CHMOD(3POSIX)

💖 CHMOD(3POSIX) — the POSIX Programmer's Manual, bestie edition 💖

okay so first things first, bestie: this page lives in the POSIX Programmer's Manual, and there's a little ✨ PROLOG ✨ tea you need to know before we go any further.

🫶 Heads up: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (so go check the corresponding Linux manual page for the deets on Linux behavior), or honestly the interface might not even be implemented on Linux at all. 😭

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✨ NAME ✨

chmod, fchmodat — change mode of a file

like, we're changing the mode. of a file. main character behavior tbh. 👑

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🌸 SYNOPSIS 🌸

#include <sys/stat.h>

int chmod(const char *path, mode_t mode);

#include <fcntl.h>

int fchmodat(int fd, const char *path, mode_t mode, int flag);

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🦋 DESCRIPTION 🦋

so the chmod() function shall change S_ISUID, S_ISGID, S_ISVTX, and the file permission bits of the file named by the pathname pointed to by the path argument — setting them to the corresponding bits in the mode argument. 💅

and the application shall make sure that the effective user ID of the process matches the owner of the file, OR that the process has appropriate privileges, in order to do this. no cap, that's the vibe check you have to pass.

S_ISUID, S_ISGID, S_ISVTX, and the file permission bits are all described over in <sys/stat.h>. go read that if you need the lore.

okay so here's a little conditional situation 🫠 — if the calling process does not have appropriate privileges, AND if the group ID of the file does not match the effective group ID or one of the supplementary group IDs, AND if the file is a regular file, then bit S_ISGID (that's set-group-ID on execution) in the file's mode shall be cleared upon successful return from chmod().

also, additional implementation-defined restrictions may cause the S_ISUID and S_ISGID bits in mode to just... be ignored. 🙈 it's giving "we'll allow it, but on our terms."

upon successful completion, chmod() shall mark the last file status change timestamp of the file for update. ⏰

now, fchmodat() ✨ the function shall be equivalent to the chmod() function, except in the case where 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. iconic.

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. but if the access mode is O_SEARCH, the function shall not perform that check. no notes. 💅

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

• AT_SYMLINK_NOFOLLOW — if path names a symbolic link, then the mode of the symbolic link is changed. 🪄

and if fchmodat() gets passed the special value AT_FDCWD in the fd parameter, the current working directory shall be used. and if flag is also zero, the behavior shall be identical to a call to chmod(). periodt. 💖

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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. and if -1 is returned, no change to the file mode occurs. so like, don't worry, nothing happened. 💀

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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. (yes that's a long one, breathe 🧘)
• EPERM — the effective user ID does not match the owner of the file and the process does not have appropriate privileges. 🚫
• EROFS — the named file resides on a read-only file system. 🔒

the fchmodat() 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:

• EINTR — a signal was caught during execution of the function. 😵
• EINVAL — the value of the mode argument is invalid.
• 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 fchmodat() function may fail if:

• EINVAL — the value of the flag argument is invalid.
• EOPNOTSUPP — the AT_SYMLINK_NOFOLLOW bit is set in the flag argument, path names a symbolic link, and the system does not support changing the mode of a symbolic link. 🫠

📢 the following sections are informative. (so like, they're the commentary track, not the rules.)

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💫 EXAMPLES 💫

💜 Setting Read Permissions for User, Group, and Others

this one sets read permissions for the owner, group, and others. simple, clean, slay.

#include <sys/stat.h>

const char *path;
...
chmod(path, S_IRUSR|S_IRGRP|S_IROTH);

💜 Setting Read, Write, and Execute Permissions for the Owner Only

this one sets read, write, and execute permissions for the owner, and no permissions for group and others. gatekeeping permissions? we love to see it. 💅

#include <sys/stat.h>

const char *path;
...
chmod(path, S_IRWXU);

💜 Setting Different Permissions for Owner, Group, and Other

this one sets owner permissions for CHANGEFILE to read, write, and execute, group permissions to read and execute, and other permissions to read.

#include <sys/stat.h>

#define CHANGEFILE "/etc/myfile"
...
chmod(CHANGEFILE, S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH);

💜 Setting and Checking File Permissions

this one sets the file permission bits for a file named /home/cnd/mod1, then calls the stat() function to verify the permissions. verify your work bestie, always. ✅

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

int status;
struct stat buffer
...
chmod("/home/cnd/mod1", S_IRWXU|S_IRWXG|S_IROTH|S_IWOTH);
status = stat("/home/cnd/mod1", &buffer);

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🌟 APPLICATION USAGE 🌟

so, in order to ensure that the S_ISUID and S_ISGID bits are set, an application requiring this should use stat() after a successful chmod() to verify it. don't just assume, check. trust but verify, bestie. 🔍

also, any file descriptors currently open by any process on the file could possibly become invalid if the mode of the file is changed to a value which would deny access to that process. one situation where this could occur is on a stateless file system. this behavior will not occur in a conforming environment. 😌

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🧠 RATIONALE 🧠

okay, lore drop. this volume of POSIX.1‐2017 specifies that the S_ISGID bit is cleared by chmod() on a regular file under certain conditions. this is specified on the assumption that regular files may be executed, and the system should prevent users from making executable setgid() files perform with privileges that the caller does not have. so basically, no sneaky privilege grabbing. 🚫👑

on implementations that support execution of other file types, the S_ISGID bit should be cleared for those file types under the same circumstances.

implementations that use the S_ISUID bit to indicate some other function (for example, mandatory record locking) on non-executable files need not clear this bit on writing. they should clear the bit for executable files and any other cases where the bit grants special powers to processes that change the file contents. similar comments apply to the S_ISGID bit. 💅

and the purpose of the fchmodat() function? to enable changing the mode of files in directories other than the current working directory without exposure to race conditions. because any part of the path of a file could be changed in parallel to a call to chmod(), resulting in unspecified behavior. 😭 by opening a file descriptor for the target directory and using the fchmodat() function, it can be guaranteed that the changed file is located relative to the desired directory. iconic, actually. ✨

some implementations might allow changing the mode of symbolic links. this is not supported by the interfaces in the POSIX specification. systems with such support provide an interface named lchmod(). and to support such implementations, fchmodat() has a flag parameter. 🪄

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🔮 FUTURE DIRECTIONS 🔮

None. that's it. nothing. the future is... empty. 💀 (but in a chill way.)

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👀 SEE ALSO 👀

access(), chown(), exec, fstatat(), fstatvfs(), mkdir(), mkfifo(), mknod(), open()

the Base Definitions volume of POSIX.1‐2017: <fcntl.h>, <sys_stat.h>, <sys_types.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 one came to you from IEEE/The Open Group, 2017 — yeah, 2017, it's vintage, it's classic, it's giving legacy but make it ✨slay✨. mwah, love u, go change some file modes responsibly 💖🦋🌈