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	 > 嵌入式系統(tǒng) > 設(shè)計(jì)應(yīng)用 > linux內(nèi)核中的文件描述符(二)--socket和文件描述符
            
    
            

            

            


	
	
	

            
	
            
		
            
			
            linux內(nèi)核中的文件描述符(二)--socket和文件描述符
            
						
            
				作者：
				時(shí)間：2016-11-22
				來(lái)源：網(wǎng)絡(luò)
				
				
				
				
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			Kernel version：2.6.14
            CPU architecture：ARM920T
            本文引用地址：http://www.biyoush.com/article/201611/319998.htm
            Author：ce123(http://blog.csdn.net/ce123)
            socket和文件系統(tǒng)緊密相關(guān)，我們可以通過(guò)文件系統(tǒng)的open、read、write和close等操作socket。下面是一個(gè)簡(jiǎn)單的例子。
            [plain]view plaincopy
            print?
            1. /****************************************************************************/
            2. /*簡(jiǎn)介：TCPServer示例*/
            3. /****************************************************************************/
            4. #include
            5. #include
            6. #include
            7. #include
            8. #include
            9. #include
            10. #include
            11. #include
            12. intmain(intargc,char*argv[])
            13. {
            14. intsockfd,new_fd;
            15. structsockaddr_inserver_addr;
            16. structsockaddr_inclient_addr;
            17. intsin_size,portnumber;
            18. constcharhello[]="Hellon";
            20. if(argc!=2)
            21. {
            22. fprintf(stderr,"Usage:%sportnumberan",argv[0]);
            23. exit(1);
            24. }
            25. if((portnumber=atoi(argv[1]))<0)
            26. {
            27. fprintf(stderr,"Usage:%sportnumberan",argv[0]);
            28. exit(1);
            29. }
            30. /*服務(wù)器端開始建立socket描述符*/
            31. if((sockfd=socket(AF_INET,SOCK_STREAM,0))==-1)
            32. {
            33. fprintf(stderr,"Socketerror:%sna",strerror(errno));
            34. exit(1);
            35. }
            36. /*服務(wù)器端填充sockaddr結(jié)構(gòu)*/
            37. bzero(&server_addr,sizeof(structsockaddr_in));
            38. server_addr.sin_family=AF_INET;
            39. server_addr.sin_addr.s_addr=htonl(INADDR_ANY);
            40. server_addr.sin_port=htons(portnumber);
            41. /*捆綁sockfd描述符*/
            42. if(bind(sockfd,(structsockaddr*)(&server_addr),sizeof(structsockaddr))==
            43. -1)
            44. {
            45. fprintf(stderr,"Binderror:%sna",strerror(errno));
            46. exit(1);
            47. }
            48. /*監(jiān)聽sockfd描述符*/
            49. if(listen(sockfd,5)==-1)
            50. {
            51. fprintf(stderr,"Listenerror:%sna",strerror(errno));
            52. exit(1);
            53. }
            54. while(1)
            55. {
            56. /*服務(wù)器阻塞,直到客戶程序建立連接*/
            57. sin_size=sizeof(structsockaddr_in);
            58. if((new_fd=accept(sockfd,(structsockaddr*)(&client_addr),&sin_size))==-1)
            59. {
            60. fprintf(stderr,"Accepterror:%sna",strerror(errno));
            61. exit(1);
            62. }
            63. fprintf(stderr,"Servergetconnectionfrom%sn",
            64. inet_ntoa(client_addr.sin_addr));
            65. if(write(new_fd,hello,strlen(hello))==-1)
            66. {
            67. fprintf(stderr,"WriteError:%sn",strerror(errno));
            68. exit(1);
            69. }
            70. /*這個(gè)通訊已經(jīng)結(jié)束*/
            71. close(new_fd);
            72. /*循環(huán)下一個(gè)*/
            73. }
            74. close(sockfd);
            75. exit(0);
            76. }
            下圖說(shuō)明了socket和fd是怎樣聯(lián)系起來(lái)的。
