【发布时间】：2022-01-19 05:51:27
【问题描述】：
此函数不返回预期结果（交换a 和b）。
#include<stdio.h>
#include<stdlib.h>
void swap_one(int *x, int *y) {
int *tmp;
tmp = x;
x = y;
y = tmp;
printf("x = %d y = %d\n", *x, *y);
}
void swap_two(int *x, int *y) {
}
int main() {
int a = 5;
int b = 100;
printf("Before a = %d b = %d\n\n", a, b);
int *p = (int *) malloc(sizeof(int));
int *q = (int *) malloc(sizeof(int));
p = &a;
q = &b;
swap_one(p, q);
printf("After a = %d b = %d\n", a, b);
return 0;
}
但下面的代码按预期工作。
#include <stdlib.h>
#include <stdio.h>
typedef struct ListElmt_ {
void *data;
struct ListElmt_ *next;
} ListElmt;
typedef struct List_ {
int size;
int (*match) (const void *key1, const void *key2);
void (*destroy) (void *data);
ListElmt *head;
ListElmt *tail;
} List;
void list_init (List *list) {
list->size = 0;
list->match = NULL;
list->destroy = NULL;
list->head = NULL;
list->tail = NULL;
}
int list_ins_next(List *list, ListElmt *element, void *data) {
ListElmt *new_element;
/* Alocate storage for the element. */
if ((new_element = (ListElmt *) malloc(sizeof(ListElmt))) == NULL) return -1;
/* new_element->data is of type void *. So we use (void *) data */
new_element->data = (void *)data;
if (element == NULL) {
/* Handle insertion at the head of the list */
if (list->size == 0) list->tail = new_element;
new_element->next = list->head;
list->head = new_element;
} else {
if (element->next == NULL) list->tail = new_element;
new_element->next = element->next;
element->next = new_element;
}
list->size++;
return 0;
}
/* Print the list */
static void print_list(const List *list) {
ListElmt *element;
int *data;
int i;
/* Display the linked list */
fprintf(stdout, "List size is %d\n", list->size);
i = 0;
element = list->head;
while (1) {
data = element->data;
fprintf(stdout, "list[%03d] = %03d\n", i, *data);
i++;
if (element->next == NULL) {
break;
} else {
element = element->next;
}
}
}
int main(int argc, char **argv) {
List list;
ListElmt *element;
int *data;
int i;
/* list = (List *) malloc(sizeof(List)); */
/* Initialize the linked list */
List *listPtr;
listPtr = &list;
list_init(listPtr);
/* Perform some linked list operations */
element = listPtr->head;
for (i = 10; i > 0; i--) {
if ( (data = (int *) malloc(sizeof(int))) == NULL) return 1;
*data = i;
if (list_ins_next(listPtr, NULL, data) != 0) return 1;
}
print_list(listPtr);
fprintf(stdout, "Value in *data is:%d\n", *data);
return 0;
}
问题是：在swap_one函数中，x=y类似于new_element->next = element->next或element->next = new_element。为什么new_element->next = element->next 和element->next = new_element 可以工作，但swap_one 函数中的x =y 不能交换a 和b？
很抱歉有很多代码，但我真的对这个感到困惑。
谢谢。
【问题讨论】：
标签： c pass-by-reference call-by-value