CODEWITHSUNDEEP
cpointersmemory-managementmallocrealloc
Omayma Benali
Omayma Benali

Reputation: 119

does the pointer reallocate memory as it increments?

how can i stop *str from filling memory that i didnt give to it, without having to add an ending condition which in this case is : i <= n

do{
   //instructions;
} while (... && i <= n);

in this exemple i reserved only 3 bytes of memory to *str but when i run my code and input more than 3 characters it still works... how does that happen shouldnt it give an error cuz there isnt enough memory for the rest of the characters ? what if the selected empty adresses were xxxxx1 xxxxx2 xxxxx3 and then xxxxx4 is full will it stop and output only the 3 characters without an error ? P.s : I am aware of the function gets() but i dont want to use it because it reallocates memory. i thought that by entering character by character i will solve the problem and stop the user from filling the pointer because this time there is no memory reallocation and *str only has 3 blocks of memory so the rest will go to the buffer and *str will stop at *(str + 2) hope u understood the problem and thank u for answering

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

int main()
{
    int i = -1, n = 3;
    char *str = (char *)malloc(n*sizeof(char));

    printf("Enter a string: ");
    do
    {
        i++;
        str[i] = getchar();
    } while (str[i] != '\n' && i < n);

    str[i] = '\0';

    printf("Entered string is: %s", str);
    return 0;
}

Upvotes: 0

Views: 61

Answers (2)

August Karlstrom
August Karlstrom

Reputation: 11377

Your example doesn't work if the input string is longer than two characters since it then tries to write beyond the array. What will happen when you try to write outside of the array is undefined, which means that it may work by pure chance under some circumstances. Try this safe function instead which always reads the entire line and truncates the result if necessary:

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

void ReadLine(char result[], int resultLen)
{
    int ch, i;

    assert(resultLen > 0);

    i = 0;
    ch = getchar();
    while ((ch != '\n') && (ch != EOF)) {
        if (i < resultLen - 1) {
            result[i] = ch;
            i++;
        }
        ch = getchar();
    }
    result[i] = '\0';
}


int main(void)
{
    int n = 3;
    char *str = malloc(n);

    printf("Enter a string: ");
    ReadLine(str, n);
    printf("Entered string is: %s\n", str);
    free(str);
    return 0;
}

Upvotes: 0

dbush
dbush

Reputation: 224882

C doesn't perform any type of bounds checking on arrays or allocated memory. That's part of what makes it fast.

That also means that reading or writing past the end of an array causes undefined behavior which basically means there's no guarantee what the program will do. The language trusts you to do the proper thing, so it's up to you to ensure that you don't do something you shouldn't.

Also, gets doesn't reallocate memory. In fact, it shouldn't be used at all specifically because it doesn't perform any bounds checking.

Upvotes: 1

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