C Questions and Answers • Hacker News Board

Re: C Questions and Answers

#11

Post by belovedeagle » Wed, Mar 04, 2015, 6:56 AM UTC

> (especially C programmers)

Author needs to stop his anti-intellectual everyone-is-as-ignorant-as-me bullshit. I see that a lot re programming to justify a lot of silly positions. If you only know Javascript, that's great, I rather like having shiny things in my browser (I like it too much, even). That doesn't mean that C programming is obsolete; some of us know C. For example, I got #5 and #9 wrong, and the rest I got right including the general idea of the justifications. (10/12 is pretty good for someone who grew up in the Java era, but I want to get better.)

Re: C Questions and Answers

#12

Post by gamegoblin » Wed, Mar 04, 2015, 6:57 AM UTC

_RPM wrote: post #2

This reminds me of tests I took in earlier CS classes. Knowing those things are utterly useless in practice.

My CS101 prof would give 50-100 line blocks of code, and hidden in them somewhere would be something like:

    if(condition); {
        some stuff
    }

Note the semicolon after the if-statement.

The tests became games of "find the semicolon or = in place of ==". Ugh.

Re: C Questions and Answers

#13

Post by kenjackson » Wed, Mar 04, 2015, 6:58 AM UTC

I just got the first one wrong and haven't written C code in a decade. I have to admit I haven't seen that construct used in any programs. I guess, no harm no foul, but still seems like an odd construct to allow.

Re: C Questions and Answers

#14

Post by ternaryoperator » Wed, Mar 04, 2015, 7:02 AM UTC

This reminds of me of the Quiz books that were popular years ago. They'd show some code that inadvertently tripped some obscure corner of the language.

Rarely did the quizzes provide great insight. Rather, they confirmed the benefits of keeping your code idiomatic.

Re: C Questions and Answers

#15

Post by bgvopg » Wed, Mar 04, 2015, 7:05 AM UTC

kdoherty wrote: post #10

Unsigned int >= 0 in a decrementing for-loop is a classic trap

Some would twitch at this:

for(i = length - 1; i but it is completely valid.

Re: C Questions and Answers

#16

Post by bottled_poe » Wed, Mar 04, 2015, 7:07 AM UTC

Does anyone know of a tool which can automatically scan source code for these types of oversights?

Assuming of course the compiler doesn't already check for all of these...

Re: C Questions and Answers

#17

Post by fdik » Wed, Mar 04, 2015, 7:07 AM UTC

Already the first example is wrong. So I stopped reading.

% cc -std=c11 -o dingens dingens.c dingens.c:6:9: error: redefinition of 'i' int i = 10; ^ dingens.c:5:9: note: previous definition is here int i; ^ 1 error generated. % cat dingens.c

#include

int main() { int i; int i = 10;

    printf("hello, world\n");
    return 0;

}

%

Re: C Questions and Answers

#18

Post by fizixer » Wed, Mar 04, 2015, 7:08 AM UTC

sjolsen wrote: post #6

fizixer wrote: post #5

He's discussing subtle points of the C language and yet there is no mention of which compiler he's using, whether the results might be different for different compilers, hardware platforms, and how they correlate with multiple C standards (C89, C99, C11/C1X, etc). He succeeded in convincing me that he does not know C!

There is no mention of which compiler he's using or whether the results might be different for different compilers and/or hardware platforms precisely because he's discussing the C language— not the idiosyncrasies of particular implementations. >and how they correlate with multiple C standards There's nothing going on here specific to any particular revision of ISO C (that I can see).

Too much of a coincidence!

Did he make sure the behavior he mentioned is uniform across all ISO standards? What is his source for coming up with a certain answer to a certain piece of code? any of the standard documents? K&R book? gcc output?

As an example:

In the answer to point 2, he claims:

> ... the compiler thinks that x cannot be a null pointer ...

First of all, this gives a strong indication that he's analyzing a compiler output, a compiler that he didn't reveal in the article.

But even if we ignore that, and he truly is going by a rule that is uniform across all of K&R, C89, C99, and so on, could you or him point me to any page in the C99 standard document where it is explicitly stated along the lines that the compiler "should assume" the pointer to be not NULL after an undefined dereferencing in line (1) and hence ignore (2) and (3)? (Based on my experience, I have a very strong hunch that a standard would not enforce assumptions as a result of an undefined operation.)

If you could, you/he "may" have a point ("may", because I still have 11 other points to critique). If not, he and you clearly have no idea what you guys are talking about!

Re: C Questions and Answers

#19

Post by anant » Wed, Mar 04, 2015, 7:09 AM UTC

fdik wrote: post #17

Already the first example is wrong. So I stopped reading. % cc -std=c11 -o dingens dingens.c dingens.c:6:9: error: redefinition of 'i' int i = 10; ^ dingens.c:5:9: note: previous definition is here int i; ^ 1 error generated. % cat dingens.c #include int main() { int i; int i = 10; printf("hello, world\n"); return 0; } %

He clearly mentions that the statements are not in a function body, rather just in the global portion of a C file.

Re: C Questions and Answers

#20

Post by pcwalton » Wed, Mar 04, 2015, 7:11 AM UTC

bottled_poe wrote: post #16

Does anyone know of a tool which can automatically scan source code for these types of oversights? Assuming of course the compiler doesn't already check for all of these...

You can use the clang sanitization flags to catch them dynamically (at least, the null pointer dereference and signed overflow).