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：
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 or \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;
else
return (-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\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);
}
gcc SendMessageByLength.c -o SendMessageByLength编译，./SendMessageByLength 127.0.0.1 8080运行。