chmod, fchmod --
change mode of file
百度 《从行政推动到内源发展：中国农业农村的再出发》，郁建兴等著，北京师范大学出版社2013年4月出版。
Synopsis
#include <sys/types.h>
#include <sys/stat.h>
int chmod(const char path, mode_t mode);
int fchmod(int fildes, mode_t mode);
Description
chmod and fchmod set the
access permission portion of the mode of
the file whose name is given by path
or referenced by the descriptor fildes
to the bit pattern contained in
mode.
If path or fildes
are symbolic links, the access permissions of the
target of the symbolic links are set.
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
Save text image after execution (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 an
OR 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 P_OWNER privilege,
to change the mode of the file.
If the process does not have the P_OWNER privilege and
the file is not a directory, mode bit 01000 (save text image on
execution) is cleared on the file.
If the effective group ID of the process does not match the group ID
of the file and the process does not have the P_OWNER privilege,
mode bit 02000 (set group ID on execution) is cleared on the file.
If a 0410 executable file
has the sticky bit (mode bit 01000) set,
the operating system
will not delete the program text from the swap area
when the last user process terminates.
If a 0413 or ELF executable file
has the sticky bit set, the operating system
will not delete the program text from memory
when the last user process terminates.
In either case, if the sticky bit is set
the text will already be available
(either in a swap area or in memory)
when the next user of the file executes it,
thus making execution faster.
If a directory is writable and the sticky bit, S_ISVTX,
is set on the directory, a process may remove or rename files
within that directory only if one or more of the following
is true:
the effective user ID of the process is the
same as that of the owner ID of the file
the effective user ID of the process is the
same as that of the owner ID of the directory
the process has write permission for the file
the process has the P_OWNER privilege
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. This may affect
future calls to
open(2),
creat(2),
read(2),
and
write(2)
on this file.
Additional access permissions beyond those indicated by the mode bits may
be placed on the file. [See
acl(2).]
The following environment variables affect the execution of chmod
LC_MESSAGES
Determines the locale to be used for diagnostic messages.
If available, these messages will be retrieved
from the message data base, uxcore.abi.
LC_ALL
If a non-empty string, this overrides the values
of all the other internationalization variables.
LANG
The default value for internationalization variables
that are unset or null.
Return values
On success, chmod and fchmod return 0 and mark for update
the st_ctime field of the file.
On failure, chmod and fchmod return ``-1'',
set errno to identify the error, and the file mode is unchanged.
In the following conditions, chmod fails and sets errno to:
EACCES
Search permission is denied on a component of the path prefix of
path.
EACCES
Write permission on the named file is denied.
EFAULT
path
points outside the allocated address space of the process.
EINTR
A signal was caught during execution of the system call.
EINVAL
An attempt was made to set file/record locking on a
file on an NFS mounted filesystem.
EIO
An I/O error occurred while reading from or writing to the file system.
ELOOP
Too many symbolic links were encountered in translating
path.
EMULTIHOP
Components of path require hopping to multiple remote machines
and file system type does not allow it.
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.
ENOTDIR
A component of the prefix of path
is not a directory.
ENOENT
Either a component of the path prefix, or the file referred to by
path does not exist or is a null pathname.
ENOLINK
fildes
points to a remote machine and the link to that machine is no longer
active.
EPERM
The effective user ID does not match the owner of the file and the
process does not have the P_OWNER privilege.
EROFS
The file referred to by path
resides on a read-only file system.
In the following conditions, fchmod fails and sets errno to:
EBADF
fildes
is not an open file descriptor
EIO
An I/O error occurred while reading from or writing to the file system.
EINTR
A signal was caught during execution of the fchmod system call.
ENOLINK
path
points to a remote machine and the link to that machine is no longer
active.
EPERM
The effective user ID does not match the owner of the file and the
process does not have the P_OWNER privilege.
EROFS
The file referred to by fildes resides on a read-only file system.
path, mode_t mode);