[HN Gopher] A primer on some C obfuscation tricks
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A primer on some C obfuscation tricks
Author : simonpure
Score : 54 points
Date : 2020-04-23 20:39 UTC (2 hours ago)
(HTM) web link (github.com)
(TXT) w3m dump (github.com)
| TimSchumann wrote:
| Thanks for sharing this!
| jackhalford wrote:
| > x[index] is *(x+index) > index[x] is legal C and equivalent too
|
| Can't be unseen, I can't believe I never thought of that.
| guitmz wrote:
| I find that after learning assembly language, things like this
| become very obvious when seen in different languages, specially
| C
| roywiggins wrote:
| The main reason that works is historic, I think.
|
| It could just as easily throw a compilation error to index a
| constant with an array rather than the other way around. I
| don't think this works in Rust even if the resulting machine
| code for array indexing is the same.
| eMSF wrote:
| The main reason is that in C, "indexing" an array is purely
| syntactic sugar for pointer arithmetic, which itself is
| commutative; that is. ((A)[B]) is equivalent to ((A)+(B)),
| which itself is equivalent to ((B)+(A)) (assuming one of
| them has an integral type and the other a pointer to
| complete object type).
|
| Now, of course an array type isn't a pointer type, but as
| "indexing" isn't one of the very few cases where an
| expression that has an array type isn't converted to an
| expression with a pointer type, you aren't really indexing
| an array, but a pointer to its first element.
| saagarjha wrote:
| I see no reason why this should be obvious; it's just a
| historical quirk of array indexing in C being a _literal_
| translation to pointer arithmetic. In C++, for example, there
| is no way to support this with the subscript operator.
| inetknght wrote:
| Today I learned... _magic_ #include <cstdio>
| void sw(int s) { switch (s) while (0) {
| case 0: printf("zero\n"); continue;
| case 1: printf("one\n"); continue;
| case 2: printf("two\n"); continue;
| } }
|
| [0] https://gcc.godbolt.org/z/Q26LWG
| lostmsu wrote:
| Huh, what does it do?
| strbean wrote:
| Looking at the link, it generates the exact same assembly
| (and behavior of course) as void sw(int s)
| noexcept { switch (s) {
| case 0: printf("zero\n"); break;
| case 1: printf("one\n"); break;
| case 2: printf("two\n"); break;
| } }
| saagarjha wrote:
| thatsthejoke.jpg
| strbean wrote:
| I mean, parent seemed an awful lot like someone asking
| what it did. Damn me for missing the invisible /s. I'm
| sure no other reader would ever be curious!
| saagarjha wrote:
| It's not the comment that is the joke, but the example in
| the context of the "C obfuscation tricks" article where
| it's just a convoluted way to write the same thing as a
| simple C construct.
| MauranKilom wrote:
| Hey wait, that's just Duff's... Oh.
| saagarjha wrote:
| Is it bad that I've written some of these in my normal code? In
| particular, I use the concepts in these when they're relevant:
| *(c ? &x : &y) = v; return (char *[]){"No",
| "Yes"}[!!x];
|
| (Note: for the second one, I use the "selecting from a temporary
| array" and !! separately; if I only had two options then I'd use
| x ? "Yes" : "No" of course.)
| bear8642 wrote:
| >return (char *[]){"No", "Yes"}[!!x];
|
| Is !!x equivalent to x here?
| saagarjha wrote:
| No, it "collapses" an integer to 0 or 1. (Essentially, it's x
| != 0.)
| jackewiehose wrote:
| > Is it bad that I've written some of these in my normal code?
|
| yes
| moonchild wrote:
| ({_:&&_;}); ({}); ({;});
|
| All of these rely on a gcc extension ('statement expressions').
|
| ________________________________ a = '-'-'-'
|
| You can also do: a = '/'/'/'
|
| To generate a 1 instead of a 0.
|
| ________________________________ printf("%d
| %d\n", 0 == sizeof(count = 2, count++), count);
|
| This works because:
|
| 1. There are two forms of sizeof; sizeof(T) where T is a type,
| and sizeof x where x is an expression.
|
| 2. There are 'comma expressions'; if you have (x, y) where x and
| y are expressions, then x is executed and the expression
| evaluates to y.
|
| 3. Parameters to sizeof are not evaluated (which is important
| because otherwise the value of the 3rd argument would be
| undefined, since there's no sequence point between evaluation of
| function parameters).
| saagarjha wrote:
| > Parameters to sizeof are not evaluated
|
| Oh, but they are ;) Try running sizeof on a VLA sometime.
| sigjuice wrote:
| $ cat sizeof.c #include <stdio.h> int
| main(int argc, char *argv[]) { printf("%zu\n",
| sizeof(int[argc - 2])); } $ make -B sizeof
| cc sizeof.c -o sizeof $ ./sizeof foo 0
| $ ./sizeof 17179869180
| freefal wrote:
| What does the provided example do?
|
| 5) Surprising math:
|
| int x = 0xfffe+0x0001;
| [deleted]
| moonchild wrote:
| It looks a bit like a binary exponent, but according to clang
| and gcc it's erroneous: 'invalid suffix '+0x0001' on integer
| constant'.
| monocasa wrote:
| It doesn't compile.
|
| It's trying to parse the e as scientific notation I think.
| surprising_math.c: In function 'main':
| surprising_math.c:4:10: error: invalid suffix "+0x0001" on
| integer constant int x = 0xfffe+0x0001;
| ^~~~~~~~~~~~~
| inetknght wrote:
| Indeed, I think you're exactly right. Changing it to
| `0xffff+0x0001` lets it compile.
| inetknght wrote:
| It produces an error message.
|
| _error: invalid suffix '+0x0001' on integer constant_
|
| [0] https://gcc.godbolt.org/z/pR5L6v
| rightbyte wrote:
| That is really stupid. How is that not a bug? I can't belive
| GCC wont compile it. What does other compilers do.
|
| Edit: Clang also gives an error. Mscv seems to compile. I
| wonder who follows spec. I assume not mscv ...
| faehnrich wrote:
| That index[x] was used in one of my favorite IOCCC one-liners.
|
| http://faehnri.ch/have-fun/
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