c - 在先前 malloc ed 指针上使用 realloc 会导致段错误

Question

我对 C 有问题，这段代码抛出堆栈转储。我不知道怎么了。

char *text;
text = (char *) malloc(sizeof (char));
int size = 1;
char c = 'a';
char *new;
while (1) {
    c = getchar();
    putchar(c);
    if (c == 10) {
        text[size - 1] = '\0';
        break;
    }
    text[size - 1] = c;
    size++;
    new = (char *) realloc(text, size * sizeof (*new));
    free(text);
    text = new;
}

Answer 1

在您的代码中，您将 text 作为第一个参数传递给 realloc()，随后，在不检查是否失败的情况下，将相同的传递给 free( )。这就是导致这里出现问题的原因。

根据 C11，章节 §7.22.3.5

The realloc function deallocates the old object pointed to by ptr and returns a pointer to a new object that has the size specified by size. [...] If memory for the new object cannot be allocated, the old object is not deallocated and its value is unchanged.

所以，如果 realloc() 成功，之后调用

free(text);

调用 undefined behavior .您无需再为 text 操心，删除对 free() 的调用。

相关，对于free()，§7.22.3.3

[...] Otherwise, if the argument does not match a pointer earlier returned by a memory management function, or if the space has been deallocated by a call to free or realloc, the behavior is undefined.

TL;DR 首先检查realloc()是否成功，如果成功，不要碰text，如果 realloc() 失败，则需要调用 free(text);