sys_stat.h man page

Prolog

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sys/stat.h — data returned by the stat() function

Synopsis

#include <sys/stat.h>

Description

The <sys/stat.h> header shall define the structure of the data returned by the fstat(), lstat(), and stat() functions.

The <sys/stat.h> header shall define the stat structure, which shall include 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.
struct timespec st_atim Last data access timestamp.
struct timespec st_mtim Last data modification timestamp.
struct timespec st_ctim Last file status change timestamp.
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 <sys/stat.h> header shall define the blkcnt_t, blksize_t, dev_t, ino_t, mode_t, nlink_t, uid_t, gid_t, off_t, and time_t types as described in <sys/types.h>.

The <sys/stat.h> header shall define the timespec structure as described in <time.h>. Times shall be given in seconds since the Epoch.

Which structure members have meaningful values depends on the type of file. For further information, see the descriptions of fstat(), lstat(), and stat() in the System Interfaces volume of POSIX.1‐2008.

For compatibility with earlier versions of this standard, the st_atime macro shall be defined with the value st_atim.tv_sec. Similarly, st_ctime and st_mtime shall be defined as macros with the values st_ctim.tv_sec and st_mtim.tv_sec, respectively.

The <sys/stat.h> header shall define the following symbolic constants for the file types encoded in type mode_t. The values shall be suitable for use in #if preprocessing directives:

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.

The <sys/stat.h> header shall define the following symbolic constants for the file mode bits encoded in type mode_t, with the indicated numeric values. These macros shall expand to an expression which has a type that allows them to be used, either singly or OR'ed together, as the third argument to open() without the need for a mode_t cast. The values shall be suitable for use in #if preprocessing directives.

NameNumeric ValueDescription
S_IRWXU0700Read, write, execute/search by owner.
S_IRUSR0400Read permission, owner.
S_IWUSR0200Write permission, owner.
S_IXUSR0100Execute/search permission, owner.
S_IRWXG070Read, write, execute/search by group.
S_IRGRP040Read permission, group.
S_IWGRP020Write permission, group.
S_IXGRP010Execute/search permission, group.
S_IRWXO07Read, write, execute/search by others.
S_IROTH04Read permission, others.
S_IWOTH02Write permission, others.
S_IXOTH01Execute/search permission, others.
S_ISUID04000Set-user-ID on execution.
S_ISGID02000Set-group-ID on execution.
S_ISVTX01000On directories, restricted deletion flag.

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 <sys/stat.h> header shall define the following symbolic constants as distinct integer values outside of the range [0,999999999], for use with the futimens() and utimensat() functions: UTIME_NOW UTIME_OMIT

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    fchmodat(int, const char *, mode_t, int);
int    fstat(int, struct stat *);
int    fstatat(int, const char *restrict, struct stat *restrict, int);
int    futimens(int, const struct timespec [2]);
int    lstat(const char *restrict, struct stat *restrict);
int    mkdir(const char *, mode_t);
int    mkdirat(int, const char *, mode_t);
int    mkfifo(const char *, mode_t);
int    mkfifoat(int, const char *, mode_t);
int    mknod(const char *, mode_t, dev_t);
int    mknodat(int, const char *, mode_t, dev_t);
int    stat(const char *restrict, struct stat *restrict);
mode_t umask(mode_t);
int    utimensat(int, const char *, const struct timespec [2], int);

The following sections are informative.

Application Usage

Use of the macros is recommended for determining the type of a file.

Rationale

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 POSIX.1‐2008 unless the standard requires that they do, except in the case of documented extensions to the standard.

Upon assignment, file timestamps are immediately converted to the resolution of the file system by truncation (i.e., the recorded time can be older than the actual time). For example, if the file system resolution is 1 microsecond, then a conforming stat() must always return an st_mtim.tv_nsec that is a multiple of 1000. Some older implementations returned higher-resolution timestamps while the inode information was cached, and then spontaneously truncated the tv_nsec fields when they were stored to and retrieved from disk, but this behavior does not conform.

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 POSIX.1‐2008. 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.

Some earlier versions of this standard did not specify values for the file mode bit macros. The expectation was that some implementors might choose to use a different encoding for these bits than the traditional one, and that new applications would use symbolic file modes instead of numeric. This version of the standard specifies the traditional encoding, in recognition that nearly 20 years after the first publication of this standard numeric file modes are still in widespread use by application developers, and that all conforming implementations still use the traditional encoding.

Future Directions

No new S_IFMT symbolic names for the file type values of mode_t will be defined by POSIX.1‐2008; if new file types are required, they will only be testable through S_ISxx() or S_TYPEISxxx() macros instead.

See Also

<sys_statvfs.h>, <sys_types.h>, <time.h>

The System Interfaces volume of POSIX.1‐2008, chmod(), fchmod(), fstat(), fstatat(), futimens(), mkdir(), mkfifo(), mknod(), umask()

Info

2013 IEEE/The Open Group POSIX Programmer's Manual