linux内核中的文件描述符(二)--socket和文件描述符

1. dmsg(a0,(structmsghdr__user*)a1,a[2]);
2. break;
3. caseSYS_RECVMSG:
4. err=sys_recvmsg(a0,(structmsghdr__user*)a1,a[2]);
5. break;
6. default:
7. err=-EINVAL;
8. break;
9. }
10. returnerr;
11. }/*Itmaybealreadyanotherdescriptor8)Notkernelproblem.*/
12. returnretval;
14. out_release:
15. sock_release(sock);
16. returnretval;
17. }

当应用程序使用socket()创建一个socket时，会执行sys_socket，其定义如下

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1. asmlinkagelongsys_socket(intfamily,inttype,intprotocol)
2. {
3. intretval;
4. structsocket*sock;
6. retval=sock_create(family,type,protocol,&sock);//创建socket
7. if(retval<0)
8. gotoout;
10. retval=sock_map_fd(sock);//分配一个未使用的文件描述符fd，并将socket和fd建立联系
11. if(retval<0)
12. gotoout_release;
14. out:
15. /*Itmaybealreadyanotherdescriptor8)Notkernelproblem.*/
16. returnretval;
18. out_release:
19. sock_release(sock);
20. returnretval;
21. }

结构体socket的定义如下(include\linux\net.h)：

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1. structsocket{
2. socket_statestate;
3. unsignedlongflags;
4. structproto_ops*ops;
5. structfasync_struct*fasync_list;
6. structfile*file;//通过这个和文件描述符建立联系
7. structsock*sk;
8. wait_queue_head_twait;
9. shorttype;
10. };

下面我们再来看看sock_map_fd函数

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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();//分配一个未使用的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;//建立联系
38. file->f_op=SOCK_INODE(sock)->i_fop=&socket_file_ops;//socket操作函数，当使用文件系统的IO函数时，其实使用的是socket的IO函数
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. };