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sys/stat.h - data returned by the stat() function
The <sys/stat.h> header shall define the structure of the data returned by the functions fstat(), lstat(), and stat(). The stat structure shall contain at least the following members: dev_t st_dev Device ID of device containing file. ino_t st_ino File serial number. mode_t st_mode Mode of file (see below). nlink_t st_nlink Number of hard links to the file. uid_t st_uid User ID of file. gid_t st_gid Group ID of file. dev_t st_rdev Device ID (if file is character or block special). off_t st_size For regular files, the file size in bytes. For symbolic links, the length in bytes of the pathname contained in the symbolic link. For a shared memory object, the length in bytes. For a typed memory object, the length in bytes. For other file types, the use of this field is unspecified. time_t st_atime Time of last access. time_t st_mtime Time of last data modification. time_t st_ctime Time of last status change. blksize_t st_blksize A file system-specific preferred I/O block size for this object. In some file system types, this may vary from file to file. blkcnt_t st_blocks Number of blocks allocated for this object. The st_ino and st_dev fields taken together uniquely identify the file within the system. The blkcnt_t, blksize_t, dev_t, ino_t, mode_t, nlink_t, uid_t, gid_t, off_t, and time_t types shall be defined as described in <sys/types.h> . Times shall be given in seconds since the Epoch. Unless otherwise specified, the structure members st_mode, st_ino, st_dev, st_uid, st_gid, st_atime, st_ctime, and st_mtime shall have meaningful values for all file types defined in IEEE Std 1003.1-2001. For symbolic links, the st_mode member shall contain meaningful information, which can be used with the file type macros described below, that take a mode argument. The st_size member shall contain the length, in bytes, of the pathname contained in the symbolic link. File mode bits and the contents of the remaining members of the stat structure are unspecified. The value returned in the st_size field shall be the length of the contents of the symbolic link, and shall not count a trailing null if one is present. The following symbolic names for the values of type mode_t shall also be defined. File type: S_IFMT Type of file. S_IFBLK Block special. S_IFCHR Character special. S_IFIFO FIFO special. S_IFREG Regular. S_IFDIR Directory. S_IFLNK Symbolic link. S_IFSOCK Socket. File mode bits: S_IRWXU Read, write, execute/search by owner. S_IRUSR Read permission, owner. S_IWUSR Write permission, owner. S_IXUSR Execute/search permission, owner. S_IRWXG Read, write, execute/search by group. S_IRGRP Read permission, group. S_IWGRP Write permission, group. S_IXGRP Execute/search permission, group. S_IRWXO Read, write, execute/search by others. S_IROTH Read permission, others. S_IWOTH Write permission, others. S_IXOTH Execute/search permission, others. S_ISUID Set-user-ID on execution. S_ISGID Set-group-ID on execution. S_ISVTX On directories, restricted deletion flag. The bits defined by S_IRUSR, S_IWUSR, S_IXUSR, S_IRGRP, S_IWGRP, S_IXGRP, S_IROTH, S_IWOTH, S_IXOTH, S_ISUID, S_ISGID, and S_ISVTX shall be unique. S_IRWXU is the bitwise-inclusive OR of S_IRUSR, S_IWUSR, and S_IXUSR. S_IRWXG is the bitwise-inclusive OR of S_IRGRP, S_IWGRP, and S_IXGRP. S_IRWXO is the bitwise-inclusive OR of S_IROTH, S_IWOTH, and S_IXOTH. Implementations may OR other implementation-defined bits into S_IRWXU, S_IRWXG, and S_IRWXO, but they shall not overlap any of the other bits defined in this volume of IEEE Std 1003.1-2001. The file permission bits are defined to be those corresponding to the bitwise-inclusive OR of S_IRWXU, S_IRWXG, and S_IRWXO. The following macros shall be provided to test whether a file is of the specified type. The value m supplied to the macros is the value of st_mode from a stat structure. The macro shall evaluate to a non-zero value if the test is true; 0 if the test is false. S_ISBLK(m) Test for a block special file. S_ISCHR(m) Test for a character special file. S_ISDIR(m) Test for a directory. S_ISFIFO(m) Test for a pipe or FIFO special file. S_ISREG(m) Test for a regular file. S_ISLNK(m) Test for a symbolic link. S_ISSOCK(m) Test for a socket. The implementation may implement message queues, semaphores, or shared memory objects as distinct file types. The following macros shall be provided to test whether a file is of the specified type. The value of the buf argument supplied to the macros is a pointer to a stat structure. The macro shall evaluate to a non-zero value if the specified object is implemented as a distinct file type and the specified file type is contained in the stat structure referenced by buf. Otherwise, the macro shall evaluate to zero. S_TYPEISMQ(buf) Test for a message queue. S_TYPEISSEM(buf) Test for a semaphore. S_TYPEISSHM(buf) Test for a shared memory object. The implementation may implement typed memory objects as distinct file types, and the following macro shall test whether a file is of the specified type. The value of the buf argument supplied to the macros is a pointer to a stat structure. The macro shall evaluate to a non-zero value if the specified object is implemented as a distinct file type and the specified file type is contained in the stat structure referenced by buf. Otherwise, the macro shall evaluate to zero. S_TYPEISTMO(buf) Test macro for a typed memory object. The following shall be declared as functions and may also be defined as macros. Function prototypes shall be provided. int chmod(const char *, mode_t); int fchmod(int, mode_t); int fstat(int, struct stat *); int lstat(const char *restrict, struct stat *restrict); int mkdir(const char *, mode_t); int mkfifo(const char *, mode_t); int mknod(const char *, mode_t, dev_t); int stat(const char *restrict, struct stat *restrict); mode_t umask(mode_t); The following sections are informative.
Use of the macros is recommended for determining the type of a file.
A conforming C-language application must include <sys/stat.h> for functions that have arguments or return values of type mode_t, so that symbolic values for that type can be used. An alternative would be to require that these constants are also defined by including <sys/types.h>. The S_ISUID and S_ISGID bits may be cleared on any write, not just on open(), as some historical implementations do. System calls that update the time entry fields in the stat structure must be documented by the implementors. POSIX-conforming systems should not update the time entry fields for functions listed in the System Interfaces volume of IEEE Std 1003.1-2001 unless the standard requires that they do, except in the case of documented extensions to the standard. Note that st_dev must be unique within a Local Area Network (LAN) in a ``system'' made up of multiple computers' file systems connected by a LAN. Networked implementations of a POSIX-conforming system must guarantee that all files visible within the file tree (including parts of the tree that may be remotely mounted from other machines on the network) on each individual processor are uniquely identified by the combination of the st_ino and st_dev fields. The unit for the st_blocks member of the stat structure is not defined within IEEE Std 1003.1-2001. In some implementations it is 512 bytes. It may differ on a file system basis. There is no correlation between values of the st_blocks and st_blksize, and the f_bsize (from <sys/statvfs.h>) structure members. Traditionally, some implementations defined the multiplier for st_blocks in <sys/param.h> as the symbol DEV_BSIZE.
No new S_IFMT symbolic names for the file type values of mode_t will be defined by IEEE Std 1003.1-2001; if new file types are required, they will only be testable through S_ISxx() or S_TYPEISxxx() macros instead.
<sys/statvfs.h> , <sys/types.h> , the System Interfaces volume of IEEE Std 1003.1-2001, chmod(), fchmod(), fstat(), lstat(), mkdir(), mkfifo(), mknod(), stat(), umask()
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .