String Replacement (str_replace)

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
//replaces all occurences with another occurence
char *str_replace(char * t1, char * t2, char * t6){
	char*t4;
	char*t5=malloc(0);
	while(strstr(t6,t1)){
		t4=strstr(t6,t1);
		strncpy(t5+strlen(t5),t6,t4-t6);
		strcat(t5,t2);
		t4+=strlen(t1);
		t6=t4;
	}
	return strcat(t5,t4);
}
int main(){
	printf("%s",str_replace("hello","goodbye","hello worldhello hello hello helloworld hello hello hello hello hellohello worldhello hello hello helloworld hello hello hello hello hellohello worldhello hello hello helloworld hello hello hello hello hello,and then it gets to the new line?\n"));
	return 0;
}