CHFLAGS(2)

💅 CHFLAGS(2) — the chflags girlies of the System Calls Manual

okay bestie, grab a lil snack because we're diving into chflags, lchflags, fchflags, chflagsat — the freebsd system calls that let you set file flags ✨ this is straight out of the System Calls Manual, and yes we're doing the whole thing, no skips, no cap 🫶

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🌸 NAME 🌸

chflags, lchflags, fchflags, chflagsat — set file flags

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📚 LIBRARY 📚

Standard C Library (libc, -lc) — she's iconic, she's everywhere 💖

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⭐ SYNOPSIS ⭐

like, here's the whole cast, literally:

#include <sys/stat.h>
#include <unistd.h>

int
chflags(const char *path, unsigned long flags);

int
lchflags(const char *path, unsigned long flags);

int
fchflags(int fd, unsigned long flags);

int
chflagsat(int fd, const char *path, unsigned long flags, int atflag);

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🦋 DESCRIPTION 🦋

so the file whose name is given by path, or referenced by the descriptor fd, has its flags changed to flags. that's the whole vibe. 🎀

lchflags() is like chflags() except — and this is the plot twist — when the named file is a symbolic link, lchflags() changes the flags of the link itself, not the file it points to. she doesn't follow, she stays in her lane 💅

chflagsat() is equivalent to either chflags() or lchflags() depending on the atflag, except when path specifies a relative path. in that case the file to be changed is determined relative to the directory associated with the file descriptor fd instead of the current working directory. main character energy honestly. the values for atflag 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, then the flags of the symbolic link are changed 🪞
• AT_RESOLVE_BENEATH — only walk paths below the directory specified by the fd descriptor. see the description of the O_RESOLVE_BENEATH flag in the open(2) manual page 👀
• AT_EMPTY_PATH — if the path argument is an empty string, operate on the file or directory referenced by the descriptor fd. and if fd is equal to AT_FDCWD, operate on the current working directory 🏠

if chflagsat() is passed the special value AT_FDCWD in the fd parameter, the current working directory is used. and if atflag is also zero, the behavior is identical to a call to chflags(). twins fr fr 👯

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✨ the flags themselves ✨

the flags specified are formed by or'ing the following values:

• SF_APPEND — the file may only be appended to 🖇️
• SF_ARCHIVED — the file has been archived. this flag means the opposite of the DOS, Windows and CIFS FILE_ATTRIBUTE_ARCHIVE attribute. > ⚠️ heads up bestie: this flag has been deprecated, and may be removed in a future release 💀
• SF_IMMUTABLE — the file may not be changed 🔒
• SF_NOUNLINK — the file may not be renamed or deleted 🚫
• SF_SNAPSHOT — the file is a snapshot file 📸
• UF_APPEND — the file may only be appended to 🖇️
• UF_ARCHIVE — the file needs to be archived. this flag has the same meaning as the DOS, Windows and CIFS FILE_ATTRIBUTE_ARCHIVE attribute. filesystems in FreeBSD may or may not have special handling for this flag. for instance, ZFS tracks changes to files and will set this bit when a file is updated. UFS only stores the flag, and relies on the application to change it when needed 🗂️
• UF_HIDDEN — the file may be hidden from directory listings at the application's discretion. the file has the DOS, Windows and CIFS FILE_ATTRIBUTE_HIDDEN attribute 🙈
• UF_IMMUTABLE — the file may not be changed 🔒
• UF_NODUMP — do not dump the file 📦
• UF_NOUNLINK — the file may not be renamed or deleted 🚫
• UF_OFFLINE — the file is offline, or has the Windows and CIFS FILE_ATTRIBUTE_OFFLINE attribute. filesystems in FreeBSD store and display this flag, but do not provide any special handling when it is set 📴
• UF_OPAQUE — the directory is opaque when viewed through a union stack 🫥
• UF_READONLY — the file is read only, and may not be written or appended. filesystems may use this flag to maintain compatibility with the DOS, Windows and CIFS FILE_ATTRIBUTE_READONLY attribute 📖
• UF_REPARSE — the file contains a Windows reparse point and has the Windows and CIFS FILE_ATTRIBUTE_REPARSE_POINT attribute 🔁
• UF_SPARSE — the file has the Windows FILE_ATTRIBUTE_SPARSE_FILE attribute. this may also be used by a filesystem to indicate a sparse file 🕳️
• UF_SYSTEM — the file has the DOS, Windows and CIFS FILE_ATTRIBUTE_SYSTEM attribute. filesystems in FreeBSD may store and display this flag, but do not provide any special handling when it is set 🖥️

