can pick one)

Also, I don’t know whether it (¿still?) is valid C, but I’ve used negative indexes in the past in graphics applications, where it is convenient to have an 'array' with indices that dont start at zero. One-dimensional example:

   #define N 100
   char a[2 * N + 1];
   char * b = a + 100;
   ...
   for(int x = -N; x <= N; ++x) b[x] = 42;

tpoacher• 5 years ago

omg that looks horrible! don't do that! xD

avianes• 5 years ago

There is an example of negative array indexing within standard lib implementations.

Functions like isprint(int c), which checks whether c is a printable ASCII character, can take as input numbers between -128 and 127 or between 0 and 255.

To achieve this, implementations usually rely on single array indexable from -128 to 255 (a lookup table).

Example from the newlib: https://sourceware.org/git/?p=newlib-cygwin.git;a=blob;f=new...

And the array came (indirectly) from: https://sourceware.org/git/?p=newlib-cygwin.git;a=blob;f=new...

b3morales• 5 years ago

And the counterpoint, [What Every Compiler Writer Should Know About Programmers][0] (PDF only, unfortunately).

Also for further reading see much of John Regehr's blog, e.g. [Taming Undefined Behavior in LLVM][1].

[0] https://c9x.me/compile/bib/ubc.pdf

[1] https://blog.regehr.org/archives/1496/

What Every C Programmer Should Know About Undefined Behavior

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