c - 什么时候在 C 中未定义指针减法？

Question

char *buf = malloc(bufsize)
char *ptr = buf;
…
while(condition) {
    ptrdiff_t offset = ptr - buf;    // <========== THIS LINE

    // offset will never be negative because we only ever *increase* ptr
    if ((size_t)offset > bufsize) {
        // we need more room
        bufsize += 128;
        buf = realloc(buf, bufsize);
        ptr = buf + offset;  // buf might be in a completely new location
    }
    *ptr++ = …  // write this byte
}

这是有效还是未定义？

我本以为它是有效的，但我读到一些关于它未定义的信息，所以我用谷歌搜索了它。这些链接似乎不可避免地声称它是未定义的:

• Secure coding
• Is subtraction of pointers not pointing to different elements of same array valid in C?

但是，这些 SO 问题中没有提及它:

• Pointer subtraction confusion
• size_t return pointer subtraction
• Pointer Arithmetic In C

这些都在谈论没有两个指针在同一个“数组”中。这是否真的意味着堆栈上有一个普通的旧 C 数组？

如果它是未定义的，我觉得很奇怪……当我可以访问一个常量指针和一个移动指针时，为什么要强制我携带一个计数器变量？

Answer 1

指向由 malloc 返回的内存块的指针算作在同一个数组中:

c11

7.22.3 Memory management functions

1 - The pointer returned [from malloc] if the allocation succeeds [...] may be assigned to a pointer to any type of object [...] and then used to access such an object or an array of such objects in the space allocated (until the space is explicitly deallocated).