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👑 who's allowed to do what 👑

if one of SF_IMMUTABLE, SF_APPEND, or SF_NOUNLINK is set, a non-super-user cannot change any flags — and even the super-user can change flags only if securelevel is 0. (see init(8) for details.) 😤

the UF_IMMUTABLE, UF_APPEND, UF_NOUNLINK, UF_NODUMP, and UF_OPAQUE flags may be set or unset by either the owner of a file or the super-user. so like, you got this 🫶

the SF_IMMUTABLE, SF_APPEND, SF_NOUNLINK, and SF_ARCHIVED flags may only be set or unset by the super-user. attempts to toggle these flags by non-super-users are rejected. these flags may be set at any time, but normally may only be unset when the system is in single-user mode. (see init(8) for details.) 🔥

⚠️ okay real talk: the implementation of all flags is filesystem-dependent. see the description of the UF_ARCHIVE flag above for one example of the differences in behavior. care should be exercised when writing applications to account for support or lack of support of these flags in various filesystems. do not assume, bestie 💀

the SF_SNAPSHOT flag is maintained by the system and cannot be toggled. periodt 🚫

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🌈 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. slay ✨

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💀 ERRORS 💀

chflags() will fail if:

• [ENOTDIR] — a component of the path prefix is not a directory
• [ENAMETOOLONG] — a component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters
• [ENOENT] — the named file does not exist 😭
• [EACCES] — search permission is denied for a component of the path prefix
• [ELOOP] — too many symbolic links were encountered in translating the pathname 🌀
• [EPERM] — the effective user ID does not match the owner of the file and the effective user ID is not the super-user
• [EPERM] — one of SF_IMMUTABLE, SF_APPEND, or SF_NOUNLINK is set and the user is either not the super-user or securelevel is greater than 0
• [EPERM] — a non-super-user attempted to toggle one of SF_ARCHIVED, SF_IMMUTABLE, SF_APPEND, or SF_NOUNLINK
• [EPERM] — an attempt was made to toggle the SF_SNAPSHOT flag
• [EROFS] — the named file resides on a read-only file system
• [EFAULT] — the path argument points outside the process's allocated address space
• [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
• [EOPNOTSUPP] — the underlying file system does not support file flags, or does not support all of the flags set in flags

fchflags() will fail if:

• [EBADF] — the descriptor is not valid
• [EINVAL] — the fd argument refers to a socket, not to a file 🧦
• [EPERM] — the effective user ID does not match the owner of the file and the effective user ID is not the super-user
• [EPERM] — one of SF_IMMUTABLE, SF_APPEND, or SF_NOUNLINK is set and the user is either not the super-user or securelevel is greater than 0
• [EPERM] — a non-super-user attempted to toggle one of SF_ARCHIVED, SF_IMMUTABLE, SF_APPEND, or SF_NOUNLINK
• [EPERM] — an attempt was made to toggle the SF_SNAPSHOT flag
• [EROFS] — the file resides on a read-only file system
• [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
• [EOPNOTSUPP] — the underlying file system does not support file flags, or does not support all of the flags set in flags
• [ENOTCAPABLE] — path is an absolute path, or contained a ".." component leading to a directory outside of the directory hierarchy specified by fd, and the process is in capability mode or the AT_RESOLVE_BENEATH flag was specified 🔐

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🧚 SEE ALSO 🧚

chflags(1), fflagstostr(3), strtofflags(3), init(8), mount_unionfs(8) — go read them, iykyk 📖💖

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🎀 HISTORY 🎀

• chflags() and fchflags() system calls first appeared in 4.4BSD 🦕
• lchflags() system call first appeared in FreeBSD 5.0 ✨
• chflagsat() system call first appeared in FreeBSD 10.0 🌟

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and that's the whole page bestie, you ate that 🍓 this lil rewrite comes to you courtesy of the Debian build, dated March 30, 2021 — mwah, go set some flags responsibly 💜🫶