Question

I am asked to implement an analogue to the string.h library. However, I am not allowed to use the '\0' inside my library to end my string; that’s why I am supposed to use a structure for strings in my program.
I have this in mystring.h file:

#include <stdio.h>
#define MAXSTRLEN 100 // Defining the maximum lenght of the string as being 100

typedef struct scell *mystring_t;
mystring_t makemystring (char cs[]); // This function stores a given string into the mystring structure

I have this in mystring.cfile:

#include <stdlib.h>
#include "mystring.h" // including the header file of mystring library

struct scell {
    char *string;
    int length;
};

mystring_t makemystring (char cs[]){ //Storing a string into the structure

    int i = 0;
    mystring_t ns;

    ns->string=(char*)calloc(MAXSTRLEN ,sizeof(char));

    // printf ("I allocated memory for the string");

    while (cs[i] != '\0')
    {
        printf ("\nI entered into the while\n");
        ns->string[i] = cs[i];
        printf ("I inserted\n");
        i++;
        printf ("I incremented the count\n");
    }
    ns->length=i; // storing the length of the string into the structure
    printf ("%d\n", ns->length);
    printf ("refreshed the length\n");
    printf ("%d", ns->length);
    return ns;
}

I have this in the main.c file:

#include "mystring.h"
#include <stdlib.h>
int main () {

int result;
mystring_t S1;
mystring_t S2;
    // create two strings
S2 = makemystring("Bye");
printf("I got out of the makemystring function\n");
S1 = makemystring("Hi");

These printf() calls are just debugging statements.
It seems to me that the function makemystring works correctly but I have a crash at the level of returning.
Can anyone please help?

Answer 1

ns is an uninitialized pointer when it is dereferenced:

mystring_t ns;

ns->string = (char*)calloc(MAXSTRLEN ,sizeof(char));

as mystring_t is a typedef for struct cell*. Allocate memory for ns before using:

mystring_t ns = malloc(sizeof(*ns)); /* No cast on return value required. */
if (ns)
{
    ns->string = calloc(MAXSTRLEN, 1);
    ns->length = 0;
}

FWIW, this is one of the reasons I dislike hiding pointers in typedefs as it is not obvious at point of use.

Protect going beyond the bounds of ns->string in the condition of the while loop.