c - 扫描与数组类型相同的 N 个成员的结构是否安全？

Question

我有这个结构:

typedef struct V2f {
    float x, y; } V2f;

在我的机器(64位Lubuntu 15.10)上，我可以使用此功能扫描它，没有任何问题:

#include <stdbool.h>
#include <stdio.h>

bool scan_struct_as_arr(void *dst, int n, size_t sz, char *format) {
    for(int i = 0; i < n; ++i) {
        if(!scanf(format, (char *) dst + i * sz)) {
            fputs("Failed to scan struct as an array", stderr);
            return false; } }
    return true; }

这种行为的可移植性有多大？为什么？

如果仅使用大小为 4 或 8 的变量，人们可以信任此代码吗？

Answer 1

一般来说，这不安全或不便携:

C.11 6.7.2.1 Structure and union specifiers

14. Each non-bit-field member of a structure or union object is aligned in an implementation defined manner appropriate to its type.
15. Within a structure object, the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared. ... There may be unnamed padding within a structure object, but not at its beginning.

给定的实现可能会决定，对于结构体，每个其他浮点应该与前一个浮点相距 16 位，而对于数组，它使它们保持连续。

更可移植的解决方案是仅以数组或变量参数的形式传入地址:

bool scan_addr_array (void *addrs[], size_t N, const char *fmt);
bool scan_addr_va (const char *fmt, ... /* NULL terminated */);