File and Directories. Advanced Programming in the UNIX Environment

Transcription

1 File and Directories Advanced Programming in the UNIX Environment

2 stat Function #include <sys/stat.h> int stat(const char *restrict pathname, struct stat *restrict buf ); int fstat(int fd, struct stat *buf); int lstat(const char *restrict pathname, struct stat *restrict buf ); int fstatat(int fd, const char *restrict pathname, struct stat *restrict buf, int flag); All four return: 0 if OK, 1 on error the stat function returns a structure of information about the named file.

3 stat Structure

4 File Types Regular file. The most common type of file, which contains data of some form. Directory file. A file that contains the names of other files and pointers to information on these files. Block special file. A type of file providing buffered I/O access in fixed-size units to devices such as disk drives. Character special file. A type of file providing unbuffered I/O access in variable-sized units to devices. FIFO. A type of file used for communication between processes. It s sometimes called a named pipe. Socket. A type of file used for network communication between processes. Symbolic link. A type of file that points to another file.

5 File type macros

6 Set-User-Id and Set-Group-Id

7 Set-User-Id and Set-Group-Id When we execute a program file, the effective user ID of the process is usually the real user ID, and the effective group ID is usually the real group ID. However, we can also set a special flag in the file s mode word (st_mode) that says, When this file is executed, set the effective user ID of the process to be the owner of the file (st_uid). Similarly, we can set another bit in the file s mode word that causes the effective group ID to be the group owner of the file (st_gid). These two bits in the file s mode word are called the set-user-id bit and the set-group-id bit.

8 File Access Permissions The st_mode value also encodes the access permission bits for the file.

9 Read vs Execute for a Directory Read permission for a directory and execute permission for a directory mean different things. Read permission lets us read the directory, obtaining a list of all the filenames in the directory. Execute permission lets us pass through the directory when it is a component of a pathname that we are trying to access.

10 access and faccessat Functions Used when a process wants to test accessibility based on the real user and group IDs. When set-user- ID or the set-group-id feature it might want to verify he real user can access a given file. #include <unistd.h> int access(const char *pathname, int mode); int faccessat(int fd, const char *pathname, int mode, int flag); Both return: 0 if OK, 1 on error

11 umask Function The umask function sets the file mode creation mask for the process and returns the previous value. #include <sys/stat.h> mode_t umask(mode_t cmask); Returns: previous file mode creation mask

12 umask Function Users can set the umask value to control the default permissions on the files they create. This value is expressed in octal, with one bit representing one permission to be masked off.

13 chmod #include <sys/stat.h> int chmod(const char *pathname, mode_t mode); int fchmod(int fd, mode_t mode); int fchmodat(int fd, const char *pathname, mode_t mode, int flag); All three return: 0 if OK, 1 on error The chmod function operates on the specified file, whereas the fchmod function operates on a file that has already been opened.

14 chmod

15 Sticky Bit If it was set for an executable program file, then the first time the program was executed, a copy of the program s text was saved in the swap area when the process terminated. On contemporary systems, the use of the sticky bit has been extended. The Single UNIX Specification allows the sticky bit to be set for a directory. If the bit is set for a directory, a file in the directory can be removed or renamed only if the user has write permission for the directory and meets one of the following criteria: Owns the file Owns the directory Is the superuser

16 File Systems

17 link, unlink, remove, and rename Functions #include <unistd.h> int link(const char *existingpath, const char *newpath); Returns: 0 if OK, 1 on error #include <unistd.h> int unlink(const char *pathname); Returns: 0 if OK, 1 on error

18 #include <stdio.h> int remove(const char *pathname); Returns 0 if OK, 1 on error #include <stdio.h> int rename(const char *oldname, const char *newname); Returns: 0 if OK, 1 on error

19 symlink and readlink Functions #include <unistd.h> int symlink(const char *actualpath, const char *sympath); Returns: 0 if OK, 1 on error #include <unistd.h> ssize_t readlink(const char* restrict pathname, char *restrict buf, size_t bufsize); Returns: number of bytes read if OK, 1 on error

20 utime Function #include <utime.h> int utime(const char *pathname, const struct utimbuf *times); Returns: 0 if OK, 1 on error struct utimbuf { time_t actime; /* access time */ time_t modtime; /* modification time */ }

21 mkdir and rmdir Functions #include <sys/stat.h> int mkdir(const char *pathname, mode_t mode); Returns: 0 if OK, 1 on error #include <unistd.h> int rmdir(const char *pathname); Returns: 0 if OK, 1 on error

22 Reading Directories #include <dirent.h> DIR *opendir(const char *pathname); Returns: pointer if OK, NULL on error struct dirent *readdir(dir *dp); Returns: pointer if OK, NULL at end of directory or error void rewinddir(dir *dp); int closedir(dir *dp); Returns: 0 if OK, 1 on error long telldir(dir *dp); Returns: current location in directory associated with dp void seekdir(dir *dp, long loc);

23 struct dirent { ino_t d_ino; /* i-node number */ char d_name[name_max + 1]; /* null-terminated filename */ }

24 Programs Write a program check file types #include "apue.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"; else if (S_ISLNK(buf.st_mode)) ptr = "symbolic link"; else if (S_ISSOCK(buf.st_mode)) ptr = "socket"; else ptr = "** unknown mode **"; printf("%s\n", ptr); } exit(0); }

25 Utime program #include "apue.h" #include <fcntl.h> #include <utime.h> int main(int argc, char *argv[]) { int i, fd; struct stat statbuf; struct utimbuf timebuf; for (i = 1; i < argc; i++) { if (stat(argv[i], &statbuf) < 0) { /* fetch current times */ err_ret("%s: stat error", argv[i]); continue; } if ((fd = open(argv[i], O_RDWR O_TRUNC)) < 0) { /* truncate */ err_ret("%s: open error", argv[i]); continue; } close(fd); timebuf.actime = statbuf.st_atime; timebuf.modtime = statbuf.st_mtime; if (utime(argv[i], &timebuf) < 0) { /* reset times */ err_ret("%s: utime error", argv[i]); continue; } } exit(0); }

26 Programs WAP to check whether the file exist or not using access function WAP to mask permission bits WAP to change the mode using chmod WAP to create hard link and soft link WAP to copy one time to another time Emulate ls command Emulate ls li