realloc does not copy values correctly

user_1139541 0
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I am trying to read a text file with unknown size into an array. I do that by reading one character at a time into the array and reallocate memory if the the end of the array is reached.

Starting from the increase in array size from 16 to 32, the realloc method copies only the first 4 characters.

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


int main()
{
    int *text_arr = malloc(1*sizeof(int));
    text_arr[0] = '\0';
    int text_len=1;
    int i=0;
    FILE *fp;

    fp = fopen("1.txt", "r");
    
    while ((text_arr[i] = fgetc(fp)) != EOF) {
        /* increment i for next loop */
        i++;

        /* extend the array if needed */
        if (i >= text_len) {
            /* debug prints */
            printf("Text: ");
            for (int n=0; n<i; n++)
                printf("%c,", text_arr[n]);
            printf("\n\n");
            
            /* double array size */
            text_len *= 2;
            text_arr = (int*)realloc(text_arr, text_len);
        }
    }
    text_arr[i] = '\0';

    fclose(fp);
    
    return 0;
}

What could be the problem?

c
3 Answers

You are working with sizeof (int) sized objects, you need to make sure to multiply your requested number of bytes by this size when you reallocate.

realloc(text_arr, sizeof (int) * text_len);
user_25059650

realloc takes the size in bytes as argument. However, you are passing the size in elements, which will be lower by a factor of the size of int (usually 4) since you have ints as elements. That means that you are actually making the array smaller after the first resize, and then you keep writing beyond the end of the array.

Adding * sizeof(int) just like you have it in your malloc line should fix it:

text_arr = (int*)realloc(text_arr, text_len * sizeof(int));
user_18710330

Consider what happens the first time text_len is increased (from 1 to 2). You had originally allocated sizeof(int) bytes to text_arr. However, when you call realloc(text_arr, text_len) and text_len is 2, you're truncating the array from 4 bytes to 2 (assuming that sizeof(int) is 4).

text_arr shouldn't be an int* since you're reading individual characters into it and referencing them by text_arr[n]. Instead, it should be a char* and allocated by malloc(1*sizeof(char)) (or just malloc(1) since sizeof(char) is defined to be 1).

Also, with this change, you won't be able to store the return value of fgetc into the array when it's equal to EOF. Therefore, you should store the return value into an actual int variable and then, if it's not EOF, store it into the array.

user_80755400
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