What's goes wrong with this code? What is runtime error?

Question

When I submit this code on URI Online judge, they show me runtime error. I have no idea about runtime error. Please someone can explain that

#include <stdio.h>
#include <math.h>

int main(int argc, char const *argv[])
{
    double x1, y1, x2, y2, result;
    scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);

    result = sqrt(pow(x2 - x1, 2) + pow(y2 - y1 , 2));

    printf("%.4lf\n", result);
}

Run it in a real environment on your computer to get a better error message. — Carcigenicate, Oct 06 '20 at 13:49
Very hard to say. I cannot see that this would cause any runtime error as long as you give correct input. If you pass something else than 4 floats, well that would be trouble. — klutt, Oct 06 '20 at 13:54
The format `%lf` specifies `double` when given to `scanf()` and `long double` when given to `printf()`. — Weather Vane, Oct 06 '20 at 13:54
@WeatherVane Maybe in Microsoft land, but not in C standard land. — Ian Abbott, Oct 06 '20 at 13:55
@IanAbbot MS dosn't even implement `long double` except as `double`. — Weather Vane, Oct 06 '20 at 13:56
Could you tell us which online judge exactly you're using? [Edit] the question and put all relevant information _into the question_ — Jabberwocky, Oct 06 '20 at 13:57
@WeatherVane That's true! `%lf` still requires a `double` in `printf()` though. — Ian Abbott, Oct 06 '20 at 13:57
@IanAbbot I can't see anything else wrong with it. And `float` can be passed where `double` is expected in variadic functions due to type promotion — Weather Vane, Oct 06 '20 at 13:59
@WeatherVane Yes, `float` can be passed where `double` is expected in variadic functions, but `long double` cannot. (`%lf` behaves the same as `%f` in `printf()`.) — Ian Abbott, Oct 06 '20 at 14:03
Perhaps with some negative values close to `0` the `pow()` function can return `-0` which then faults `sqrt()`. You should not use `pow()` for a simple square anyway. — Weather Vane, Oct 06 '20 at 14:09

Antti Haapala -- Слава Україні · Answer 1 · 2020-10-06T14:25:56.623

The site says "ANSI C" which is a commonly used misnomer for the C89/C90 standard revision... It is the obsolete deprecated version that no one should use anywhere - it does not accept for example // comment characters.

The thing is that the implicit return 0 was added in C99. In C89 if main does not return a value, behaviour is undefined. It seems that the solution is not accepted if main does not explicitly return 0. URI Online Judge uses GCC 4.8.5. GCC 4.8.5 defaults to gnu89 "dialect" and without explicit return from main the exit code is completely random. On the other hand calling it Ansi C is not appropriate either as it still does support things like // for comments.

Thus:

#include <stdio.h>
#include <math.h>

int main(int argc, char const *argv[])
{
    double x1, y1, x2, y2, result;
    scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);

    result = sqrt(pow(x2 - x1, 2) + pow(y2 - y1 , 2));

    printf("%.4lf\n", result);
    return 0;
}

works but

#include <stdio.h>
#include <math.h>

int main(int argc, char const *argv[])
{
    double x1, y1, x2, y2, result;
    scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);

    result = sqrt(pow(x2 - x1, 2) + pow(y2 - y1 , 2));

    printf("%.4lf\n", result);
    return 42;
}

gives "Runtime error".

Another thing C89 does not accept either is %lf to signify a double for printf - l is valid for only d, i , o, u, x, and X conversion specifiers. Notably, f, is not listed! However either one works in URI, because glibc has for long supported C99.

MikeCAT · Answer 2 · 2020-10-06T14:04:23.677

0

In some old C specification, values returned from the process will be indeterminate when returned from the function main without using return statement. (0 will be returned in C99 or later).

Also there are online judge systems that treat being returned non-zero from the process as runtime error.

Add return 0; after the printf statement to avoid this error.

Also using %lf in printf is not allowed (invokes undefined behavior) in old C. Use %f (%.4f in this case) instead. (Using %lf for reading double in scanf is good)

edited Oct 06 '20 at 14:04

answered Oct 06 '20 at 13:50

MikeCAT

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Are you sure this is the reason? – user253751 Oct 06 '20 at 13:52
1

@user253751 Do you have some other candidates of the reason? – MikeCAT Oct 06 '20 at 13:54
@user253751 The [FAQ](https://www.urionlinejudge.com.br/judge/en/faqs/about/judge) says that ANSI C is used on the system and this is one of "some old C specification". – MikeCAT Oct 06 '20 at 13:55

What's goes wrong with this code? What is runtime error?

2 Answers2