            下面通過(guò)來(lái)具體分析一下。sys_socket是socket相關(guān)函數(shù)的總?cè)肟凇?/p>
            [plain]view plaincopy
            print?
            1. net/socket.c
            2. /*
            3. *Systemcallvectors.
            4. *
            5. *Argumentcheckingcleanedup.Saved20%insize.
            6. *Thisfunctiondoesntneedtosetthekernellockbecause
            7. *itissetbythecallees.
            8. */
            10. asmlinkagelongsys_socketcall(intcall,unsignedlong__user*args)
            11. {
            12. unsignedlonga[6];
            13. unsignedlonga0,a1;
            14. interr;
            16. if(call<1||call>SYS_RECVMSG)
            17. return-EINVAL;
            19. /*copy_from_usershouldbeSMPsafe.*/
            20. if(copy_from_user(a,args,nargs[call]))
            21. return-EFAULT;
            23. err=audit_socketcall(nargs[call]/sizeof(unsignedlong),a);
            24. if(err)
            25. returnerr;
            27. a0=a[0];
            28. a1=a[1];
            30. switch(call)
            31. {
            32. caseSYS_SOCKET:
            33. err=sys_socket(a0,a1,a[2]);
            34. break;
            35. caseSYS_BIND:
            36. err=sys_bind(a0,(structsockaddr__user*)a1,a[2]);
            37. break;
            38. caseSYS_CONNECT:
            39. err=sys_connect(a0,(structsockaddr__user*)a1,a[2]);
            40. break;
            41. caseSYS_LISTEN:
            42. err=sys_listen(a0,a1);
            43. break;
            44. caseSYS_ACCEPT:
            45. err=sys_accept(a0,(structsockaddr__user*)a1,(int__user*)a[2]);
            46. break;
            47. caseSYS_GETSOCKNAME:
            48. err=sys_getsockname(a0,(structsockaddr__user*)a1,(int__user*)a[2]);
            49. break;
            50. caseSYS_GETPEERNAME:
            51. err=sys_getpeername(a0,(structsockaddr__user*)a1,(int__user*)a[2]);
            52. break;
            53. caseSYS_SOCKETPAIR:
            54. err=sys_socketpair(a0,a1,a[2],(int__user*)a[3]);
            55. break;
            56. caseSYS_SEND:
            57. err=sys_send(a0,(void__user*)a1,a[2],a[3]);
            58. break;
            59. caseSYS_SENDTO:
            60. err=sys_sendto(a0,(void__user*)a1,a[2],a[3],
            61. (structsockaddr__user*)a[4],a[5]);
            62. break;
            63. caseSYS_RECV:
            64. err=sys_recv(a0,(void__user*)a1,a[2],a[3]);
            65. break;
            66. caseSYS_RECVFROM:
            67. err=sys_recvfrom(a0,(void__user*)a1,a[2],a[3],
            68. (structsockaddr__user*)a[4],(int__user*)a[5]);
            69. break;
            70. caseSYS_SHUTDOWN:
            71. err=sys_shutdown(a0,a1);
            72. break;
            73. caseSYS_SETSOCKOPT:
            74. err=sys_setsockopt(a0,a1,a[2],(char__user*)a[3],a[4]);
            75. break;
            76. caseSYS_GETSOCKOPT:
            77. err=sys_getsockopt(a0,a1,a[2],(char__user*)a[3],(int__user*)a[4]);
            78. break;
            79. caseSYS_SENDMSG:
            80. err=sys_sendmsg(a0,(structmsghdr__user*)a1,a[2]);
            81. break;
            82. caseSYS_RECVMSG:
            83. err=sys_recvmsg(a0,(structmsghdr__user*)a1,a[2]);
            84. break;
            85. default:
            86. err=-EINVAL;
            87. break;
            88. }
            89. returnerr;
            90. }/*Itmaybealreadyanotherdescriptor8)Notkernelproblem.*/
            91. returnretval;
            93. out_release:
            94. sock_release(sock);
            95. returnretval;
            96. }
            當(dāng)應(yīng)用程序使用socket()創(chuàng)建一個(gè)socket時(shí)，會(huì)執(zhí)行sys_socket，其定義如下
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            1. asmlinkagelongsys_socket(intfamily,inttype,intprotocol)
