Why dynamically resizing a string causes a crash?

Question

Consider code:

char *word = NULL;                                      // Pointer at buffered string.
int size = 0;                                           // Size of buffered string.
int index = 0;                                          // Write index.

char c;                                                 // Next character read from file.

FILE *file = fopen(fileDir, "r");
if (file)
{
    while ((c = getc(file)) != EOF)
    {
        printf("Current index: %d, size: %d, Word: %s\n", index, size, word);
        if (isValidChar(c))
        {
            appendChar(c, &word, &size, &index);
        }
        else if (word) // Any non-valid char is end of word. If (pointer) word is not null, we can process word.
        {
            // Processing parsed word.
            size = 0;                                   // Reset buffer size.
            index = 0;                                  // Reset buffer index.
            free(word);                                 // Free memory.
            word = NULL;                                // Nullify word.
            // Next word will be read
        }
    }
}
fclose(file);

/* Appends c to string, resizes string, inceremnts index. */
void appendChar(char c, char **string, int *size, int *index)
{
    printf("CALL\n");
    if (*size <= *index)                                // Resize buffer.
    {
        *size += 1; // Words are mostly 1-3 chars, that's why I use +1.
        char *newString = realloc(*string, *size);          // Reallocate memory.

        printf("REALLOC\n");

        if (!newString)                                     // Out of memory?
        {
            printf("[ERROR] Failed to append character to buffered string.");
            return;
        }

        *string = newString;
        printf("ASSIGN\n");
    }

    *string[*index] = c;
    printf("SET\n");
    (*index)++;
    printf("RET\n");
}

For Input:

BLOODY

Output:

Current index: 0, size: 0, Word: <null>
CALL
REALLOC
ASSIGN
SET
RET
Current index: 1, size: 1, Word: B** // Where * means "some random char" since I am NOT saving additional '\0'. I don't need to, I have my size/index.
CALL
REALLOC
ASSIGN
CRASH!!!

So basically - *string[*index] = 'B' works, when index is first letter, it crashes at second one. Why? I probably messed up allocation or pointers, I don't really know (novice) :C

Thank you!

EDIT I also ment to ask - is there anything else wrong with my code?

Answer 1

This expression is incorrect:

*string[*index] = c;

Since []'s precedence is higher than *'s, the code tries to interpret the double pointer string as an array of pointers. When *index is zero, you get the right address, so the first iteration works by pure coincidence.

You can fix this by forcing the right order of operations with parentheses:

(*string)[*index] = c;