[英]C: Trouble with GMP pointers in a struct
I'm fairly new to c and I'm implementing a DHT.我对 c相当陌生,我正在实施 DHT。 For that, I have to store node IDs in a list.
为此,我必须将节点 ID 存储在列表中。
Node IDs are < 2^160 whole numbers(thus resulting in me using GMP), but I'm setting 123 as an ID here to keep the example clean.节点 ID 是 < 2^160 整数(因此导致我使用 GMP),但我在这里将 123 设置为 ID 以保持示例清洁。
I am able to set the Node's ID just fine, and I also verify it with mpz_out_str
.我可以很好地设置节点的 ID,我还使用
mpz_out_str
验证它。 When I iterate over it with g_slist_foreach
and print the ID, it outputs a HUGE number, which is completely irrelevant.当我用
g_slist_foreach
迭代它并打印 ID 时,它会输出一个巨大的数字,这完全不相关。
I suspect it has something to do with the cast from gpointer
to Node
, or that I may be storing it incorrectly(very new to pointers )我怀疑它与从
gpointer
到Node
的转换有关,或者我可能存储不正确(对指针来说非常新)
Code:代码:
#include <glib.h>
#include <gmp.h>
#include <stdio.h>
typedef struct {
mpz_t *node_id;
} Node;
void print_iterator(gpointer item, gpointer prefix) {
// Also, since the item is of type gpointer,
// It has to be casted back to Node.
Node *node = (Node *)item;
// outputs a HUGE, completely random number.
mpz_out_str(stdout, 10, node->node_id);
printf("\n");
};
GSList *add_node(GSList *nodes) {
Node *node = malloc(sizeof(Node));
mpz_t id;
mpz_init(id);
char *f = "123";
mpz_set_str(id, f, 10);
node->node_id = &id;
// prints correct value
mpz_out_str(stdout, 10, node->node_id);
nodes = g_slist_append(nodes, node);
return nodes;
}
int main(int argc, char const *argv[]) {
GSList *nodes = NULL;
nodes = add_node(nodes);
g_slist_foreach(nodes, print_iterator, "‑‑>");
return 0;
}
Relevant links:相关链接:
https://developer.gnome.org/glib/stable/glib-Singly-Linked-Lists.html https://developer.gnome.org/glib/stable/glib-Singly-Linked-Lists.html
https://gmplib.org/manual/ https://gmplib.org/manual/
With node->node_id = &id;
使用
node->node_id = &id;
your node_id points at a local variable.你的 node_id 指向一个局部变量。 When
add_node
returns, id
is destroyed and your pointer is dangling.当
add_node
返回时, id
被销毁并且您的指针悬空。 Dereferencing it results in Undefined Behavior.取消引用它会导致未定义的行为。
A simple solution is to store the id in Node
rather than storing a pointer to it.一个简单的解决方案是将 id 存储在
Node
中,而不是存储指向它的指针。
typedef struct {
mpz_t node_id;
} Node;
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