Manpage of CHMOD

CHMOD

Section: Linux Programmer's Manual (2)
Updated: 2016-03-15
Index
 

NAME

chmod, fchmod, fchmodat - change permissions of a file  

SYNOPSIS

#include <sys/stat.h>int chmod(const char *pathname, mode_t mode);
int fchmod(int fd, mode_t mode);#include <fcntl.h> /* Definition of AT_* constants */ #include <sys/stat.h>int fchmodat(int dirfd, const char *pathname, mode_t mode, int flags);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

fchmod():

/* Since glibc 2.16: */ _POSIX_C_SOURCE
    || /* Glibc versions <= 2.19: */ _BSD_SOURCE
    || /* Glibc versions <= 2.15: */ _XOPEN_SOURCE >= 500
    || /* Glibc 2.12 to 2.15: */ _POSIX_C_SOURCE >= 200809L

fchmodat():

Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_ATFILE_SOURCE
 

DESCRIPTION

The chmod() and fchmod() system calls change a files mode bits. (The file mode consists of the file permission bits plus the set-user-ID, set-group-ID, and sticky bits.) These system calls differ only in how the file is specified:
*
chmod() changes the mode of the file specified whose pathname is given in pathname, which is dereferenced if it is a symbolic link.
*
fchmod() changes the mode of the file referred to by the open file descriptor fd.

The new file mode is specified in mode, which is a bit mask created by ORing together zero or more of the following:

S_ISUID (04000)
set-user-ID (set process effective user ID on execve(2))
S_ISGID (02000)
set-group-ID (set process effective group ID on execve(2); mandatory locking, as described in fcntl(2); take a new file's group from parent directory, as described in chown(2) and mkdir(2))
S_ISVTX (01000)
sticky bit (restricted deletion flag, as described in unlink(2))
S_IRUSR (00400)
read by owner
S_IWUSR (00200)
write by owner
S_IXUSR (00100)
execute/search by owner ("search" applies for directories, and means that entries within the directory can be accessed)
S_IRGRP (00040)
read by group
S_IWGRP (00020)
write by group
S_IXGRP (00010)
execute/search by group
S_IROTH (00004)
read by others
S_IWOTH (00002)
write by others
S_IXOTH (00001)
execute/search by others

The effective UID of the calling process must match the owner of the file, or the process must be privileged (Linux: it must have the CAP_FOWNERcapability).

If the calling process is not privileged (Linux: does not have the CAP_FSETIDcapability), and the group of the file does not match the effective group ID of the process or one of its supplementary group IDs, the S_ISGIDbit will be turned off, but this will not cause an error to be returned.

As a security measure, depending on the filesystem, the set-user-ID and set-group-ID execution bits may be turned off if a file is written. (On Linux this occurs if the writing process does not have the CAP_FSETIDcapability.) On some filesystems, only the superuser can set the sticky bit, which may have a special meaning. For the sticky bit, and for set-user-ID and set-group-ID bits on directories, see stat(2).

On NFS filesystems, restricting the permissions will immediately influence already open files, because the access control is done on the server, but open files are maintained by the client. Widening the permissions may be delayed for other clients if attribute caching is enabled on them.  

fchmodat()

The fchmodat() system call operates in exactly the same way as chmod(), except for the differences described here.

If the pathname given in pathnameis relative, then it is interpreted relative to the directory referred to by the file descriptor dirfd(rather than relative to the current working directory of the calling process, as is done by chmod() for a relative pathname).

If pathnameis relative and dirfdis the special value AT_FDCWD, then pathnameis interpreted relative to the current working directory of the calling process (like chmod()).

If pathnameis absolute, then dirfdis ignored.

flagscan either be 0, or include the following flag:

AT_SYMLINK_NOFOLLOW
If pathnameis a symbolic link, do not dereference it: instead operate on the link itself. This flag is not currently implemented.

See openat(2) for an explanation of the need for fchmodat().  

RETURN VALUE

On success, zero is returned. On error, -1 is returned, and errnois set appropriately.  

ERRORS

Depending on the filesystem, errors other than those listed below can be returned.

The more general errors for chmod() are listed below:

EACCES
Search permission is denied on a component of the path prefix. (See also path_resolution(7).)
EFAULT
pathnamepoints outside your accessible address space.
EIO
An I/O error occurred.
ELOOP
Too many symbolic links were encountered in resolving pathname.
ENAMETOOLONG
pathnameis too long.
ENOENT
The file does not exist.
ENOMEM
Insufficient kernel memory was available.
ENOTDIR
A component of the path prefix is not a directory.
EPERM
The effective UID does not match the owner of the file, and the process is not privileged (Linux: it does not have the CAP_FOWNERcapability).
EROFS
The named file resides on a read-only filesystem.

The general errors for fchmod() are listed below:

EBADF
The file descriptor fdis not valid.
EIO
See above.
EPERM
See above.
EROFS
See above.

The same errors that occur for chmod() can also occur for fchmodat(). The following additional errors can occur for fchmodat():

EBADF
dirfdis not a valid file descriptor.
EINVAL
Invalid flag specified in flags.
ENOTDIR
pathnameis relative and dirfdis a file descriptor referring to a file other than a directory.
ENOTSUP
flagsspecified AT_SYMLINK_NOFOLLOW, which is not supported.
 

VERSIONS

fchmodat() was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4.  

CONFORMING TO

chmod(), fchmod(): 4.4BSD, SVr4, POSIX.1-2001i, POSIX.1-2008.

fchmodat(): POSIX.1-2008.  

NOTES

 

C library/kernel differences

The GNU C library fchmodat() wrapper function implements the POSIX-specified interface described in this page. This interface differs from the underlying Linux system call, which does nothave a flagsargument.  

Glibc notes

On older kernels where fchmodat() is unavailable, the glibc wrapper function falls back to the use of chmod(). When pathnameis a relative pathname, glibc constructs a pathname based on the symbolic link in /proc/self/fdthat corresponds to the dirfdargument.  

SEE ALSO

chown(2), execve(2), open(2), stat(2), path_resolution(7), symlink(7)


 

Index

NAME
SYNOPSIS
DESCRIPTION
fchmodat()
RETURN VALUE
ERRORS
VERSIONS
CONFORMING TO
NOTES
C library/kernel differences
Glibc notes
SEE ALSO

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