此实例是客户端向服务器端下载文件：

服务器端代码实现：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <errno.h>
#include <fcntl.h>

#define FD_SERVER_PORT 8088 /*set port*/
#define FD_BUFFERSIZE 1024 /*set buffer size*/
#define FD_LISTENQ 10 /*set listen max conncent*/

int main(int argc,char *argv[])
{
int fd_listenfd,fd_connfd,fd_filefd; /*descriptor*/
int fd_read,fd_write;
int struct_size;
struct sockaddr_in fd_servaddr,fd_cliaddr;
char *fd_filename;
char *p;
char buffer[FD_BUFFERSIZE];

fd_listenfd=socket(AF_INET,SOCK_STREAM,0); /*create socket*/
if(fd_listenfd==-1){
perror("fd_socket");
exit(1);
}

memset(&fd_servaddr,0,sizeof(fd_servaddr)); /*servaddr set 0*/

fd_servaddr.sin_family=AF_INET; /*init serveraddr*/
fd_servaddr.sin_addr.s_addr=INADDR_ANY;
fd_servaddr.sin_port=htons(FD_SERVER_PORT);

/*bind fd_listenfd*/

if(-1==(bind(fd_listenfd,(struct sockaddr *)&fd_servaddr, sizeof(fd_servaddr)))){
perror("fd_bind");
exit(1);
}

/*listen fd_listenfd*/
if(-1==(listen(fd_listenfd,FD_LISTENQ))){
perror("fd_listen");
exit(1);
}

/*file download server start*/
while(1){
printf("file download server starting......\n");

memset(&fd_cliaddr,0,sizeof(fd_cliaddr));
struct_size=sizeof(fd_cliaddr);

fd_connfd=accept(fd_listenfd,(struct sockaddr *)&fd_cliaddr,&struct_size);
if(-1==fd_connfd){
perror("fd_accpet");
continue;
}

fd_filename="/root/tmp.txt";
printf("will download file name is:%s\n",fd_filename);
fd_filefd=open(fd_filename,O_RDONLY);
if(-1==fd_filefd){
perror("open will download file");
continue;
}
while(fd_read=read(fd_filefd,buffer,FD_BUFFERSIZE)){
if(-1==fd_read){
perror("read will download file");
break;
}

p=buffer;
while(fd_write=write(fd_connfd,p,fd_read)){
if(-1==fd_write){
perror("write client file");
break;
}
else if(fd_read==fd_write) break;
else if(fd_write>0){
p+=fd_write;
fd_read-=fd_write;
}
}
}
if(-1==fd_read||-1==fd_write) continue;
close(fd_filefd);
close(fd_connfd);
printf("file already is downloaded!\n");
}
close(fd_listenfd);
return 0;
}

客户端代码实现：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <errno.h>
#include <fcntl.h>

#define FD_SERVER_PORT 8088 /*set port*/
#define FD_BUFFERSIZE 1024 /*set buffer size*/

int main(int argc,char *argv[])
{
int fd_sockfd,fd_filefd; /*descriptor*/
int fd_read,fd_write;
int struct_size;
struct sockaddr_in fd_sockaddr;
char *fd_filename;
char *p;
char buffer[FD_BUFFERSIZE];

if (argc!=2) {
perror("fd_Usage: <./client> <server IP>\n");
exit(1);
}
fd_sockfd=socket(AF_INET,SOCK_STREAM,0); /*create socket*/
if(fd_sockfd==-1){
perror("fd_socket");
exit(1);
}

memset(&fd_sockaddr,0,sizeof(fd_sockaddr)); /*servaddr set 0*/

fd_sockaddr.sin_family=AF_INET; /*init serveraddr*/
inet_aton(argv[1],&fd_sockaddr.sin_addr);
fd_sockaddr.sin_port=htons(FD_SERVER_PORT);

if(-1==(connect(fd_sockfd,(struct sockaddr *)&fd_sockaddr,sizeof(fd_sockaddr)))){
perror("fd_connect");
exit(1);
}
printf("start connecting..........\n");

fd_filename="./backup.txt";
fd_filefd=open(fd_filename,O_RDWR|O_CREAT|O_TRUNC,S_IRWXU);
if (-1==fd_filefd) {
perror("open the backup file");
exit(1);
}
while(fd_read=read(fd_sockfd,buffer,FD_BUFFERSIZE)) {
if(-1==fd_read){
perror("read server file");
exit(1);
}

/*p is used to wirte error*/
p=buffer;
while(fd_write=write(fd_filefd,p,fd_read)){
if(-1==fd_write){
perror("write localhost file");
exit(1);
}
else if(fd_read==fd_write) break;
else if(fd_write>0){
p+=fd_write;
fd_read-=fd_write;
}
}
}
close(fd_filefd);
close(fd_sockfd);
printf("finsh file download!\n");
return 0;

}