STAT(2)

✨ STAT(2) ✨ — get file status

okay so bestie, this one comes straight outta the System Calls Manual, and it's genuinely one of the most iconic pages in the whole Unix cinematic universe 💅 every single program that's ever been like "hiii what is this file actually" is doing this. let's get into it.

---

🎀 NAME 🎀

stat, lstat, fstat, fstatat — get file status

📚 LIBRARY 📚

Standard C Library (libc, -lc)

🧾 SYNOPSIS 🧾

#include <sys/stat.h>

int
stat(const char * restrict path, struct stat * restrict sb);

int
lstat(const char * restrict path, struct stat * restrict sb);

int
fstat(int fd, struct stat *sb);

int
fstatat(int fd, const char *path, struct stat *sb, int flag);

---

💖 DESCRIPTION 💖

The stat() system call obtains information about the file pointed to by path. And here's the slay part: you do not need read, write or execute permission on the named file itself. But — and this is the catch, bestie — all directories listed in the path name leading to the file must be searchable. so the whole hallway has to let you through, even if the door at the end doesn't care. 🚪✨

The lstat() system call is like stat() except when the named file is a symbolic link, in which case lstat() returns information about the link, while stat() returns information about the file the link references. so like… are we describing the girl or the girl she's pointing at? 👀 different answers, different functions.

The fstat() system call obtains the same information about an open file known by the file descriptor fd.

The fstatat() system call is equivalent to stat() and lstat() except when the path specifies a relative path. In this case the status is retrieved from a file relative to the directory associated with the file descriptor fd instead of the current working directory. it's giving "relative to you, not relative to here" 🫶

The values for the flag are constructed by a bitwise-inclusive OR of flags from this list, defined in <fcntl.h>:

• AT_SYMLINK_NOFOLLOW — If path names a symbolic link, the status of the symbolic link is returned. (i.e. do not chase the link, look at the link itself, no notes 💅)

If fstatat() is passed the special value AT_FDCWD in the fd parameter, the current working directory is used and the behavior is identical to a call to stat() or lstat() respectively, depending on whether or not the AT_SYMLINK_NOFOLLOW bit is set in flag.

The sb argument is a pointer to a stat structure as defined by <sys/stat.h> and into which information is placed concerning the file.

🗂️ the file-system fields of struct stat

• st_dev — Numeric ID of the device containing the file.
• st_ino — The file's inode number.
• st_nlink — Number of hard links to the file.
• st_flags — Flags enabled for the file. See chflags(2) for the list of flags and their description.

The st_dev and st_ino fields together identify the file uniquely within the system. (it's their joint slay, no one else can do it 🔥)

⏰ the time-related fields of struct stat

• st_atim — Time when file data was last accessed. Changed implicitly by syscalls such as read(2) and readv(2), and explicitly by utimes(2).
• st_mtim — Time when file data was last modified. Changed implicitly by syscalls such as truncate(2), write(2), and writev(2), and explicitly by utimes(2). Also, any syscall which modifies directory content changes the st_mtim for the affected directory. For instance, creat(2), mkdir(2), rename(2), link(2), and unlink(2).
• st_ctim — Time when file status was last changed (inode data modification). Changed implicitly by any syscall that affects file metadata, including st_mtim, such as chflags(2), chmod(2), chown(2), truncate(2), utimes(2), and write(2). Also, any syscall which modifies directory content changes the st_ctim for the affected directory. For instance, creat(2), mkdir(2), rename(2), link(2), and unlink(2).
• st_birthtim — Time when the inode was created. the actual birthday 🎂✨

These time-related macros are defined for compatibility:

#define st_atime                st_atim.tv_sec
#define st_mtime                st_mtim.tv_sec
#define st_ctime                st_ctim.tv_sec
#ifndef _POSIX_SOURCE
#define st_birthtime            st_birthtim.tv_sec
#endif

#ifndef _POSIX_SOURCE
#define st_atimespec            st_atim
#define st_mtimespec            st_mtim
#define st_ctimespec            st_ctim
#define st_birthtimespec        st_birthtim
#endif

📏 size-related fields of the struct stat

• st_size — File size in bytes.
• st_blksize — Optimal I/O block size for the file.
• st_blocks — Actual number of blocks allocated for the file in 512-byte units. As short symbolic links are stored in the inode, this number may be zero. (short symlinks live rent-free inside the inode, so yeah, zero blocks, iconic behaviour 🌸)

🔐 access-related fields of struct stat

• st_uid — User ID of the file's owner.
• st_gid — Group ID of the file.
• st_mode — Status of the file (see below).

