Error in assigning values to memory location with for loop [duplicate]

Question

I have just entered the world of C programming, and I'm currently learning how to use malloc & free.

I've written a short excercise code to printf the string entered with scanf, and I am having trouble compiling it.

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

int main(void)
{
    char *keyword;
    printf("keyword: ");
    scanf("%s", keyword);
    if (keyword == NULL)
        return 1;

    char *t = malloc(strlen(keyword) + 1);
    if (t == NULL)
        return 1;

    for (int i = 0, n = strlen(keyword) + 1; i < n; i++)
        t[i] = keyword[i];

    printf("t: %s\n", t);

    free(t);
    return 0;
}

To debug it, I wrote the same code but without the for loop(code below), and then the code worked well. This led me to conclude that the issue might be with the for loop which is used to assign characters to corresponding memory locations, but I am still unable to find a solution.

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

int main(void)
{
    char *keyword;
    printf("keyword: ");
    scanf("%s", keyword);
    if (keyword == NULL)
        return 1;

    char *t = malloc(strlen(keyword) + 1);
    if (t == NULL)
        return 1;

    t[0] = keyword[0];
    t[1] = keyword[1];
    t[2] = keyword[2];
    t[3] = keyword[3];
    t[4] = keyword[4];
    t[5] = keyword[5];
    printf("t: ");
    printf("%c", t[0]);
    printf("%c", t[1]);
    printf("%c", t[2]);
    printf("%c", t[3]);
    printf("%c", t[4]);
    printf("%c", t[5]);
    printf("\n");

    free(t);
    return 0;
}

I am pretty sure this is a simple error, but I cannot figure out the reason of this error with debugger or online. I would really appreciate it if someone could help me solve this problem.

Answer 1

The following lines:

char *keyword;
printf("keyword: ");
scanf("%s", keyword);
if (keyword == NULL)
     return 1;

Have several problems:

1. keyword is not initialized. It must be initialized to point to some buffer/memory in order to scanf into it in scanf("%s", keyword);.
   Accessing it uninitialized causes undefined behavior (UB).
   In order to initialize it you should determine the maximum length you allow for it, and then initialize it by:
   (1) Using malloc
   (2) Using stack allocation (if the max size is relatively small)
   (3) Setting it to point to some existing buffer
   Don't forget to add 1 to the max length when you allocate, for zero termination.

2. After determining the maximum length of keyword you should pass this value to scanf to make sure the buffer is not overrun, e.g.:

   ... = scanf("%33s", keyword);  // here the maximum length is 33
   

3. You should always check the return value of scanf. If it succeeds it returns:

   Number of receiving arguments successfully assigned

   (here it should be 1).

4. (keyword == NULL) is unlikely to be true (unless by chance), regardless of whether scanf succeeded or not. And if you'll initialize it properly as explained above, it will never be NULL.

Answer 2

and then the code worked well. ...
IT IS NOT Work well !!
It's just because of ... you are a lucky guy....

First of all, when you compile your code, you might get a warning message about this code :

char *keyword;
scanf("%s", keyword);

Because,

1. You are NOT assign(initial) the value(pointer) of keyword.
2. When the pointer of keyword is uncertain, you use it as the input pointer for scanf()

So, where to store the data from the input of 'scanf()' ?? It's uncertainly memory place !!

So, You are a lucky guy that not cause your program to crash...

Do NOT ignore warning message for all situation!

For your case, it's better to change it into :

char keyword[1048575];
scanf("%s", &keyword);

-or-

char *keyword = malloc(1048575);
scanf("%s", keyword);

Then, you might run into for lop smoothly.
~enjoy~

Answer 3

You did not allocate memory for keyword, which sometimes may not cause an error, but it is essential to initialize a pointer. An uninitialized pointer can point to any location in memory, leading to undefined behavior and potentially modifying important memory.