            2. {
            3. intretval;
            4. structsocket*sock;
            6. retval=sock_create(family,type,protocol,&sock);//創(chuàng)建socket
            7. if(retval<0)
            8. gotoout;
            10. retval=sock_map_fd(sock);//分配一個(gè)未使用的文件描述符fd，并將socket和fd建立聯(lián)系
            11. if(retval<0)
            12. gotoout_release;
            14. out:
            15. /*Itmaybealreadyanotherdescriptor8)Notkernelproblem.*/
            16. returnretval;
            18. out_release:
            19. sock_release(sock);
            20. returnretval;
            21. }
            結(jié)構(gòu)體socket的定義如下(includelinuxnet.h)：
            [plain]view plaincopy
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            1. structsocket{
            2. socket_statestate;
            3. unsignedlongflags;
            4. structproto_ops*ops;
            5. structfasync_struct*fasync_list;
            6. structfile*file;//通過(guò)這個(gè)和文件描述符建立聯(lián)系
            7. structsock*sk;
            8. wait_queue_head_twait;
            9. shorttype;
            10. };
            下面我們?cè)賮?lái)看看sock_map_fd函數(shù)
            [plain]view plaincopy
            print?
            1. intsock_map_fd(structsocket*sock)
            2. {
            3. intfd;
            4. structqstrthis;
            5. charname[32];
            7. /*
            8. *Findafiledescriptorsuitableforreturntotheuser.
            9. */
            11. fd=get_unused_fd();//分配一個(gè)未使用的fd
            12. if(fd>=0){
            13. structfile*file=get_empty_filp();
            15. if(!file){
            16. put_unused_fd(fd);
            17. fd=-ENFILE;
            18. gotoout;
            19. }
            21. this.len=sprintf(name,"[%lu]",SOCK_INODE(sock)->i_ino);
            22. this.name=name;
            23. this.hash=SOCK_INODE(sock)->i_ino;
            25. file->f_dentry=d_alloc(sock_mnt->mnt_sb->s_root,&this);
            26. if(!file->f_dentry){
            27. put_filp(file);
            28. put_unused_fd(fd);
            29. fd=-ENOMEM;
            30. gotoout;
            31. }
            32. file->f_dentry->d_op=&sockfs_dentry_operations;
            33. d_add(file->f_dentry,SOCK_INODE(sock));
            34. file->f_vfsmnt=mntget(sock_mnt);
            35. file->f_mapping=file->f_dentry->d_inode->i_mapping;
            37. sock->file=file;//建立聯(lián)系
            38. file->f_op=SOCK_INODE(sock)->i_fop=&socket_file_ops;//socket操作函數(shù)，當(dāng)使用文件系統(tǒng)的IO函數(shù)時(shí)，其實(shí)使用的是socket的IO函數(shù)
            39. file->f_mode=FMODE_READ|FMODE_WRITE;
            40. file->f_flags=O_RDWR;
            41. file->f_pos=0;
            42. file->private_data=sock;
            43. fd_install(fd,file);
            44. }
            46. out:
            47. returnfd;
            48. }
            50. staticstructfile_operationssocket_file_ops={
            51. .owner=THIS_MODULE,
            52. .llseek=no_llseek,
            53. .aio_read=sock_aio_read,
            54. .aio_write=sock_aio_write,
            55. .poll=sock_poll,
            56. .unlocked_ioctl=sock_ioctl,
            57. .mmap=sock_mmap,
            58. .open=sock_no_open,/*specialopencodetodisallowopenvia/proc*/
            59. .release=sock_close,
            60. .fasync=sock_fasync,
            61. .readv=sock_readv,
            62. .writev=sock_writev,
            63. .sendpage=sock_sendpage
            64. }; 

            
				
            
                
			
							
            
                
			              
              
            
            
		
            
		
            
		
            
		
            
		
            
			
              
				
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