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What the File Attribute Values Mean

When you read the attributes of a file, they come back in a structure called struct stat. This section describes the names of the attributes, their data types, and what they mean. For the functions to read the attributes of a file, see section Reading the Attributes of a File.

The header file `sys/stat.h' declares all the symbols defined in this section.

Data Type: struct stat

The stat structure type is used to return information about the attributes of a file. It contains at least the following members:

mode_t st_mode

Specifies the mode of the file. This includes file type information (see section Testing the Type of a File) and the file permission bits (see section The Mode Bits for Access Permission).

ino_t st_ino

The file serial number, which distinguishes this file from all other files on the same device.

dev_t st_dev

Identifies the device containing the file. The st_ino and st_dev, taken together, uniquely identify the file.

nlink_t st_nlink

The number of hard links to the file. This count keeps track of how many directories have entries for this file. If the count is ever decremented to zero, then the file itself is discarded. Symbolic links are not counted in the total.

uid_t st_uid

The user ID of the file's owner. See section File Owner.

gid_t st_gid

The group ID of the file. See section File Owner.

off_t st_size

This specifies the size of a regular file in bytes. For files that are really devices and the like, this field isn't usually meaningful.

time_t st_atime

This is the last access time for the file. See section File Times.

unsigned long int st_atime_usec

This is the fractional part of the last access time for the file. See section File Times.

time_t st_mtime

This is the time of the last modification to the contents of the file. See section File Times.

unsigned long int st_mtime_usec

This is the fractional part of the time of last modification to the contents of the file. See section File Times.

time_t st_ctime

This is the time of the last modification to the attributes of the file. See section File Times.

unsigned long int st_ctime_usec

This is the fractional part of the time of last modification to the attributes of the file. See section File Times.

unsigned int st_nblocks

This is the amount of disk space that the file occupies, measured in units of 512-byte blocks.

The number of disk blocks is not strictly proportional to the size of the file, for two reasons: the file system may use some blocks for internal record keeping; and the file may be sparse--it may have "holes" which contain zeros but do not actually take up space on the disk.

You can tell (approximately) whether a file is sparse by comparing this value with st_size, like this:

(st.st_blocks * 512 < st.st_size)

This test is not perfect because a file that is just slightly sparse might not be detected as sparse at all. For practical applications, this is not a problem.

unsigned int st_blksize

The optimal block size for reading of writing this file. You might use this size for allocating the buffer space for reading of writing the file.

Some of the file attributes have special data type names which exist specifically for those attributes. (They are all aliases for well-known integer types that you know and love.) These typedef names are defined in the header file `sys/types.h' as well as in `sys/stat.h'. Here is a list of them.

Data Type: mode_t

This is an integer data type used to represent file modes. In the GNU system, this is equivalent to unsigned int.

Data Type: ino_t

This is an arithmetic data type used to represent file serial numbers. (In Unix jargon, these are sometimes called inode numbers.) In the GNU system, this type is equivalent to unsigned long int.

Data Type: dev_t

This is an arithmetic data type used to represent file device numbers. In the GNU system, this is equivalent to int.

Data Type: nlink_t

This is an arithmetic data type used to represent file link counts. In the GNU system, this is equivalent to unsigned short int.

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