URLs in C (2011)(susam.in)

63 pointsby susamhá 6 anos23 comments

susam.in

URLs in C (2011)

https://susam.in/blog/urls-in-c/

23 comments

mastrsushihá 6 anos

TLDR http: is parsed as a label and the rest of the url following // becomes a comment.

Nothing special

simiashá 6 anos

There are literally 4 sentences in the article before the answer is introduced, I don't think a TLDR is warranted...

ktpsnshá 6 anos

goto https;

// nothing to see here

jasodehá 6 anos

Related wikipedia: https://en.wikipedia.org/wiki/Polyglot_(computing)

And C/C++ also has the "-->" operator which, of course, is the "goes to zero" operator: https://stackoverflow.com/questions/1642028/what-is-the-oper...

And the famous joke thread from Stackoverflow with examples of file format abuse: https://stackoverflow.com/questions/5508110/why-is-this-prog...

Saint_Genethá 6 anos

Just wish the answer was hidden so I could have given it 10 seconds of thought without accidentally reading it.

nayukihá 6 anos

The gotcha is that if in the same function you put in another URL with the same protocol, the compiler will refuse to compile the code due to a duplicate label.

  int main() {
      https://acceptable/
      http://okay/
      https://duplicate.error/
  }

sgthá 6 anos

Luckily there are other protocols we can work with to bypass this limitation.

  int main() {
      gopher://myhost:151/hello.txt
      ftp://ftp.acc.umu.se/pub/netbsd
      ldap://yesherewego
      dav://getmyfiles
  }

klodolphhá 6 anos

Another gotcha is that a statement must follow. The following is an error:

    int main() {
        https://error/
        int x = 3;
    }

colejohnson66há 6 anos

Why?

eindiranhá 6 anos

In C, a label requires a statement[0] to come after it. A declaration in C is not a statement (though it is in C++).

gcc for example gives the error "error: a label can only be part of a statement and a declaration is not a statement" for the code in OP's comment.

g++ happily compiles it.

See the first answer here: [1]

[0] https://en.wikipedia.org/wiki/Statement_(computer_science)

[1] https://stackoverflow.com/questions/49861965/declarations-de...

tyingqhá 6 anos

Capitalizing different parts of the scheme would give you some mileage. The standard allows for it.

iforgotpasswordhá 6 anos

Due to a duplicate protocol

kevincoxhá 6 anos

Somehow I quickly recognized the `https:` as a label. But I could not for the life of me figure out how `//` would compile. I was thinking it must be some sort of operator.

Brains are funny sometimes, and it is amazing how much we rely on context to remember and parse things.

Kurajhá 6 anos

And how much we take syntax coloring for granted :)

rkangelhá 6 anos

Whereas I saw it the other way round. I parsed that as a comment before I parsed it as a URL.

robalnihá 6 anos

This reminds me of when I tried to write HTML code in bash.

<html> hi </html> a
This code when run as a bash script reads from a file with name "html" and writes to file "hi". It also reads from file "html" (in the root directory) and writes to a file called "a". The "a" in the end is just to avoid bash errors about unexpected newline.

When I played with this a few years ago I actually discovered a bug in bash. If I had a bash alias with the same name as one of the filenames, bash would do alias expansion on the filename. This bug was triggered by executing a line with two redirection operators before any command.

So if you follow these steps:

$ alias a='abc xyz' $ </ >a
you would get the error (in addition to a file created with name "abc"):

bash: xyz: command not found
This happened because bash expanded the second line to:

$ </ >abc xyz
and therefore treated "xyz" as a command (and "abc" and the root directory as filenames). This bug doesn't work anymore in new versions of bash because it has been fixed.

abetuskhá 6 anos

Yes, while gotos are frowned upon, they're still allowed in C/C++.

Where to goto? A label, of course.

astrobe_há 6 anos

I'm guessing it is not compatible with calling conventions in C, but one could goto a function name.

chriszhanghá 6 anos

Can you explain "goto a function name" further?

I wrote this:

#include <stdio.h> void f() { printf("f"); } int main() { goto f; return 0; }
It creates error:

foo.c:8:10: error: use of undeclared label 'f' goto f; ^ 1 error generated.
How can we goto a function name?

urygahá 6 anos

i think GP means that conceptually, you could imagine goto-ing to a function's code - they do compile to labels after all.

fun note: C-- has something like that! iirc it's like calling a function, but reusing the current stackframe:

jump f(expr1,. . .,exprn);
> "the `jump` statement performs a control jump but carrying parameters. It has as target [...] a procedure address and can optionally transfer arguments to that procedure."

https://www.microsoft.com/en-us/research/wp-content/uploads/...

piscisaureushá 6 anos

This is the first time I've heard about a language called C--, thank you for that.

The concept is not so unique as you might think tho; the more commonly used term is "tail call".

urygahá 6 anos

i always thought "tail call" means a function call that can be optimized in this way (located in a tail position), not the act of call-by-jumping itself. both uses make sense though

re: C--, i think it's mostly used as a compiler IR. (notably in Haskell's GHC)

aidenn0há 6 anos

That reminds me of the expression in C that will evaluate to 0 or 1 depending on whether or not "//" style comments are supported:

int x = 1 //* //*/2 ;
Without "//" comments this is whitespace equivalent to

int x = 1 / 2 ;
With "//" comments this is whitespace equivalent to

int x = 1 ;
[edit]

Full working demo: https://gist.github.com/jasom/41ab3712d83d7edbdc03bb8e43a7af...