Retrieving File Information: stat

The stat, lstat, and fstat system calls retrieve information about a file.

#include <sys/stat.h>
int stat(const char *pathname, struct stat *statbuf);
int lstat(const char *pathname, struct stat *statbuf);
int fstat(int fd, struct stat *statbuf); // All return 0 on success, or –1 on error

They differ in the way the file is specified:

• stat for information about a named file

• lstat for information about a link

• fstat for information about a file descriptor

• stat and lstat don’t require permissions on the file itself, but they require search permissions on all parent directories.

All these system calls fill in a stat struct, which looks like this:

struct stat {
    dev_t st_dev; /* IDs of device on which file resides */
    ino_t st_ino; /* I-node number of file */
    mode_t st_mode; /* File type and permissions */
    nlink_t st_nlink; /* Number of (hard) links to file */
    uid_t st_uid; /* User ID of file owner */
    gid_t st_gid; /* Group ID of file owner */
    dev_t st_rdev; /* IDs for device special files */
    off_t st_size; /* Total file size (bytes) */
    blksize_t st_blksize; /* Optimal block size for I/O (bytes) */
    blkcnt_t st_blocks; /* Number of (512B) blocks allocated */
    time_t st_atime; /* Time of last file access */
    time_t st_mtime; /* Time of last file modification */
    time_t st_ctime; /* Time of last status change */
};

• st_dev identifies the device on which the file resides.
• st_ino contains the inode number of the file.
• st_uid and st_gid respectively identify the user ID and group ID to which the file belongs.
• st_nlink is the number of hard links to the file.
• st_mode is a bitmask for identifying the file type and specifying file permissions.
• st_size is the total size of the file in bytes for a normal file, and for a link, the length of the pathname pointed to.
• st_blocks indicates the number of pointer blocks (512-bytes).
• st_blksize is the optimal block size for I/O in this file system. It’s normally 4KB, 4096.
• st_atime, st_mtime, and st_ctime contain the last file access, last file modification, last status change.

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