some dark corners of c

Some dark corners of C-std=c99 -pedantic -Wall -Warn-me-harder

Stolen from Rob Kendrick <rjek+dark corners@rjek.com>

Rob Kendrick <rob.kendrick@codethink.co.uk>

Dark corner of C

Here it is!

Shadowy escape

Returns 3840 Returns 42

Bizarre keyword reusevoid foo(int x[static 10]);

Passed array must be at least 10 entries.

void foo(char x[static 1]);

Free compile-time non-NULL check!

Warning: Don't breathe this

Yes, that's right, it makes demons fly out of your nose!

(On my system, 1,179,602,721.)

If you do this, I will find you.

Yes, single quotes.

[ ] is just + in disguise

Pointer aliasing

Compiler doesn't

know that *z won't also be *x, so loads it twice :(

Pointer aliasing: Fixed?

Only one load, at the

cost of beauty.

Pointer aliasing: Fixed!

'restrict' means we

promise not to mess about.

Counting up ...

... vs counting down

const confusion

"Constant" integer foo with value 10.And a "constant" pointer to it?

Oops. binds left. But if there's nothing to its left, it binds to the right.

Munchy munchy.

The C specification says that when there is such an ambiguity, munch as much as possible. (The "greedy lexer rule".)

Munchy munchy. Again.

Alas, not

But

Parser go boom.

C keywordsauto break case char const continue default do

double else enum extern float for goto if int long register restrict return short signed sizeof static struct switch typedef union unsigned void

volatile while

Wait, what? is a bizarreness left over from B. The only place it's valid, it's also the default. And that's to say a

variable should be automatically managed, ie, placed on the stack. is its antonym. Ish.

Smallest C programWhat's the smallest C program that will compile and link?

(Don't run this.)

Alas it produces a warning. So we can do this:

Global variables are filthyfoo1.c:

foo2.c:

Global variables are filthyfoo1.c:

foo2.c:

Global variables are filthyfoo1.c:

foo2.c:

// comments are evil

C89: 5 C++: 10 C99: 10

Portable lossless floats

Ruin somebody's dayWhat would you sneak into somebody's headers to drive them mad?

No valid C program will fail to compile with these.

Recommended reads

Sadly very few modern books on the subject of C :(

Thanks for ideas/input...David Thomas

Daniel SilverstoneClive Jones

Peter Van Der LindenErlend Hamberg