My C program stops when iteration variable reaches 2100 why?

Asked Sep 05 '20 at 15:21

Active Sep 05 '20 at 15:37

Viewed 70 times

I have written a simple C code like this:

#include <stdio.h>

int main()
{
    long iteration = 0;
    
    while(1)
    {
        iteration += 10;
        
        if(iteration % 100000000 == 0)
        {
            printf("Iteration %ld\n", iteration / 1000000);
        }
    }

    return 0;
}

When the iteration reaches to 2100, the program stops (doesn't print anything). It seems that the iteration is considered as int by the compiler (GNU gcc), but I defined it as long. What is the problem?

2100 * 1000000 = 2100000000 (2.1 billions) and it is int overflow but my variable has been defined as long.

edited Sep 05 '20 at 15:37

GSerg

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asked Sep 05 '20 at 15:21

abc def

1

Your compiler should be generating a few warnings for this. Read them. – 3Dave Sep 05 '20 at 15:25
5

`long` != `long long`. – GSerg Sep 05 '20 at 15:25
1

Hint: print `sizeof(long)`. – dbush Sep 05 '20 at 15:30
Will it work if you use long long or unsigned long? I'm not sure if it is the same with C, but in C# and C++, long has the same range as int. Source: https://learn.microsoft.com/en-us/cpp/cpp/data-type-ranges?view=vs-2019 – mberna Sep 05 '20 at 15:30
@ GSerg: many thanks it's the answer. but why doesn't "long iteration" work ? it's 8 bytes length – abc def Sep 05 '20 at 15:31
many thanks. "long long iteration" works. but why doesn't "long iteration" work? long is 8 bytes length – abc def Sep 05 '20 at 15:34
`it's 8 bytes length` - is it though? Have you verified, and have you visited the links? – GSerg Sep 05 '20 at 15:35
@Gserg: here is the link: https://www.tutorialspoint.com/cprogramming/c_data_types.htm – abc def Sep 05 '20 at 15:37
1

@abcdef Not a very good one then. Please see the ones at the top of the question instead. – GSerg Sep 05 '20 at 15:38
please read [signed values in modulo](https://stackoverflow.com/questions/43686864/unexpected-modulo-behavior-with-long-integer) – user3629249 Sep 05 '20 at 15:43
The code you posted will hit the `printf` even if `iteration` is 32 bits. – Jim Rhodes Sep 05 '20 at 15:45
Also referring to @3Dave 's comment - always seek extra warning from your compiler , use flags such as : `-Wall -Wextra -pedantic -Wconvert` – user13863346 Sep 05 '20 at 22:40

My C program stops when iteration variable reaches 2100 why?

0 Answers0