C Program Compiles but Segfaults [Commented Code]

[LtStaffel]

Hello everyone,

 

This is a prime number generator up to a given number. It compiles but segmentation faults 70% of the time and does not print when it doesn't segfault. Any idea what's wrong with it? This is a C version of what I already wrote (and is working) in Python. I suspect it has to do with my use of sizeof().

 

Thanks in advance

 

Code:

Spoiler

// obvious include
#include <stdio.h>

// main
int main(int argc, char *argv[]){
    // grab limiting number from first CLI argument
	int limit = (int)argv[1];
    // I want each value to be the same as its index
	int nums[limit+1];
    // populating the array
	for(int i = 0; i <= sizeof(nums); i++){
		nums[i] = i;
	}

    // I don't want to use the number 1 because it will mark off all my numbers
	nums[1] = 0;

    // start a loop
	for(int k = 0; k < sizeof(nums); k++){
        // if the number is not marked
		if(nums[k] != 0){
            // print the number we are on that is not marked
			printf("%d ",nums[k]);
            // mark every multiple of our current number
			for(int j = k; j+k <= limit; j+=k){
                // the marking
				nums[j] = 0;
			}
		}
	}
	// return
	return 0;
}

 

 

[79wjd]

You need to divide the array by the size of an int. sizeof(array) will return the length of the array*the size of the data. So sizeof() on an array with four ints would return 16 instead of 4.

 

for(int i=0; i<=sizeof(nums)/sizeof(int); i++)

 

[fizzlesticks]

int limit = (int)argv[1];

argv is an array of "strings", you can't just use a cast to get the int value.

[LtStaffel]

8 minutes ago, fizzlesticks said:

int limit = (int)argv[1];

argv is an array of "strings", you can't just use a cast to get the int value.

Thanks!

I was told not to use atoi(), what would be the best route?

[LtStaffel]

10 minutes ago, djdwosk97 said:

You need to divide the array by the size of an int. sizeof(array) will return the length of the array*the size of the data. So sizeof() on an array with four ints would return 16 instead of 4.

for(int i=0; i<=sizeof(nums)/sizeof(int); i++)

 

Thanks!

[fizzlesticks]

1 hour ago, LtStaffel said:

Thanks!

I was told not to use atoi(), what would be the best route?

http://en.cppreference.com/w/c/string/byte/strtol probably, but I'm not really into C so someone else might know better.