linux内核中的文件描述符(二)--socket和文件描述符

CPU architecture：ARM920T

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

socket和文件系统紧密相关，我们可以通过文件系统的open、read、write和close等操作socket。下面是一个简单的例子。

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1. /****************************************************************************/
2. /*简介：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[]="Hello\n";
20. if(argc!=2)
21. {
22. fprintf(stderr,"Usage:%sportnumber\a\n",argv[0]);
23. exit(1);
24. }
25. if((portnumber=atoi(argv[1]))<0)
26. {
27. fprintf(stderr,"Usage:%sportnumber\a\n",argv[0]);
28. exit(1);
29. }
30. /*服务器端开始建立socket描述符*/
31. if((sockfd=socket(AF_INET,SOCK_STREAM,0))==-1)
32. {
33. fprintf(stderr,"Socketerror:%s\n\a",strerror(errno));
34. exit(1);
35. }
36. /*服务器端填充sockaddr结构*/
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. /*捆绑sockfd描述符*/
42. if(bind(sockfd,(structsockaddr*)(&server_addr),sizeof(structsockaddr))==
43. -1)
44. {
45. fprintf(stderr,"Binderror:%s\n\a",strerror(errno));
46. exit(1);
47. }
48. /*监听sockfd描述符*/
49. if(listen(sockfd,5)==-1)
50. {
51. fprintf(stderr,"Listenerror:%s\n\a",strerror(errno));
52. exit(1);
53. }
54. while(1)
55. {
56. /*服务器阻塞,直到客户程序建立连接*/
57. sin_size=sizeof(structsockaddr_in);
58. if((new_fd=accept(sockfd,(structsockaddr*)(&client_addr),&sin_size))==-1)
59. {
60. fprintf(stderr,"Accepterror:%s\n\a",strerror(errno));
61. exit(1);
62. }
63. fprintf(stderr,"Servergetconnectionfrom%s\n",
64. inet_ntoa(client_addr.sin_addr));
65. if(write(new_fd,hello,strlen(hello))==-1)
66. {
67. fprintf(stderr,"WriteError:%s\n",strerror(errno));
68. exit(1);
69. }
70. /*这个通讯已经结束*/
71. close(new_fd);
72. /*循环下一个*/
73. }
74. close(sockfd);
75. exit(0);
76. }

下图说明了socket和fd是怎样联系起来的。

下面通过来具体分析一下。sys_socket是socket相关函数的总入口。

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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_sen