sendfile

参考
http://calmness.iteye.com/blog/378463
在传统的文件传输里面（read/write方式），在实现上其实是比较复杂的，需要经过多次上下文的切换，我们看一下如下两行代码：
Java代码 
read(file, tmp_buf, len);     
       write(socket, tmp_buf, len);     
    
        以上两行代码是传统的read/write方式进行文件到socket的传输。

当需要对一个文件进行传输的时候，其具体流程细节如下：
1、调用read函数，文件数据被copy到内核缓冲区
2、read函数返回，文件数据从内核缓冲区copy到用户缓冲区
3、write函数调用，将文件数据从用户缓冲区copy到内核与socket相关的缓冲区。
4、数据从socket缓冲区copy到相关协议引擎。

以上细节是传统read/write方式进行网络文件传输的方式，我们可以看到，在这个过程当中，文件数据实际上是经过了四次copy操作：

硬盘—>内核buf—>用户buf—>socket相关缓冲区—>协议引擎

而sendfile系统调用则提供了一种减少以上多次copy，提升文件传输性能的方法。Sendfile系统调用是在2.1版本内核时引进的：
Java代码 
sendfile(socket, file, len); 

运行流程如下：
1、sendfile系统调用，文件数据被copy至内核缓冲区
2、再从内核缓冲区copy至内核中socket相关的缓冲区
3、最后再socket相关的缓冲区copy到协议引擎

相较传统read/write方式，2.1版本内核引进的sendfile已经减少了内核缓冲区到user缓冲区，再由user缓冲区到socket相关缓冲区的文件copy，而在内核版本2.4之后，文件描述符结果被改变，sendfile实现了更简单的方式，系统调用方式仍然一样，细节与2.1版本的不同之处在于，当文件数据被复制到内核缓冲区时，不再将所有数据copy到socket相关的缓冲区，而是仅仅将记录数据位置和长度相关的数据保存到socket相关的缓存，而实际数据将由DMA模块直接发送到协议引擎，再次减少了一次copy操作。
★★★★例子代码

[root@VM_253_237_tlinux sendfile]# cat sendclient.c 
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/sendfile.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <fcntl.h>
#include <stdio.h>
#include <sys/un.h>
#include <unistd.h>
#include <stdlib.h>
#include <time.h>

int main (int argc, char *argv[])
{
        int a=clock();
        printf("this is used %d\n",a);
        struct sockaddr_un address;
        int sockfd;
        int len, result;
        int i, bytes;
        struct stat buf;
        off_t off;
        ssize_t res, total = 0;
        int wfd;
        char rwbuf[4096];
        wfd = open ("src_sendfile_save", O_WRONLY);
        if (wfd < 0) {
                perror ("open");
                exit (EXIT_FAILURE);
        }

        /*..socket,AF_UNIX....,SOCK_STREAM....*/
        if ((sockfd = socket(AF_UNIX, SOCK_STREAM, 0)) == -1) {
                perror ("socket");
                exit (EXIT_FAILURE);
        }

        address.sun_family = AF_UNIX;
        strcpy (address.sun_path, "server_socket");
        len = sizeof (address);

        /*..........*/
        result = connect (sockfd, (struct sockaddr *)&address, len);
        if (result == -1) {
                printf ("ensure the server is up\n");
                perror ("connect");
                exit (EXIT_FAILURE);
        }

        while ((res = read (sockfd, rwbuf, 4096)) > 0) {
                total += res;
                write (wfd, rwbuf, 4096);
        }
        printf ("total %u bytes received from server snedfile\n", total);
        close (sockfd);
        close (wfd);

        int b=clock();
        printf("this is used %d\n",b);
        int c=b-a;
        printf("this is used    %d\n",c);
        return (0);
}
[root@VM_253_237_tlinux sendfile]# 

[root@VM_253_237_tlinux sendfile]# cat sendserver.c
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/un.h>
#include <sys/sendfile.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>

int main (int argc, char **argv)
{
        struct sockaddr_un sin1;
        int server_sockfd, client_sockfd;
        int server_len, client_len;
        ssize_t bytes, res=0;
        ssize_t rtotal = 0;
        FILE *stream;
        int in_fd;
        struct stat buf;
        off_t off = 0;

        unlink ("server_socket");
        unlink ("src_sendfile_save");
        stream = fopen ("src_sendfile_save", "w");
        if (!stream) {
                perror ("fopen");
                exit (EXIT_FAILURE);
        }
        fclose (stream);

        if ((in_fd = open ("src", O_RDONLY)) < 0) {
                printf ("Can't open 'src' file");
                exit (EXIT_FAILURE);
        }
        if (fstat (in_fd, &buf) == -1) {
                printf ("Can't stat 'src' file\n");
                exit (EXIT_FAILURE);
        }
        printf ("Get file size are %u bytes\n", buf.st_size);

        server_sockfd = socket (AF_UNIX, SOCK_STREAM, 0);
        if (server_sockfd < 0) {
                perror ("socket");
                exit (EXIT_FAILURE);
        }
        sin1.sun_family = AF_UNIX;
        strcpy (sin1.sun_path, "server_socket");

        server_len = sizeof (sin1);
        if (bind (server_sockfd, (struct sockaddr *)&sin1, server_len) < 0) {
                perror ("bind");
                exit (EXIT_FAILURE);
        }
        if (listen (server_sockfd, 5) < 0) {
                perror ("listen");
                exit (EXIT_FAILURE);
        }

        printf ("The server is waiting for client connect...\n");

        client_sockfd = accept (server_sockfd, (struct sockaddr *)&sin1, (socklen_t *)&client_len);

        if (client_sockfd == -1 ) {
                perror ("accept");
                exit (EXIT_FAILURE);
        }
        while (off < buf.st_size) {
                if ((res = sendfile (client_sockfd, in_fd, &off, buf.st_size)) < 0 ) {
                        printf ("sendfile failed\n");
                        exit (EXIT_FAILURE);
                } else {
                        rtotal += res;
                }
        }

        printf ("server sendfile total %u bytes\n", rtotal);
        close (client_sockfd);
        unlink ("server_socket");
        return (0);
}
[root@VM_253_237_tlinux sendfile]#