Apue Study Notes-Chapter 4. Files And Directories

1. Stat, fstat, lstat

Int Stat (const char * restrict pathname, struct stat * restrict BUF ); Int fstat (INT filedes, struct stat * restrict BUF );

Int lstat (const char * restrict pathname, struct stat * restrict BUF ); All the above three functions can obtain information about the file and return it in the stat structure. Among them, stat uses the file name for retrieval, and tracks the link, while lstat displays the information of the linked file and does not trace. Fstat obtains the relevant file information through the opened file descriptor.

Struct stat { Mode_t st_mode;/* file type & mode (permissions )*/ Ino_t st_ino;/* I-node number (serial number )*/ Dev_t st_dev;/* Device number (File System )*/ Dev_t st_rdev;/* Device number for special files */ Nlink_t st_nlink;/* Number of links */ Uid_t st_uid;/* User ID of owner */ Gid_t st_gid;/* Group ID of owner */ Off_t st_size;/* size in bytes, for regular files */ Time_t st_atime;/* time of last access */ Time_t st_mtime;/* time of last modification */ Time_t st_ctime;/* time of last File status change */ Blksize_t st_blksize;/* Best I/O BLOCK SIZE */ Blkcnt_t st_blocks;/* Number of disk blocks allocated */ }; 2. Each process has six user IDs: real user id real group ID who we really are valid tive user id valid tive group ID Supplementary group IDs used for file access permission checks saved set-user-id saved set-group-id saved by exec functions, which is a copy of valid user ID and valid group ID. 3. File Access Permissions S_irusr User-read S_iwusr User-write S_ixusr User-Execute S_irgrp Group-read S_iwgrp Group-write S_ixgrp Group-Execute S_iroth Other-read S_iwoth Other-write S_ixoth Other-Execute When accessing related files, we must have the running permission for all the folders on the path. If we have the read permission, we can open the file in read-only mode, If you have the write permission, you can open the file as a write. To create or delete a file under a directory, you must have the write and run permissions on the directory. 4. Access check Int access (const char * pathname, int mode ); 5. Sticky Bit By setting stick bit, after the file is run for the first time, the text content is copied to the swap area, and the next time can be quickly run. 6. File System Because each I-node is numbered inside the same file system, ln (1) cannot be linked in different file systems. In the same file system, MV and RM operations do not need to carry data. Each cylinder group can be expressed as follows: Open the file. If the link count of the file is 0, delete the file. Each inode contains the following information: File type, File Permission, file size, file Data Pointer Most of the information in stat is stored in inode. 7. Links Hard links all point to the same I-node Symbolic Links are implemented by storing the directed path name, which can be cross-file systems and direct to directories. Function Does not follow symbolic link Follows symbolic link Access   • Chdir   • Chmod   • Chown • • Creat   • Exec   • Lchown •   Link   • Lstat •   Open   • Opendir   • Pathconf   • Readlink •   Remove •   Rename •   Stat   • Truncate   • Unlink •   8. File time: Field Description Example Ls(1) Option St_atime Last-access time of file data Read -U St_mtime Last-modification time of file data Write Default St_ctime Last-Change Time of I-node status Chmod, chown -C You can use the following functions to modify access time and modification time: Int utime (const char * pathname, const struct utimbuf * times );