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man pages section 2: System Calls

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Updated: Thursday, June 13, 2019
 
 

fchmodat(2)

Name

chmod, fchmod, fchmodat - change access permission mode of file

Synopsis

#include <sys/stat.h>

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

Description

The chmod() and fchmod() functions set the access permission portion of the mode of the file whose name is given by path or referenced by the open file descriptor fildes to the bit pattern contained in mode. Access permission bits are interpreted as follows:

S_ISUID
04000
Set user ID on execution.
S_ISGID
020#0
Set group ID on execution if # is 7, 5, 3, or 1. Enable mandatory file/record locking if # is 6, 4, 2, or 0.
S_ISVTX
01000
Sticky bit.
S_IRWXU
00700
Read, write, execute by owner.
S_IRUSR
00400
Read by owner.
S_IWUSR
00200
Write by owner.
S_IXUSR
00100
Execute (search if a directory) by owner.
S_IRWXG
00070
Read, write, execute by group.
S_IRGRP
00040
Read by group.
S_IWGRP
00020
Write by group.
S_IXGRP
00010
Execute by group.
S_IRWXO
00007
Read, write, execute (search) by others.
S_IROTH
00004
Read by others.
S_IWOTH
00002
Write by others.
S_IXOTH
00001
Execute by others.

Modes are constructed by the bitwise OR operation of the access permission bits.

The effective user ID of the process must match the owner of the file or the process must have the appropriate privilege to change the mode of a file.

If the process is not a privileged process and the file is not a directory, mode bit 01000 (S_ISVTX, the sticky bit) is cleared.

If neither the process is privileged nor the file's group is a member of the process's supplementary group list, and the effective group ID of the process does not match the group ID of the file, mode bit 02000 (set group ID on execution) is cleared.

    If a directory is writable and has S_ISVTX (the sticky bit) set, files within that directory can be removed or renamed only if one or more of the following is true (see unlink(2) and rename(2)):

  • the user owns the file

  • the user owns the directory

  • the file is writable by the user

  • the user is a privileged user

If a regular file is not executable and has S_ISVTX set, the file is assumed to be a swap file. In this case, the system's page cache will not be used to hold the file's data. If the S_ISVTX bit is set on any other file, the results are unspecified.

If a directory has the set group ID bit set, a given file created within that directory will have the same group ID as the directory. Otherwise, the newly created file's group ID will be set to the effective group ID of the creating process.

If the mode bit 02000 (set group ID on execution) is set and the mode bit 00010 (execute or search by group) is not set, mandatory file/record locking will exist on a regular file, possibly affecting future calls to open(2), creat(2), read(2), and write(2) on this file.

If fildes references a shared memory object, fchmod() need only affect the S_IRUSR, S_IRGRP, S_IROTH, S_IWUSR, S_IWGRP, S_IWOTH, S_IXUSR , S_IXGRP, and S_IXOTH file permission bits.

If fildes refers to a socket, fchmod() does not fail but no action is taken.

If fildes refers to a stream that is attached to an object in the file system name space with fattach(3C), the fchmod() call performs no action and returns successfully.

Upon successful completion, chmod() and fchmod() mark for update the st_ctime field of the file.

The fchmodat() function is equivalent to chmod() 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 checks 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 does notperform the check.

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

AT_SYMLINK_NOFOLLOW

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 is used. If flag is also 0, the behavior shall be identical to a call to chmod().

Return Values

Upon successful completion, 0 is returned. Otherwise, −1 is returned, the file mode is unchanged, and errno is set to indicate the error.

Errors

The chmod(), fchmod(), and fchmodat() functions will fail if:

EPERM

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

The {PRIV_FILE_OWNER} privilege overrides constraints on ownership when changing permissions on a file.

The {PRIV_FILE_SETID} privilege overrides constraints on ownership when adding the setuid or setgid bits to an executable file or a directory. When adding the setuid bit to a root owned executable, additional restrictions apply. See privileges(7).

EROFS

The file referred to by path resides on a read-only file system.

The chmod() and fchmod() functions will fail if:

EIO

An I/O error occurred while reading from or writing to the file system.

The chmod() and fchmodat()functions will fail if:

EACCES

Search permission is denied on a component of the path prefix of path. The privilege {FILE_DAC_SEARCH} overrides file permissions restrictions in that case.

ELOOP

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

ENAMETOOLONG

The length of the path argument exceeds PATH_MAX, or the length of a path component exceeds NAME_MAX while _POSIX_NO_TRUNC is in effect.

ENOENT

Either a component of the path prefix or the file referred to by path does not exist or is a null pathname.

ENOTDIR

A component of the prefix of path is not a directory.

The chmod() function will fail if:

EFAULT

The path argument points to an illegal address.

ENOLINK

The fildes argument points to a remote machine and the link to that machine is no longer active.

The fchmod() function will fail if:

EBADF

The fildes argument is not an open file descriptor

ENOLINK

The path argument points to a remote machine and the link to that machine is no longer active.

The fchmodat() function will fail if:

EACCES

fd was not opened with 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.

The chmod(), fchmod(), and fchmodat() functions may fail if:

EINTR

A signal was caught during execution of the function.

EINVAL

The value of the mode argument is invalid.

The chmod() and fchmodat() functions may fail if:

ELOOP

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

ENAMETOOLONG

As a result of encountering a symbolic link in resolution of thepath argument, the length of the substituted pathname strings exceeds {PATH_MAX}.

The fchmod() function may fail if:

EINVAL

The fildes argument refers to a pipe and the system disallows execution of this function on a pipe.

The fchmodat() function may fail if:

EINVAL

The value of the flag argument is invalid

ENOTDIR

The path argument is not an absolute path and fd is neither AT_FDCWD nor a file descriptor associated with a directory

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.

Examples

Example 1 Set 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);
Example 2 Set 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);
Example 3 Set 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" 
...
chmod(CHANGEFILE, S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH);
Example 4 Set and Checking File Permissions

The following example sets the file permission bits for a file named /home/cnd/mod1, then calls the stat(2) function to verify the permissions.

#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;);

Usage

If chmod() or fchmod() is used to change the file group owner permissions on a file with non-trivial ACL entries, only the ACL mask is set to the new permissions and the group owner permission bits in the file's mode field (defined in mknod(2)) are unchanged. A non-trivial ACL entry is one whose meaning cannot be represented in the file's mode field alone. The new ACL mask permissions might change the effective permissions for additional users and groups that have ACL entries on the file.

Attributes

See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
Async-Signal-Safe
Standard

See Also

chmod(1), chown(2), creat(2), fcntl(2), mknod(2), open(2), read(2), rename(2), stat(2), write(2), fattach(3C), mkfifo(3C), stat.h(3HEAD), attributes(7), privileges(7), standards(7)