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linux內(nèi)核中的文件描述符(二)--socket和文件描述符

作者： 時間：2016-11-22 來源：網(wǎng)絡(luò) 收藏
Kernel version：2.6.14

CPU architecture：ARM920T

本文引用地址：http://m.butianyuan.cn/article/201611/319998.htm

Author：ce123(http://blog.csdn.net/ce123)

socket和文件系統(tǒng)緊密相關(guān)，我們可以通過文件系統(tǒng)的open、read、write和close等操作socket。下面是一個簡單的例子。

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  1. /****************************************************************************/
  2. /*簡介：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. /*這個通訊已經(jīng)結(jié)束*/
  71. close(new_fd);
  72. /*循環(huán)下一個*/
  73. }
  74. close(sockfd);
  75. exit(0);
  76. }

下圖說明了socket和fd是怎樣聯(lián)系起來的。

下面通過來具體分析一下。sys_socket是socket相關(guān)函數(shù)的總?cè)肟凇?/p>

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  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socket時，會執(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)：

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  1. structsocket{
  2. socket_statestate;
  3. unsignedlongflags;
  4. structproto_ops*ops;
  5. structfasync_struct*fasync_list;
  6. structfile*file;//通過這個和文件描述符建立聯(lián)系
  7. structsock*sk;
  8. wait_queue_head_twait;
  9. shorttype;
  10. };
下面我們再來看看sock_map_fd函數(shù)

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  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，其實使用的是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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