Segmentation fault, I dont know why?

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C

Hi guys, can someone help me, my program scrashing and when i debugging it takes "SIGSEGV Segmentation Fault" and show me my CPU window. It takes only in Dev-C++, in visual studio it goes well, but i want it to do in Dev-C++. Cursor in debug stay at line 30 (str2[k] = str1[j];). I have 32-bit OS.
Thanks for help.

<pre lang="c#">
#include <stdio.h>
#include <string.h>

//príklad 2

int main()

{
	FILE *fr,*fw;
	int c,i=0,k=0,f = 0;
	char str1[50];
	char str2[50];
	
	fr = fopen ("vstup.txt","r");
	fw = fopen ("vystup.txt","w");
	while ((c=getc(fr)) != EOF){
		str1[i] = c;
		i++;
	}
	str1[i]= '\0';
	fclose (fr);
	printf("%s",str1); 
	i=0;
	while (str1[i] != EOF)
	{		
		if(str1[i] == '\n')	
		{	
			for (int j=i-1;j>=f;j--)
			{
				str2[k] = str1[j];
				k++;
			}
			f = i+1;
			str2[k] = '\n';
			k++;
		}
		i++;
	}
	str2[k] = '\0';
	
	fprintf(fw,"%s",str2);
	fclose (fw);
	return 0;
}

What I have tried:

Cause i am a little bit new in programming i didn't know about this fault(first time i get it) i know that is something with memmory acces etc.