How come Codeblocks run this C program without errors but Codeforces show compilation error?

Question

The following code runs fine while I'm using it in my Code blocks application. However, when solving the "Even Odds" problem in Codeforces by submitting the same code, it gives me a Compilation error.

Error:invalid conversion from 'void*' to 'long long int*' [-fpermissive]

I'm quite new to C programming. So, plz help me how I can fix this code of mine.

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int main(){
long long *arr, n, k, i, n1, n2, m;
scanf("%I64d%I64d", &n, &k);
arr=malloc(sizeof(long long)*n);
    n1=1;n2=2;
for(i=0;i<=n/2&&n1<=n;i++){
    arr[i]=n1;
    n1+=2;
    }
if(n%2!=0) m=n/2+1;
else m=n/2;
for(i=m;i<=n&&n2<=n;i++){
    arr[i]=n2;
    n2+=2;
    }
printf("%I64d", arr[k-1]);
free(arr);

return 0;
}

Also note that the `I64` modifier to `printf` and `scanf` is a MSVC -specific extension, it's not standard C. — Some programmer dude, Feb 22 '20 at 07:16
Check the code is saved as .c extension or you may cast to (long long*) That is arr=(long long*)malloc(sizeof(long long)*n); — srilakshmikanthanp, Feb 22 '20 at 07:23
https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc/605858#605858 @Srilakshmikanthanp — Yunnosch, Feb 22 '20 at 07:29
ImranF, please identify the line for which you get the error and quote the error verbatim, in full and as text directly here. — Yunnosch, Feb 22 '20 at 07:31
Indeed the problem is that that error comes from GCC **C++** compiler. You need to compile a C program with a C compiler. — Antti Haapala -- Слава Україні, Feb 22 '20 at 08:11
Yes, the file is in .c extension and the error shows for line 7, i.e. arr=malloc(sizeof(long long)*n) — Imran Farid, Feb 22 '20 at 17:35

score 1 · Accepted Answer · edited Feb 22 '20 at 13:55

1

In Codeblocks, you must be compiling it as a c file. But in Codeforces, the code is being compiled as cpp.

arr=malloc(sizeof(long long)*n);

When you compile that line in C, the malloc will throw a pointer of void* which will be automatically converted into long long*

But in the case of cpp, explicit conversion should be performed. To run the code in cpp:

arr = (long long*)malloc(sizeof(long long) * n);

That will do the job. Read this article for clarification.

edited Feb 22 '20 at 13:55

Ardent Coder

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answered Feb 22 '20 at 11:43

Kashish Dhawan

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Thanks a lot! That really helped me understand with clarity. – Imran Farid Feb 22 '20 at 17:39

How come Codeblocks run this C program without errors but Codeforces show compilation error?

1 Answers1