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chmod, fchmodat — change mode of a file relative to directory file descriptor


#include <sys/stat.h>

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


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 to the corresponding bits in the mode argument. The application shall ensure that the effective user ID of the process matches the owner of the file or the process has appropriate privileges in order to do this.

S_ISUID, S_ISGID, S_ISVTX, and the file permission bits are described in <sys/stat.h>.

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, bit S_ISGID (set-group-ID on execution) in the file’s mode shall be cleared upon successful return from chmod().

Additional implementation-defined restrictions may cause the S_ISUID and S_ISGID bits in mode to be ignored.

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

The fchmodat() function shall be equivalent to the chmod() function 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 file descriptor was opened without O_SEARCH, the function shall check whether directory searches are permitted using the current permissions of the directory underlying the file descriptor. If the file descriptor was opened with O_SEARCH, the function shall not perform the check.

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

If path names a symbolic link, then the mode of the symbolic link is changed.

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


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 change to the file mode occurs.


These functions shall fail if:


Search permission is denied on a component of the path prefix.


A loop exists in symbolic links encountered during resolution of the path argument.


The length of a component of a pathname is longer than {NAME_MAX}.


A component of path does not name an existing file or path is an empty string.


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.


The effective user ID does not match the owner of the file and the process does not have appropriate privileges.


The named file resides on a read-only file system.

The fchmodat() function shall fail if:


fd was not opened with O_SEARCH and the permissions of the directory underlying fd do not permit directory searches.


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.


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:


A signal was caught during execution of the function.


The value of the mode argument is invalid.


More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.


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:


The value of the flag argument is invalid.


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.


Setting Read Permissions for User, Group, and Others
The following example sets read permissions for the owner, group, and others.

#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
The following example sets read, write, and execute permissions for the owner, and no permissions for group and others.

#include <sys/stat.h>

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

Setting Different Permissions for Owner, Group, and Other
The following example 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"

Setting and Checking File Permissions
The following example sets the file permission bits for a file named /home/cnd/mod1, then calls the stat() function to verify the permissions.

#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;);


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 this.

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.


This volume of POSIX.1-2008 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. 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.

The purpose of the fchmodat() function is to enable changing the mode 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 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. 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(). To support such implementations fchmodat() has a flag parameter.




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

The Base Definitions volume of POSIX.1-2008, <fcntl.h>, <sys_stat.h>, <sys_types.h>


Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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 .

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