#include <stdio.h>
int main() {


    /***Declaration of non-local variable in 'for' loop***/
    for (struct {
             int i;
         } s = {0}; s.i < 25; ++s.i) {
        printf("-----------\n");
    }

    /**等價方式**/
    {
        struct {
            int i;
        } s = {0};
        for
 
 (; s.i < 25; ++s.i) {
            printf("-----------\n");
        }
    }


    return 0;
}

參考自：https://stackoverflow.com/questions/53468223/declare-structure-within-for

這是我在使用一份程式碼[https://github.com/DavidLeeds/hashmap]的時候發現的問題：

#define __HASHMAP_FOREACH(x, key, data, h)                              \
    for
 
 (HASHMAP_ITER(*(h)) __HASHMAP_UNIQUE(x, it) = hashmap_iter(h, &__HASHMAP_UNIQUE(x, it)); \
        ((key) = hashmap_iter_get_key(&__HASHMAP_UNIQUE(x, it))) &&     \
            ((data) = hashmap_iter_get_data(&__HASHMAP_UNIQUE(x, it))); \
        hashmap_iter_next(&__HASHMAP_UNIQUE(x, it)))
 
#define __HASHMAP_FOREACH_SAFE(x, key, data, h, temp_ptr)               \
    for (HASHMAP_ITER(*(h)) __HASHMAP_UNIQUE(x, it) = hashmap_iter(h, &__HASHMAP_UNIQUE(x, it)); \
        ((temp_ptr) = (void *)((key) = hashmap_iter_get_key(&__HASHMAP_UNIQUE(x, it)))) && \
            ((data) = hashmap_iter_get_data(&__HASHMAP_UNIQUE(x, it)));           \
        ((temp_ptr) == (void *)hashmap_iter_get_key(&__HASHMAP_UNIQUE(x, it))) ?  \
            hashmap_iter_next(&__HASHMAP_UNIQUE(x, it)) : __HASHMAP_ITER_RESET(&__HASHMAP_UNIQUE(x, it)))
#define __HASHMAP_FOREACH_KEY(x, key, h)                                \
    for (HASHMAP_ITER(*(h)) __HASHMAP_UNIQUE(x, it) = hashmap_iter(h, &__HASHMAP_UNIQUE(x, it)); \
        (key = hashmap_iter_get_key(&__HASHMAP_UNIQUE(x, it)));                   \
        hashmap_iter_next(&__HASHMAP_UNIQUE(x, it)))
#define __HASHMAP_FOREACH_KEY_SAFE(x, key, h, temp_ptr)                 \
    for (HASHMAP_ITER(*(h)) __HASHMAP_UNIQUE(x, it) = hashmap_iter(h, &__HASHMAP_UNIQUE(x, it)); \
        ((temp_ptr) = (void *)((key) = hashmap_iter_get_key(&__HASHMAP_UNIQUE(x, it)))); \
        ((temp_ptr) == (void *)hashmap_iter_get_key(&__HASHMAP_UNIQUE(x, it))) ?  \
            hashmap_iter_next(&__HASHMAP_UNIQUE(x, it)) : __HASHMAP_ITER_RESET(&__HASHMAP_UNIQUE(x, it)))
#define __HASHMAP_FOREACH_DATA(x, data, h)                              \
    for (HASHMAP_ITER(*(h)) __HASHMAP_UNIQUE(x, it) = hashmap_iter(h, &__HASHMAP_UNIQUE(x, it)); \
        (data = hashmap_iter_get_data(&__HASHMAP_UNIQUE(x, it)));                 \
        hashmap_iter_next(&__HASHMAP_UNIQUE(x, it)))
#define __HASHMAP_FOREACH_DATA_SAFE(x, data, h, temp_ptr)               \
    for (HASHMAP_ITER(*(h)) __HASHMAP_UNIQUE(x, it) = hashmap_iter(h, &__HASHMAP_UNIQUE(x, it)); \
        ((temp_ptr) = (void *)hashmap_iter_get_key(&__HASHMAP_UNIQUE(x, it))) &&  \
            ((data) = hashmap_iter_get_data(&__HASHMAP_UNIQUE(x, it)));           \
        ((temp_ptr) == (void *)hashmap_iter_get_key(&__HASHMAP_UNIQUE(x, it))) ?  \
            hashmap_iter_next(&__HASHMAP_UNIQUE(x, it)) : __HASHMAP_ITER_RESET(&__HASHMAP_UNIQUE(x, it)))

