CHOWN(3POSIX)

💖 CHOWN(3POSIX) — bestie edition 💖

okay so this page lives in the POSIX Programmer's Manual, and the manual name is CHOWN(3POSIX) — yes that's the whole title, we love a full-name moment. 👑

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✨ PROLOG ✨

so first things first, a lil heads up: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ — like, consult the corresponding Linux manual page for the deets on Linux behavior — or honestly the interface may not be implemented on Linux at all. 🙈 it's giving "check your local listings."

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🌸 NAME 🌸

chown, fchownat — change owner and group of a file. that's the whole vibe. changing who owns a file. iconic behavior honestly. 💅

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🎀 SYNOPSIS 🎀

#include <unistd.h>

int chown(const char *path, uid_t owner, gid_t group);

#include <fcntl.h>

int fchownat(int fd, const char *path, uid_t owner, gid_t group,
    int flag);

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🦋 DESCRIPTION 🦋

The chown() function shall change the user and group ownership of a file. like, it's handing over the keys. 🔑

The path argument points to a pathname naming a file. The user ID and group ID of the named file shall be set to the numeric values contained in owner and group, respectively.

Only processes with an effective user ID equal to the user ID of the file, or with appropriate privileges, may change the ownership of a file. If _POSIX_CHOWN_RESTRICTED is in effect for path:

• Changing the user ID is restricted to processes with appropriate privileges. 🔒
• Changing the group ID is permitted to a process with an effective user ID equal to the user ID of the file, but without appropriate privileges, if and only if owner is equal to the file's user ID or (uid_t)-1 and group is equal either to the calling process' effective group ID or to one of its supplementary group IDs.

If the specified file is a regular file, one or more of the S_IXUSR, S_IXGRP, or S_IXOTH bits of the file mode are set, and the process does not have appropriate privileges, the set-user-ID (S_ISUID) and set-group-ID (S_ISGID) bits of the file mode shall be cleared upon successful return from chown(). 😭 the bits are gone bestie.

If the specified file is a regular file, one or more of the S_IXUSR, S_IXGRP, or S_IXOTH bits of the file mode are set, and the process has appropriate privileges, it is implementation-defined whether the set-user-ID and set-group-ID bits are altered. so, like, 🤷 depends on the implementation.

If the chown() function is successfully invoked on a file that is not a regular file and one or more of the S_IXUSR, S_IXGRP, or S_IXOTH bits of the file mode are set, the set-user-ID and set-group-ID bits may be cleared.

If owner or group is specified as (uid_t)-1 or (gid_t)-1, respectively, the corresponding ID of the file shall not be changed. leave it alone, it's fine. 🫶

Upon successful completion, chown() shall mark for update the last file status change timestamp of the file — except that if owner is (uid_t)-1 and group is (gid_t)-1, the file status change timestamp need not be marked for update. nothing changed so nothing to timestamp, makes sense. ⭐

The fchownat() function shall be equivalent to the chown() and lchown() functions except in the case where path specifies a relative path. In this case the file to be changed is determined relative to the directory associated with the file descriptor fd instead of the current working directory. If the access mode of the open file description associated with the file descriptor is not O_SEARCH, the function shall check whether directory searches are permitted using the current permissions of the directory underlying the file descriptor. If the access mode is O_SEARCH, the function shall not perform the check. we trust it. ✨

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

🧚 flag options 🧚

• AT_SYMLINK_NOFOLLOW — If path names a symbolic link, ownership of the symbolic link is changed. like, we're changing the link itself, not chasing it down the rabbit hole. 🐇

If fchownat() is passed the special value AT_FDCWD in the fd parameter, the current working directory shall be used and the behavior shall be identical to a call to chown() or lchown() respectively, depending on whether or not the AT_SYMLINK_NOFOLLOW bit is set in the flag argument.