The status information word st_mode has these bits:

#define S_IFMT   0170000  /* type of file mask */
#define S_IFIFO  0010000  /* named pipe (fifo) */
#define S_IFCHR  0020000  /* character special */
#define S_IFDIR  0040000  /* directory */
#define S_IFBLK  0060000  /* block special */
#define S_IFREG  0100000  /* regular */
#define S_IFLNK  0120000  /* symbolic link */
#define S_IFSOCK 0140000  /* socket */
#define S_IFWHT  0160000  /* whiteout */
#define S_ISUID  0004000  /* set user id on execution */
#define S_ISGID  0002000  /* set group id on execution */
#define S_ISVTX  0001000  /* save swapped text even after use */
#define S_IRWXU  0000700  /* RWX mask for owner */
#define S_IRUSR  0000400  /* read permission, owner */
#define S_IWUSR  0000200  /* write permission, owner */
#define S_IXUSR  0000100  /* execute/search permission, owner */
#define S_IRWXG  0000070  /* RWX mask for group */
#define S_IRGRP  0000040  /* read permission, group */
#define S_IWGRP  0000020  /* write permission, group */
#define S_IXGRP  0000010  /* execute/search permission, group */
#define S_IRWXO  0000007  /* RWX mask for other */
#define S_IROTH  0000004  /* read permission, other */
#define S_IWOTH  0000002  /* write permission, other */
#define S_IXOTH  0000001  /* execute/search permission, other */

For a list of access modes, see <sys/stat.h>, access(2) and chmod(2). These macros are available to test whether a st_mode value passed in the m argument corresponds to a file of the specified type:

• 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_ISLNK(m) — Test for a symbolic link.
• S_ISREG(m) — Test for a regular file.
• S_ISSOCK(m) — Test for a socket.
• S_ISWHT(m) — Test for a whiteout.

The macros evaluate to a non-zero value if the test is true or to the value 0 if the test is false. true/false but make it numbers 💅

---

✅ RETURN VALUES ✅

Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error. that's the whole personality. 0 or -1 and a little errno gossip 🫶

---

🕰️ COMPATIBILITY 🕰️

Previous versions of the system used different types for the st_dev, st_uid, st_gid, st_rdev, st_size, st_blksize and st_blocks fields. so if you're porting ancient code, bestie, mind the types — they used to be built different. 😭

---

💀 ERRORS 💀

The stat() and lstat() system calls will fail if:

• [EACCES] — Search permission is denied for a component of the path prefix.
• [EFAULT] — The sb or path argument points to an invalid address.
• [EIO] — An I/O error occurred while reading from or writing to the file system.
• [EINTEGRITY] — Corrupted data was detected while reading from the file system.
• [ELOOP] — Too many symbolic links were encountered in translating the pathname.
• [ENAMETOOLONG] — A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.
• [ENOENT] — The named file does not exist.
• [ENOTDIR] — A component of the path prefix is not a directory.
• [EOVERFLOW] — The file size in bytes cannot be represented correctly in the structure pointed to by sb.

The fstat() system call will fail if:

• [EBADF] — The fd argument is not a valid open file descriptor.
• [EFAULT] — The sb argument points to an invalid address.
• [EIO] — An I/O error occurred while reading from or writing to the file system.
• [EINTEGRITY] — Corrupted data was detected while reading from the file system.
• [EOVERFLOW] — The file size in bytes cannot be represented correctly in the structure pointed to by sb.

In addition to the errors returned by the lstat(), the fstatat() may fail if:

• [EBADF] — The path argument does not specify an absolute path and the fd argument is neither AT_FDCWD nor a valid file descriptor open for searching.
• [EINVAL] — The value of the flag argument is not valid.
• [ENOTDIR] — The path argument is not an absolute path and fd is neither AT_FDCWD nor a file descriptor associated with a directory.

---

👀 SEE ALSO 👀

access(2), chmod(2), chown(2), fhstat(2), statfs(2), utimes(2), sticky(7), symlink(7)

---

📜 STANDARDS 📜

The stat() and fstat() system calls are expected to conform to ISO/IEC 9945-1:1990 (“POSIX.1”). The fstatat() system call follows The Open Group Extended API Set 2 specification. certified and spec-compliant, periodt ✨

---

🦋 HISTORY 🦋

The stat() and fstat() system calls appeared in Version 1 AT&T UNIX. The lstat() system call appeared in 4.2BSD. The fstatat() system call appeared in FreeBSD 8.0. three eras, three icons, one page 👑

---

🐛 BUGS 🐛

Applying fstat() to a socket returns a zeroed buffer, except for the blocksize field, and a unique device and inode number. so like… don't expect a whole personality from a socket, you get a mostly-empty buffer and a lil unique device + inode number. that's the deal. 🙈

---

and that's the whole page, bestie 💖 this one's from Debian, dated March 30, 2020 — go forth and stat() responsibly, mwah 🫶🎀