Chapter 4: Files and DirectoriesFiles and DirectoriesStat, fstat, lstatStat detailsFile Types IFile Types IIDevice access via the file systemFile Types IIIFile Types IVDetermining file typeFile type frequenciesSet-User-ID and Set-Group-IDFile Access PermissionsRulesFile Access Permission ChecksOwnership of new files and directoriesAccess functionUmask functionChmod and fchmodSticky bitChown, fchown, and lchownFile sizeFile truncationFile systemsLinkunlinkremoveRenameSymbolic Linkssymlink/readlinkFile timesutimeHow file operations affect timesmkdir and rmdirReading directorieschdir, fchdir, and getcwdSpecial device filessync and fsyncChapter 4: Files and DirectoriesCMPS 105: Systems ProgrammingProf. Scott BrandtT Th 2-3:45Soc Sci 2, Rm. 167Files and Directories Chapter 3 covered basic file I/O Chapter 4 covers more details stat File attributes Special files DirectoriesStat, fstat, lstat Sys/types.h, sys/stat.h Int stat (const char *pathname, struct stat *buf) Int fstat(int fildes, struct stat *buf) Int lstat(const char *pathname, struct stat *buf) All three return 0 or -1 (on error) Provide information about the named file Fstat works on open files Lstat is like stat, but provides info about symbolic link on symbolic linksStat detailsstruct stat {mode_t st_mode; /* file type and mode (perms) */ino_t st ino; /* i-node number */dev_t st_dev; /* device number (filesystem) */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 */long st_blksize;/* best I/O block size */long st_blocks;/* number of 512-byte blocks allocated */};File Types I Regular files Most common Contain data (text, binary, etc.) Kernel considers contents to be a stream of bytes (or blocks of bytes) Directory files Contains the names of other files Also contains pointers to other files Read permission = read contents of directory Write permission = create new files in the directory Execute permission = access files in the directoryFile Types II Character special file A type of file used for certain types of devices Character-oriented devices: keyboard, mouse, … Block special file A type of file used for certain types of devices Block-oriented devices: disk, tape, …Device access via the file system Devices need to be accessible to processes Devices need to be nameable by processes Devices are generally read and written File systems provide all of this We use the file system to interface to the devices The read and write calls executed by the OS are specific to the individual devicesFile Types III FIFO A type of file used for interprocesscommunication (IPC) between files Also called a named pipe Socket A type of file used for network communication between processes Can also be used for processes on the same machineFile Types IV Symbolic Link A type of file that points to another file A hard link is a name for a file Different hard links to the same file are really two different names for the file A soft link always contains the name of a file It refers to the file indirectly through the “real” name of the fileDetermining file type File type is encoded in the st_mode member of the stat data structure Macros S_ISREG() /* regular file */ S_ISDIR() /* directory file */ S_ISCHR() /* character special file */ S_ISBLK() /* block special file */ S_ISFIFO() /* pipe or FIFO */ S_ISLNK() /* symbolic link */ S_ISSOCK() /* socket */#include <sys/types.h>#include <sys/stat.h>Int main(int argc, char *argv[]) {int i;struct stat buf;char *ptr;for(i = 1; i < argc; i++) {printf(“%s: “, argv[i]);if(lstat(argv[i], &buf) < 0) {err_ret(“lstat error”);continue;} if(S_ISREG(buf.st_mode)) ptr = “regular”;else if(S_ISDIR(buf.st_mode)) ptr = “directory”;else if(S_ISCHR(buf.st_mode)) ptr = “character special”;else if(S_ISBLK(buf.st_mode)) ptr = “block special”;else if(S_ISFIFO(buf.st_mode)) ptr = “FIFO”;#ifdef S_ISLNKelse if(S_ISLNK(buf.st_mode)) ptr = “symbolic link”;#endif#ifdef S_ISSOCKelse if(S_ISSOCK(buf.st_mode)) ptr = “socket”;#endifelse ptr = “unknown”;printf(“%s\n”, ptr);}exit(0);}File type frequenciesFile Type Count PercentageRegular file 30,369 91.7%Directory 1,901 5.7%Symbolic link 416 1.3%Character special 373 1.1Block special 61 0.2Socket 5 0.0FIFO 1 0.0Set-User-ID and Set-Group-ID Every process has six or more IDs Who we really are Real user ID Real group ID Taken from our entry in the password file Don’t generally change Who we are currently pretending to be Effective user ID Effective group ID Supplementary group IDs Used for file access permission checks Normally the same as the real user and group ID Can be changed via set-uid and set-gid bits in programs Passwd is a set-uid program Saved by exec() functions Saved set-user-ID Saved set-group-ID Copies of effective user ID and effective group ID when a program is executed Only meaningful when running a set-uid or set-gid programFile Access Permissions st_mode also include access permissions for the file All file types have permissions Nine permission bits 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 */Rules To open a file, must have execute permission on the directory Directory read = read names of files Directory execute = access files Read permission for a file determines if we can read a file Write permission for a file determines if we can write the file Also needed for truncation To create a new file, must have write and execute permission for the directory To delete a file, must have write and execute permission for thedirectory Do not need read or write permission for the file itself To execute a file, must have execute permission for the file andexecute permission for the directoryFile Access Permission Checks If effective user ID is zero,