前言
工作中遇到各种各样的网络通信。有MQTT和CoAP这样的物联网应用层协议，也有各种自定义的TCP或UDP协议。使用各种不同的计算机语言和框架开发网络通信，例如Java的Netty框架，C语言原始socket，Python Socket。各有各的使用场景，难易程度相差巨大。Netty上手困难，C语言编写复杂，Python Socket上手容易。
本文将介绍如何使用select套接字实现一个UDP服务器。相较于其他手段，C语言的Socket套接字使用难度较大，但是对于嵌入式开发来说，C语言的Socket是基本功。
示例简述
- 通过recvfrom和sendto实现一个UDP Echo服务器
- 使用Python脚本实现一个UDP客户端，验证UDP服务器是否运行正确
- 开发环境 Linux，构建工具CMake，编译器Linux GCC
UDP服务器
udp-server.c
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/socket.h>
#include <netdb.h>
#include <time.h>
#define BUF_SIZE 500
/*
编译 gcc udp-server.c -o udp-server
运行 ./udp-server 50018
*/
int main(int argc, char *argv[])
{
struct addrinfo hints;
struct addrinfo *result, *rp;
int sfd = -1, s;
struct sockaddr_storage peer_addr;
socklen_t peer_addr_len;
ssize_t num_read;
char buf[BUF_SIZE];
if (argc != 2) {
fprintf(stderr, "usage: %s port\n", argv[0]);
exit(EXIT_FAILURE);
}
memset(&hints, 0, sizeof(struct addrinfo));
hints.ai_family = AF_UNSPEC; /* 允许IPv4 或者 IPv6 */
hints.ai_socktype = SOCK_DGRAM; /* UDP */
hints.ai_flags = AI_PASSIVE;
hints.ai_protocol = 0;
hints.ai_canonname = NULL;
hints.ai_addr = NULL;
hints.ai_next = NULL;
s = getaddrinfo(NULL, argv[1], &hints, &result);
if (s != 0) {
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s));
exit(EXIT_FAILURE);
}
for (rp = result; rp != NULL; rp = rp->ai_next) {
sfd = socket(rp->ai_family, rp->ai_socktype,
rp->ai_protocol);
if (sfd == -1)
continue;
if (bind(sfd, rp->ai_addr, rp->ai_addrlen) == 0)
break;
close(sfd);
}
if (rp == NULL) {
fprintf(stderr, "Could not bind\n");
exit(EXIT_FAILURE);
}
freeaddrinfo(result);
for (;;) {
peer_addr_len = sizeof(struct sockaddr_storage);
num_read = recvfrom(sfd, buf, BUF_SIZE, 0,
(struct sockaddr *)&peer_addr, &peer_addr_len);
buf[num_read] = '\0';
if (num_read == -1)
continue;
char host[NI_MAXHOST], service[NI_MAXSERV];
s = getnameinfo((struct sockaddr *)&peer_addr,
peer_addr_len, host, NI_MAXHOST,
service, NI_MAXSERV, NI_NUMERICSERV);
if (s == 0) {
printf("receive %zd bytes: \"%s\" from %s:%s\n", num_read, buf, host, service);
} else {
fprintf(stderr, "getnameinfo: %s\n", gai_strerror(s));
}
sendto(sfd, buf, (size_t)num_read, 0,
(struct sockaddr *) &peer_addr, peer_addr_len);
}
}
CMake构建
CMakeLists.txt内容如下:
cmake_minimum_required(VERSION 3.13)
set(CMAKE_C_STANDARD 99)
add_executable(udp-server udp-server.c)
编译过程
mkdir -p build
cd build
cmake ..
make
# 最终生成可执行文件udp-server
UDP客户端验证脚本
udp-client.py内容如下
import socket
import binascii
HOST = '127.0.0.1'
PORT = 50018
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
request = bytearray([0x31, 0x32, 0x33, 0x34])
s.sendto(request, (HOST, PORT))
response, server_address = s.recvfrom(1024)
print('received', binascii.hexlify(response), "from", server_address)
测试
先运行UDP服务器
# 可执行文件
cd build
./udp-server 50018
# 运行python脚本后控制台输出
receive 4 bytes: "1234" from localhost:33937
receive 4 bytes: "1234" from localhost:58370
再运行python脚本
python3 udp-client.py
# 控制台输出
received b'31323334' from ('127.0.0.1', 50018)
