c - 为什么动态调整字符串会导致崩溃？

Question

考虑代码:

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");
}

对于输入:

血腥

输出:

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!!!

所以基本上 - *string[*index] = 'B' 有效，当索引是第一个字母时，它会在第二个字母处崩溃。为什么？我可能搞砸了分配或指针，我真的不知道(新手):C

谢谢!

编辑我还想问 - 我的代码还有什么问题吗？

Answer 1

这个表达式是错误的:

*string[*index] = c;

因为 [] 的优先级高于 * 的，代码试图将双指针 string 解释为一个数组指针。当 *index 为零时，您将获得正确的地址，因此第一次迭代成功纯属巧合。

您可以通过使用括号强制执行正确的操作顺序来解决此问题:

(*string)[*index] = c;