TCP显式编码报文长度发送与接收

报文格式最重要的是如何确定报文的边界。常见的报文格式有两种方法，一种是发送端把要发送的报文长度预先通过报文告知给接收端；另一种是通过一些特殊的字符来进行边界的划分。
这篇文章中讲的是发送报文长度的方法。报文类型如下：
在这里插入图片描述

第一部分是4个字节大小的消息长度，其目的是将真正发送的字节流的大小显式通过报文告知接收端，第二部分是 4 个字节大小的消息类型，第2部分才是真正需要发送的数据。

发送端

readnMessageByLength.c里边的代码如下：

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<unistd.h>
#include<arpa/inet.h>
#include<sys/socket.h>
#include<errno.h>
#include<syslog.h>
#include<signal.h>
size_t readn(int fd, void *buffer, size_t length);
size_t read_message(int fd, char *buffer, size_t length);static int count;static void sig_int(int signo) {printf("\nreceived %d datagrams\n", count);exit(0);
}int main(int argc, char **argv) {if (argc != 2) {printf("usage: select01 <IPaddress> or <Port>\n");}int listenfd;listenfd = socket(AF_INET, SOCK_STREAM, 0);struct sockaddr_in server_addr;memset(&server_addr, 0, sizeof(server_addr));server_addr.sin_family = AF_INET;server_addr.sin_addr.s_addr = htonl(INADDR_ANY);server_addr.sin_port=htons(atoi(argv[1]));int on = 1;setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on));int rt1 = bind(listenfd, (struct sockaddr *) &server_addr, sizeof(server_addr));if (rt1 < 0) {printf("bind failed");exit(errno);}int rt2 = listen(listenfd, 5);if (rt2 < 0) {printf("listen failed");exit(errno);}signal(SIGPIPE, SIG_IGN);int connfd;struct sockaddr_in client_addr;socklen_t client_len = sizeof(client_addr);if ((connfd = accept(listenfd, (struct sockaddr *) &client_addr, &client_len)) < 0) {printf("bind failed");exit(errno);}char buf[128];count = 0;while (1) {int n = read_message(connfd, buf, sizeof(buf));if (n < 0) {printf("error read message\n");exit(errno);} else if (n == 0) {printf("client closed \n");exit(0);}buf[n] = 0;printf("received %d bytes: %s\n", n, buf);count++;}exit(0);}size_t readn(int fd, void *buffer, size_t length) {size_t count;ssize_t nread;char *ptr;ptr = buffer;count = length;while (count > 0) {nread = read(fd, ptr, count);if (nread < 0) {if (errno == EINTR)continue;elsereturn (-1);} else if (nread == 0)break;                /* EOF */count -= nread;ptr += nread;}return (length - count);        /* return >= 0 */
}size_t read_message(int fd, char *buffer, size_t length) {u_int32_t msg_length;u_int32_t msg_type;int rc;rc = readn(fd, (char *) &msg_length, sizeof(u_int32_t));if (rc != sizeof(u_int32_t))return rc < 0 ? -1 : 0;msg_length = ntohl(msg_length);rc = readn(fd, (char *) &msg_type, sizeof(msg_type));if (rc != sizeof(u_int32_t))return rc < 0 ? -1 : 0;if (msg_length > length) {return -1;}rc = readn(fd, buffer, msg_length);if (rc != msg_length)return rc < 0 ? -1 : 0;return rc;
}

gcc readnMessageByLength.c -o readnMessageByLength编译，./readnMessageByLength 8080运行。

接收端

SendMessageByLength.c里边的代码如下：

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<unistd.h>
#include<arpa/inet.h>
#include<sys/socket.h>
#include<errno.h>
#include<syslog.h>
#include<signal.h>
int main(int argc, char **argv) {if (argc != 3) {printf("usage: tcpclient <IPaddress>\n");exit(errno);}int socket_fd;socket_fd = socket(AF_INET, SOCK_STREAM, 0);struct sockaddr_in server_addr;bzero(&server_addr, sizeof(server_addr));server_addr.sin_family = AF_INET;server_addr.sin_port = htons(atoi(argv[2]));;inet_pton(AF_INET, argv[1], &server_addr.sin_addr);socklen_t server_len = sizeof(server_addr);int connect_rt = connect(socket_fd, (struct sockaddr *) &server_addr, server_len);if (connect_rt < 0) {fprintf(stderr, "error in connect: %s (%d)\n", strerror(errno), errno);exit(errno);}struct {u_int32_t message_length;u_int32_t message_type;char buf[128];} message;int n;while (fgets(message.buf, sizeof(message.buf), stdin) != NULL) {n = strlen(message.buf);message.message_length = htonl(n);message.message_type = 1;if (send(socket_fd, (char *) &message, sizeof(message.message_length) + sizeof(message.message_type) + n, 0) < 0){fprintf(stderr, "error in send: %s (%d)\n", strerror(errno), errno);exit(errno);}}exit(0);
}