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💜 RETURN VALUE 💜

Upon successful completion, these functions shall return 0. 🎉 Otherwise, these functions shall return -1 and set errno to indicate the error. If -1 is returned, no changes are made in the user ID and group ID of the file. it's all or nothing, no half-sends. 💀

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🔥 ERRORS 🔥

These functions shall fail if:

• EACCES — Search permission is denied on a component of the path prefix.
• ELOOP — A loop exists in symbolic links encountered during resolution of the path argument. 🌀
• ENAMETOOLONG — The length of a component of a pathname is longer than {NAME_MAX}.
• ENOENT — A component of path does not name an existing file or path is an empty string.
• ENOTDIR — A component of the path prefix names an existing file that is neither a directory nor a symbolic link to a directory, or the path argument contains at least one non-<slash> character and ends with one or more trailing <slash> characters and the last pathname component names an existing file that is neither a directory nor a symbolic link to a directory.
• EPERM — The effective user ID does not match the owner of the file, or the calling process does not have appropriate privileges and _POSIX_CHOWN_RESTRICTED indicates that such privilege is required.
• EROFS — The named file resides on a read-only file system. ❄️

The fchownat() function shall fail if:

• EACCES — The access mode of the open file description associated with fd is not O_SEARCH and the permissions of the directory underlying fd do not permit directory searches.
• 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 reading or searching.
• ENOTDIR — The path argument is not an absolute path and fd is a file descriptor associated with a non-directory file.

These functions may fail if:

• EIO — An I/O error occurred while reading or writing to the file system.
• EINTR — The chown() function was interrupted by a signal which was caught.
• EINVAL — The owner or group ID supplied is not a value supported by the implementation.
• ELOOP — More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.
• ENAMETOOLONG — The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic link produced an intermediate result with a length that exceeds {PATH_MAX}.

The fchownat() function may fail if:

• EINVAL — The value of the flag argument is not valid.

heads up: the following sections are informative. like, not normative. casual. 🍓

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🌈 EXAMPLES 🌈

None. 😭 not a single one. nothing. empty. the girls gave us nothing. 🙈

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🫶 APPLICATION USAGE 🫶

Although chown() can be used on some implementations by the file owner to change the owner and group to any desired values, the only portable use of this function is to change the group of a file to the effective GID of the calling process or to a member of its group set. periodt. that's the safe lane, stay in it. 💅

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👑 RATIONALE 👑

System III and System V allow a user to give away files; that is, the owner of a file may change its user ID to anything. This is a serious problem for implementations that are intended to meet government security regulations. 🚨 Version 7 and 4.3 BSD permit only the superuser to change the user ID of a file. Some government agencies (usually not ones concerned directly with security) find this limitation too confining. This volume of POSIX.1‐2017 uses may to permit secure implementations while not disallowing System V.

System III and System V allow the owner of a file to change the group ID to anything. Version 7 permits only the superuser to change the group ID of a file. 4.3 BSD permits the owner to change the group ID of a file to its effective group ID or to any of the groups in the list of supplementary group IDs, but to no others.

The POSIX.1‐1990 standard requires that the chown() function invoked by a non-appropriate privileged process clear the S_ISGID and the S_ISUID bits for regular files, and permits them to be cleared for other types of files. This is so that changes in accessibility do not accidentally cause files to become security holes. 🕳️ Unfortunately, requiring these bits to be cleared on non-executable data files also clears the mandatory file locking bit (shared with S_ISGID), which is an extension on many implementations (it first appeared in System V). These bits should only be required to be cleared on regular files that have one or more of their execute bits set.

The purpose of the fchownat() function is to enable changing ownership of files in directories other than the current working directory without exposure to race conditions. Any part of the path of a file could be changed in parallel to a call to chown() or lchown(), resulting in unspecified behavior. By opening a file descriptor for the target directory and using the fchownat() function it can be guaranteed that the changed file is located relative to the desired directory. no cap, that's the whole point. 🔥

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💀 FUTURE DIRECTIONS 💀

None. the future is empty. we live in the now. ✨

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⭐ SEE ALSO ⭐

• chmod()
• fpathconf()
• lchown()

The Base Definitions volume of POSIX.1‐2017, <fcntl.h>, <sys_types.h>, <unistd.h>

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📜 COPYRIGHT 📜

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html.

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html.

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and that's the whole page bestie!! 🎀 this edition was brought to you by IEEE/The Open Group, 2017 — yeah the footer said that, so consider it said. go change some ownership (portably!!) and slay. 💖✨🦋