Why isn't this case insensitive version of strstr() function working? - c

Question

In an effort of solving a textbook problem, I'm trying to create a case insensitive version of the function called strstr() which is in the C language. So far, I've run into two problems. The first problem being that when I make the case insensitive version of strstr() it worked, but it didn't stop at the first matching string and continued to return the string even if they didn't match.
strstr() is supposed to see the first instance of a matching character up to n counts specified and then stop. Like if I wrote: "Xehanort" in string A and "Xemnas" in string B and specified 4, as the number, it would return Xe.
The idea behind the case insensitive version is that I can write : "Xehanort" in one string and "xemnas" in the next string and have it return Xe.
However, I've run into a new problem in new code I've tried: the function doesn't seem to want to run at all. I've tested this and it turns out the function seems to be at a crash and I'm not sure how to make it stop.
I've tried editing the code, I've tried using different for loops but figured that the code doesn't need to be too sophisticated yet, I've also tried different code entirely than what you are going to read, but that resulted in the problem mentioned earlier.
#include <ctype.h>
#include <stdio.h>
#include <string.h>
#include <limits.h>
#define MAX 100
char *stristr4(const char *p1, const char *p2, size_t num);
int main() {
char c[MAX], d[MAX];
printf("Please enter the string you want to compare.");
gets(c);
printf("Please enter the next string you want to compare.");
gets(d);
printf("The first string to be obtained from \n%s, and \n%s is \n%s",
c, d, stristr4(c, d, MAX));
}
char *stristr4(const char *p1, const char *p2, size_t num) {
const char *str1 = p1;
const char *str2 = p2;
char *str3;
int counter = 0;
for (int i = 0; i < num; i++) {
for (int j = 0; j < num; j++) {
if (tolower(str1[i]) == tolower(str2[j])) {
str3[i] = str1[i];
counter++;
} else {
if (counter > 0) {
break;
} else
continue;
}
}
}
return str3;
}
The code you see will ask for the strings you want to input. Ideally, it should return the input.
Then it should do the stristr function and return the first instance of matching string with case insensitivity.
However, the function I've created doesn't even seem to run.

Answer 1

Your code has undefined behavior (in this case causing a segmentation fault), because you try to store the resulting string via an uninitialized pointer str3.
Standard function strstr returns a pointer to the matching subsequence, you should do the same. The third argument is useless if the first and second arguments are proper C strings.
Here is a modified version:
char *stristr4(const char *p1, const char *p2) {
for (;; p1++) {
for (size_t i = 0;; i++) {
if (p2[i] == '\0')
return (char *)p1;
if (tolower((unsigned char)p1[i]) != tolower((unsigned char)p2[i]))
break;
}
if (*p1 == '\0')
return NULL;
}
}
Notes:
function tolower() as other functions from <ctype.h> takes an int argument that must have the value of an unsigned char or the special negative value EOF. char arguments must be converted to unsigned char to avoid undefined behavior for negative char values. char can be signed or unsigned by default depending on the platform and the compilers settings.
you should never use gets(). This function is obsolete and cannot be used safely with uncontrolled input. Use fgets() and strip the trailing newline:
if (fgets(c, sizeof c, stdin)) {
c[strcspn(c, "\n")] = '\0'; // strip the trailing newline if any
...
}

Answer 2

A third string could be passed to the function and fill that string with the matching characters.
Use fgets instead of gets.
#include <ctype.h>
#include <stdio.h>
#include <string.h>
#define MAX 100
int stristr4(const char* p1, const char *p2, char *same);
int main( void)
{
int comp = 0;
char c[MAX] = "", d[MAX] = "", match[MAX] = "";//initialize to all zero
printf ( "Please enter the string you want to compare. ");
fflush ( stdout);//printf has no newline so make sure it prints
fgets ( c, MAX, stdin);
c[strcspn ( c, "\n")] = 0;//remove newline
printf ( "Please enter the next string you want to compare. ");
fflush ( stdout);//printf has no newline so make sure it prints
fgets ( d, MAX, stdin);
d[strcspn ( d, "\n")] = 0;//remove newline
comp = stristr4 ( c, d, match);
printf ( "Comparison of \n%s, and \n%s is \n%d\n", c, d, comp);
if ( *match) {
printf ( "The matching string to be obtained from \n%s, and \n%s is \n%s\n"
, c, d, match);
}
return 0;
}
int stristr4 ( const char *p1,const char *p2, char *same)
{
//pointers not pointing to zero and tolower values are equal
while ( *p1 && *p2 && tolower ( (unsigned char)*p1) == tolower ( (unsigned char)*p2))
{
*same = tolower ( (unsigned char)*p1);//count same characters
same++;//increment to next character
*same = 0;//zero terminate
p1++;
p2++;
}
return *p1 - *p2;//return difference
}