Run into segmentation fault when doing string manipulation in C

I'm working on removing obfuscating a substring from the main string in C. The substring has a format of xx:xx:xx:xx:xx:xx where x can be a number or a letter. The beginning of the substring always have a space, but the end of the substring doesn't always have a space.

e.x. the input can be "Found new device with wlan 33:33:33:33:33:33. Total 4 devices connected" the output should be "Found new device with wlan *. Total 4 devices connected"

I need to make a helper function to do so, and this is what I come up with:

//find the first occurency of ":" record as first_position_ptr and the last occurency of ":" record as last_position_ptr. 
// Add 2 to first_position_ptr to beginning of the substring and set it to "*"
// Subtract 2 from the last_position_ptr to get the pointer of the last charactor in substrung. 
// Iterate from 1 + first_position_ptr to last_position_ptr - 2 and set inStr to 0 to deleted the rest of substring. 

const char *proc(char *inStr) {
    const char tar[] = ":";
    char *first_position_ptr = strchr(inStr, tar[0]);
    char *last_position_ptr = strrchr(inStr, tar[0]);

    int first_position = (first_position_ptr == NULL ? -1 : first_position_ptr - inStr);
    int last_position = (last_position_ptr == NULL ? -1 : last_position_ptr - inStr);

    if (first_position != -1 && last_position != -1) {
        inStr[first_position + 2] = "*";
    }

    for (int i = (first_position + 1); i < (last_position + 2); i++) {
        inStr[i] = 0;
    }
    return inStr;
}

int main() {
    const char str[] = "Found new device with wlan 33:33:33:33:33:33. Total 4 devices connected";
    proc(*str);
    printf("%s\n", str);
    return 0;
}

But when I compile the code, I run into these warnings:

Jeff-MacBook-Pro-2:Desktop jBerman$ gcc test.c -o test
test.c:35:29: warning: incompatible pointer to integer conversion assigning to 'char' from
      'char [2]' [-Wint-conversion]
                inStr[first_position + 2] = "*";
                                          ^ ~~~
test.c:43:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
test.c:49:7: warning: incompatible integer to pointer conversion passing 'const char' to
      parameter of type 'char *' [-Wint-conversion]
        proc(*str);
             ^~~~
test.c:26:24: note: passing argument to parameter 'inStr' here
const char* proc(char *inStr){
                       ^
3 warnings generated.

When I run it, the compile says:

Segmentation fault: 11

So I tried to correct this line in main:

proc(*str) -> proc(&str);

The compiler return:

Bus error: 10

Can someone let me know what might be wrong please? Or if you help more efficient way of doing this, please let me know

You should pass str to proc, not *str and define str as a modifiable char array. The proc function should also be simplified: setting the characters in the middle to zero does not erase the middle portion, it makes the string stop after the *. To keep the end of the string, you must copy it with memmove().

Here is a modified version:

#include <stdio.h>
#include <string.h>

// replace the mac address from the input string with a *
char *proc(char *inStr) {
    char *p1 = strchr(inStr, ':');
    char *p2 = strrchr(inStr, ':');

    if (p1) {  // no need to test p2, there is at least one ':' in the string
        /* backtrack 2 characters, but not before the start of inStr */
        if (p1 > inStr) p1--;
        if (p1 > inStr) p1--;
        /* skip 3 characters, but not past the end of the string */
        p2++;
        if (*p2) p2++;
        if (*p2) p2++;
        /* replace the MAC address with a '*' */
        *p1++ = '*';
        /* move the rest of the string, including the null terminator */
        memmove(p1, p2, strlen(p2) + 1);
    }
    return inStr;
}

int main() {
    char str[] = "Found new device with wlan 33:33:33:33:33:33. Total 4 devices connected";
    printf("%s\n", proc(str));
    return 0;
}