1. 概述
Socket编程中，使用send()传送数据时，返回结果受到以下几个因素的影响：
•
Blocking模式或non-blocking模式
•
发送缓冲区的大小
•
接收窗口大小
本文档介绍通过实验的方式，得出（收发）缓冲区大小对send结果的影响。实验使用C语言。
2 数据发送和接收的过程
如下图所示，程序调用send()发送数据时，数据将首先进入发送缓冲区，等待发送。系统底层socket负责数据的传送，数据通过网络到达接收方的缓冲区。接收方缓冲区内的数据，等待应用程序调用recv()读取。
3 实验一：Blocking模式下
3.1 实验步骤
发送端：blocking模式send()发送8192字节，使用setsockopt设置SO_SNDBUF改变发送缓冲区的大小。
接收端：建立连接后进入睡眠。使用setsockopt设置SO_RCVBUF改变接收缓冲区的大小。
3.2 实验得到的数据
3.3实验结论
“已发送字节 + 缓冲区中待发送字节 > 总字节”时，send()能立即返回，否则处于阻塞等待。
4 实验二：Non-Blocking模式下
4.1 实验步骤
发送端：non-blocking模式send()发送8192字节，使用setsockopt设置SO_SNDBUF改变发送缓冲区的大小。
接收端：建立连接后进入睡眠。使用setsockopt设置SO_RCVBUF改变接收缓冲区的大小。
4.2 实验数据
4.3 实验结论
随着SNDBUF增大，send()返回已发送字节越大。接收窗口大小对结果影响不是线性的。实际已接收的只有窗口大小。
5. 实验原代码
5.1 服务器端代码
1 #include <netinet/in.h>
2 #include <sys/socket.h>
3 #include <stdio.h>
4 #include <stdlib.h>
5 #include <stdint.h>
6 #include <string.h>
7 #include <errno.h>
8
9 int
10 init_server(int type, const struct sockaddr_in *addr)
11 {
12 int fd;
13 int err = 0;
14 int reuse = 1;
15
16 if ((fd = socket(AF_INET, type, 0)) < 0) {
17 printf("Failed to create socket.\n");
18 exit(1);
19 }
20 if (bind(fd, (struct sockaddr *)addr, sizeof(*addr)) < 0) {
21 printf("Server Bind Failed: %d\n", errno);
22 exit(1);
23 }
24 if (listen(fd, 5) < 0) {
25 printf("Fail to listen\n");
26 exit(1);
27 }
28 return fd;
29 }
30
31 void
32 serve(int fd, int n_to_send, int s_buf_size, int flag)
33 {
34 int clfd, clfd_2;
35 struct sockaddr_in client_addr;
36 char *buf;
37 const char *addr;
38 socklen_t alen = sizeof(int);
39 int n=0;
40 int i;
41 ssize_t num;
42
43 /* initialize the send buffer */
44 buf = malloc(n_to_send * sizeof(char));
45 for (i = 0; i < n_to_send; i++)
46 buf[i] = '=';
47
48 /* change the send buffer size */
49 getsockopt(fd, SOL_SOCKET, SO_SNDBUF, (char *)&n, &alen);
50 printf("SEND buffer size: %d\n", n);
51 getsockopt(fd, SOL_SOCKET, SO_SNDLOWAT, (char *)&n, &alen);
52 printf("SEND LOWAT size: %d\n", n);
53 if (setsockopt(fd, SOL_SOCKET, SO_SNDBUF, (char *)&s_buf_size, sizeof(int)) < 0) {
54 printf("fail to change SNDbuf.\n");
55 exit(2);
56 }
57 getsockopt(fd, SOL_SOCKET, SO_SNDBUF, (char *)&n, &alen);
58 printf("Current SEND buffer size: %d\n", n);
59
60 /* loop: accept a client, and send buffer to it */
61 while(1) {
62 printf("waiting for clients...\n");
63 clfd = accept(fd, (struct sockaddr*)&client_addr, &alen);
64 if(clfd < 0) {
65 printf("accept error.");
66 exit(4);
67 }
68 printf("new client\n");
69 printf("Sending %d bytes...\n", n_to_send);
70 i = send(clfd, buf, n_to_send, flag);
71 printf("send %d bytes.\n", i);
72 close(clfd);
73 printf("\n\n");
74 }
75 }
76
77 int
78 main(int argc, char *argv[])
79 {
80 char host[] = "127.0.0.1";
81 struct sockaddr_in server_addr;
82 uint32_t s_addr;
83 int fd;
84 int n_to_send, s_buf_size, flag;
85
86 if (argc != 4) {
87 printf("useage %s <num to send> <send buf size> <wait_flag:1|0>\n", argv[0]);
88 exit(1);
89 }
90 n_to_send = atoi(argv[1]);
91 s_buf_size = atoi(argv[2]);
92 flag = atoi(argv[3]) ? 0 : MSG_DONTWAIT;
93
94 bzero(&server_addr, sizeof(server_addr));
95 inet_pton(AF_INET, host, &s_addr);
96 server_addr.sin_family = AF_INET;
97 server_addr.sin_addr.s_addr = s_addr;
98 server_addr.sin_port = htons(9000);
99
100 fd = init_server(SOCK_STREAM, &server_addr);
101 serve(fd, n_to_send, s_buf_size, flag);
102
103 exit(0);
104 }
5.2 客户端代码
1 #include <netinet/in.h>
2 #include <unistd.h>
3 #include <errno.h>
4 #include <sys/socket.h>
5 #include <stdio.h>
6 #include <stdlib.h>
7 #include <string.h>
8 #include <stdint.h>
9
10 #define SLEEP_TIME 120
11
12 void
13 tcp_client(int sockfd, struct sockaddr_in *s_addr, int r_buf_size)
14 {
15 int n, i;
16 socklen_t slen = sizeof(int);
17 int rcv_len;
18
19 /* change receiving buffer size */
20 getsockopt(sockfd, SOL_SOCKET, SO_RCVBUF, &rcv_len, &slen);
21 printf("Receive buffer size: %d\n", rcv_len);
22 setsockopt(sockfd, SOL_SOCKET, SO_RCVBUF, &r_buf_size, sizeof(int));
23 getsockopt(sockfd, SOL_SOCKET, SO_RCVBUF, &rcv_len, &slen);
24 printf("Current Receive buffer size: %d\n", rcv_len);
25
26 /* connect to server */
27 if(connect(sockfd, (struct sockaddr*)s_addr, sizeof(*s_addr))) {
28 printf("Can not connect to server.\n");
29 exit(2);
30 }
31
32 sleep(SLEEP_TIME);
33 }
34
35 int
36 main(int argc, char *argv[])
37 {
38 char server_ip[] = "127.0.0.1";
39 int port = 9000;
40 uint32_t s_addr;
41 int s_fd;
42 struct sockaddr_in server_addr;
43 int r_buf_size;
44
45 if (argc != 2) {
46 printf("usage: %s <recv_buf_size>\n", argv[0]);
47 exit(1);
48 }
49 r_buf_size = atoi(argv[1]);
50
51 bzero(&server_addr, sizeof(server_addr));
52 inet_pton(AF_INET, server_ip, &s_addr);
53 server_addr.sin_family = AF_INET;
54 server_addr.sin_addr.s_addr = s_addr;
55 server_addr.sin_port = htons(port);
56 if ((s_fd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
57 printf("Fail to create socket.");
58 exit(1);
59 }
60
61 tcp_client(s_fd, &server_addr, r_buf_size);
62 exit(0);
63 }