• 套接字选项详细介绍，见文章：https://blog.csdn.net/qq_41453285/article/details/89536443

一、选项简单介绍

• 当使用setsockopt来设置TCP的接收缓冲区和发送缓冲区的大小时，系统都会将其值加倍，并且不得小于某个最小值。TCP接收缓冲区的最小值是256字节（不同系统会有不同的值），而发送缓冲区的最小值是2048字节（不同系统会有不同的值）。系统这样做的目的主要是：确保一个TCP连接拥有足够的空闲缓冲区来处理拥塞（比如快速重传算法就期望TCP接收缓冲区能至少容纳4个大小为SMSS的TCP报文段）
• 此外，我们还可以直接修改内核参数/proc/sys/net/ipv4/tcp_rmem和/proc/sys/net/ipv4/tcp_wmem来强制TCP接收缓冲区和发送缓冲区的大小没有最小值限制

二、演示案例

客户端

• 客户端发送数据给服务端，并且设置了发送缓冲区的大小（大小由程序参数传入），设置完之后打印一下设置的缓冲区大小

//cli_send.c

#include <sys/socket.h>
#include <arpa/inet.h>
#include <assert.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <libgen.h>

#define BUFFER_SIZE 512

int main( int argc, char* argv[] )
{
    if( argc <= 3 )
    {
        printf( "usage: %s ip_address port_number send_bufer_size\n", basename( argv[0] ) );
        return 1;
    }
    const char* ip = argv[1];
    int port = atoi( argv[2] );

    struct sockaddr_in server_address;
    bzero( &server_address, sizeof( server_address ) );
    server_address.sin_family = AF_INET;
    inet_pton( AF_INET, ip, &server_address.sin_addr );
    server_address.sin_port = htons( port );

    int sock = socket( PF_INET, SOCK_STREAM, 0 );
    assert( sock >= 0 );

    int sendbuf = atoi( argv[3] );
    int len = sizeof( sendbuf );
    setsockopt( sock, SOL_SOCKET, SO_SNDBUF, &sendbuf, sizeof( sendbuf ) );
    getsockopt( sock, SOL_SOCKET, SO_SNDBUF, &sendbuf, ( socklen_t* )&len );
    printf( "the tcp send buffer size after setting is %d\n", sendbuf );

    if ( connect( sock, ( struct sockaddr* )&server_address, sizeof( server_address ) ) != -1 )
    {
        char buffer[ BUFFER_SIZE ];
        memset( buffer, 'a', BUFFER_SIZE );
        send( sock, buffer, BUFFER_SIZE, 0 );
    }

    close( sock );
    return 0;
}

服务端

• 服务端接收客户端的数据，并且设置了接收缓冲区的大小（大小由程序参数传入），设置完之后打印一下设置的缓冲区大小

//ser_recv.c

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <assert.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <libgen.h>

#define BUFFER_SIZE 1024

int main( int argc, char* argv[] )
{
    if( argc <= 3 )
    {
        printf( "usage: %s ip_address port_number receive_buffer_size\n", basename( argv[0] ) );
        return 1;
    }
    const char* ip = argv[1];
    int port = atoi( argv[2] );

    struct sockaddr_in address;
    bzero( &address, sizeof( address ) );
    address.sin_family = AF_INET;
    inet_pton( AF_INET, ip, &address.sin_addr );
    address.sin_port = htons( port );

    int sock = socket( PF_INET, SOCK_STREAM, 0 );
    assert( sock >= 0 );
    int recvbuf = atoi( argv[3] );
    int len = sizeof( recvbuf );
    setsockopt( sock, SOL_SOCKET, SO_RCVBUF, &recvbuf, sizeof( recvbuf ) );
    getsockopt( sock, SOL_SOCKET, SO_RCVBUF, &recvbuf, ( socklen_t* )&len );
    printf( "the receive buffer size after settting is %d\n", recvbuf );

    int ret = bind( sock, ( struct sockaddr* )&address, sizeof( address ) );
    assert( ret != -1 );

    ret = listen( sock, 5 );
    assert( ret != -1 );

    struct sockaddr_in client;
    socklen_t client_addrlength = sizeof( client );
    int connfd = accept( sock, ( struct sockaddr* )&client, &client_addrlength );
    if ( connfd < 0 )
    {
        printf( "errno is: %d\n", errno );
    }
    else
    {
        char buffer[ BUFFER_SIZE ];
        memset( buffer, '\0', BUFFER_SIZE );
        while( recv( connfd, buffer, BUFFER_SIZE-1, 0 ) > 0 ){}
        close( connfd );
    }

    close( sock );
    return 0;
}

运行程序

• 运行服务端程序，将其接收缓冲区大小设置为50字节（但是输出的大小却是2304字节，说明此系统的TCP接收端缓冲区最小值为2304）

• 运行客户端程序，将其发送缓冲区大小设置为2304字节（但是输出的大小却是4608，这与缓冲区会被系统设置加倍的概念相符合）