面试题：C语言中NAT技术在多线程环境下的应用

设计思路

线程管理：使用POSIX线程库（pthread）创建多个线程，每个线程负责一个网络连接的地址转换。
线程同步：为了避免资源竞争，使用互斥锁（mutex）来保护共享资源，如NAT表。
高效地址转换算法：可以采用哈希表来存储NAT映射关系，以提高查找和插入的效率。

关键代码实现

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#define MAX_CONNECTIONS 100
#define NAT_TABLE_SIZE 1000

// 定义NAT表项
typedef struct {
    unsigned int internal_ip;
    unsigned short internal_port;
    unsigned int external_ip;
    unsigned short external_port;
} NATEntry;

// 定义NAT表
NATEntry nat_table[NAT_TABLE_SIZE];
pthread_mutex_t nat_table_mutex;

// 初始化NAT表
void init_nat_table() {
    memset(nat_table, 0, sizeof(nat_table));
    pthread_mutex_init(&nat_table_mutex, NULL);
}

// 查找NAT表项
NATEntry* find_nat_entry(unsigned int internal_ip, unsigned short internal_port) {
    unsigned int hash = (internal_ip ^ internal_port) % NAT_TABLE_SIZE;
    while (nat_table[hash].internal_ip != 0) {
        if (nat_table[hash].internal_ip == internal_ip && nat_table[hash].internal_port == internal_port) {
            return &nat_table[hash];
        }
        hash = (hash + 1) % NAT_TABLE_SIZE;
    }
    return NULL;
}

// 添加NAT表项
void add_nat_entry(unsigned int internal_ip, unsigned short internal_port,
                   unsigned int external_ip, unsigned short external_port) {
    pthread_mutex_lock(&nat_table_mutex);
    unsigned int hash = (internal_ip ^ internal_port) % NAT_TABLE_SIZE;
    while (nat_table[hash].internal_ip != 0) {
        hash = (hash + 1) % NAT_TABLE_SIZE;
    }
    nat_table[hash].internal_ip = internal_ip;
    nat_table[hash].internal_port = internal_port;
    nat_table[hash].external_ip = external_ip;
    nat_table[hash].external_port = external_port;
    pthread_mutex_unlock(&nat_table_mutex);
}

// 线程处理函数
void* handle_connection(void* arg) {
    unsigned int internal_ip = *((unsigned int*)arg);
    unsigned short internal_port = *((unsigned short*)((char*)arg + sizeof(unsigned int)));

    NATEntry* entry = find_nat_entry(internal_ip, internal_port);
    if (entry == NULL) {
        // 假设这里分配外部IP和端口
        unsigned int external_ip = 0x12345678;
        unsigned short external_port = 12345;
        add_nat_entry(internal_ip, internal_port, external_ip, external_port);
        entry = find_nat_entry(internal_ip, internal_port);
    }

    // 处理地址转换后的网络连接
    printf("Thread handling connection: %u:%u -> %u:%u\n",
           internal_ip, internal_port, entry->external_ip, entry->external_port);

    pthread_exit(NULL);
}

int main() {
    pthread_t threads[MAX_CONNECTIONS];
    unsigned int internal_ips[MAX_CONNECTIONS];
    unsigned short internal_ports[MAX_CONNECTIONS];
    int i;

    init_nat_table();

    // 模拟多个并发网络连接
    for (i = 0; i < MAX_CONNECTIONS; i++) {
        internal_ips[i] = i;
        internal_ports[i] = i + 1000;
        char* arg = (char*)malloc(sizeof(unsigned int) + sizeof(unsigned short));
        memcpy(arg, &internal_ips[i], sizeof(unsigned int));
        memcpy(arg + sizeof(unsigned int), &internal_ports[i], sizeof(unsigned short));
        pthread_create(&threads[i], NULL, handle_connection, arg);
    }

    for (i = 0; i < MAX_CONNECTIONS; i++) {
        pthread_join(threads[i], NULL);
    }

    pthread_mutex_destroy(&nat_table_mutex);
    return 0;
}

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面试题：C语言中NAT技术在多线程环境下的应用

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设计思路

关键代码实现