Unable to resolve segmentation Fault in C program

My assignment is to write a C function void censor(char *start) that will replace all 4 letter words in the string pointed to by start with ****. Assume that words are delimited by blank chars. The beginning and the end of strings are also delimiters. I am not to use [ ] in your code. For example, the following code:

int main(){
char myStr[50] = ”this is a test for the last word in line”;
censor(myStr);
printf(”%s\n”, myStr);
}
would yield:
**** is a **** for the **** **** in ****

Here is my code:

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

char* findBlank(char *start)
{   char *tempStart = start;
    while(*tempStart != '\0') {
        if(*tempStart == ' ') {
            return tempStart;
        }
        tempStart++;
    }
    return NULL;
}

void fourStars(char *start)
{
    char *tempStart = start;
    for (int i = 0; i < 4; ++i) {
        *tempStart = '*';
        tempStart++;
    }
}

void censor(char *start)
{   
    char *temp;

    while(*start != '\0') {
        int len = 0;
        temp = findBlank(start);
        while(start < temp && *start != '\0'){
            len++;
            start++;
        }

        if(len == 4) {
            char *replace = temp - len;
            fourStars(replace);
        }
        
        if ((*start != '\0') || (*(temp+1) != '\0')) {
            start = temp + 1;
        }
    }
}

int main()
{   
    char myStr[50] = "this is a test for the last word in line";
    censor(myStr);
    printf("%s\n", myStr);

}

I tried running it through the gdb debugger and also trying to trace it but I am unable to resolve this. Here was the output from the gdb debugger:

Program received signal SIGSEGV, Segmentation fault.
0x000055555555527c in censor (start=0x1 <error: Cannot access memory at address 0x1>) at main.c:29
29          while(*start != '\0') {

I'm not allowed to change the functionality of my findBlank or fourStars functions.

Your program segfaults on while(*start != '\0') as start == 1. It was assigned that value from start = temp + 1 so temp == NULL. temp is NULL when findBlank() doesn't find any spaces. If you want the last 4 letter word censored then the minimal fix is for findBlank() return a pointer to the '\0' instead of NULL:

char *findBlank2(char *start) {
    for(; *start && *start != ' '; start++);
    return start;
}

and example run:

**** is a **** for the **** **** in ****