更正：

#define __HASHMAP_FOREACH(x, key, data, h)                              \
   HASHMAP_ITER(*(h)) __HASHMAP_UNIQUE(x, it) = hashmap_iter(h, &__HASHMAP_UNIQUE(x, it)); for (; \
        ((key) = hashmap_iter_get_key(&__HASHMAP_UNIQUE(x, it))) &&     \
            ((data) = hashmap_iter_get_data(&__HASHMAP_UNIQUE(x, it))); \
        hashmap_iter_next(&__HASHMAP_UNIQUE(x, it))) \

#define __HASHMAP_FOREACH_SAFE(x, key, data, h, temp_ptr)               \
  HASHMAP_ITER(*(h)) __HASHMAP_UNIQUE(x, it) = hashmap_iter(h, &__HASHMAP_UNIQUE(x, it)); for (; \
        ((temp_ptr) = (void *)((key) = hashmap_iter_get_key(&__HASHMAP_UNIQUE(x, it)))) && \
            ((data) = hashmap_iter_get_data(&__HASHMAP_UNIQUE(x, it)));           \
        ((temp_ptr) == (void *)hashmap_iter_get_key(&__HASHMAP_UNIQUE(x, it))) ?  \
            hashmap_iter_next(&__HASHMAP_UNIQUE(x, it)) : __HASHMAP_ITER_RESET(&__HASHMAP_UNIQUE(x, it)))
#define __HASHMAP_FOREACH_KEY(x, key, h)                                \
  HASHMAP_ITER(*(h)) __HASHMAP_UNIQUE(x, it) = hashmap_iter(h, &__HASHMAP_UNIQUE(x, it)); for (; \
        (key = hashmap_iter_get_key(&__HASHMAP_UNIQUE(x, it)));                   \
        hashmap_iter_next(&__HASHMAP_UNIQUE(x, it)))
#define __HASHMAP_FOREACH_KEY_SAFE(x, key, h, temp_ptr)                 \
   HASHMAP_ITER(*(h)) __HASHMAP_UNIQUE(x, it) = hashmap_iter(h, &__HASHMAP_UNIQUE(x, it)); for (; \
        ((temp_ptr) = (void *)((key) = hashmap_iter_get_key(&__HASHMAP_UNIQUE(x, it)))); \
        ((temp_ptr) == (void *)hashmap_iter_get_key(&__HASHMAP_UNIQUE(x, it))) ?  \
            hashmap_iter_next(&__HASHMAP_UNIQUE(x, it)) : __HASHMAP_ITER_RESET(&__HASHMAP_UNIQUE(x, it)))
#define __HASHMAP_FOREACH_DATA(x, data, h)                              \
   HASHMAP_ITER(*(h)) __HASHMAP_UNIQUE(x, it) = hashmap_iter(h, &__HASHMAP_UNIQUE(x, it)); for (; \
        (data = hashmap_iter_get_data(&__HASHMAP_UNIQUE(x, it)));                 \
        hashmap_iter_next(&__HASHMAP_UNIQUE(x, it)))
#define __HASHMAP_FOREACH_DATA_SAFE(x, data, h, temp_ptr)               \
   HASHMAP_ITER(*(h)) __HASHMAP_UNIQUE(x, it) = hashmap_iter(h, &__HASHMAP_UNIQUE(x, it));  for (; \
        ((temp_ptr) = (void *)hashmap_iter_get_key(&__HASHMAP_UNIQUE(x, it))) &&  \
            ((data) = hashmap_iter_get_data(&__HASHMAP_UNIQUE(x, it)));           \
        ((temp_ptr) == (void *)hashmap_iter_get_key(&__HASHMAP_UNIQUE(x, it))) ?  \
            hashmap_iter_next(&__HASHMAP_UNIQUE(x, it)) : __HASHMAP_ITER_RESET(&__HASHMAP_UNIQUE(x, it)))