Originální popis anglicky:
chmod - change mode of a file
Návod, kniha: POSIX Programmer's Manual
#include <sys/stat.h>
int chmod(const char *
path, mode_t
mode );
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.
The effect on file descriptors for files open at the time of a call to
chmod() is implementation-defined.
Upon successful completion,
chmod() shall mark for update the
st_ctime field of the file.
Upon successful completion, 0 shall be returned; otherwise, -1 shall be returned
and
errno set to indicate the error. If -1 is returned, no change to
the file mode occurs.
The
chmod() function 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 the path argument exceeds {PATH_MAX}
or a pathname component is longer than {NAME_MAX}.
- ENOTDIR
- A component of the path prefix is not a directory.
- ENOENT
- A component of path does not name an existing file
or path is an empty string.
- 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
chmod() function 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
- As a result of encountering a symbolic link in resolution
of the path argument, the length of the substituted pathname
strings exceeded {PATH_MAX}.
The following sections are informative.
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);
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);
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);
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 IEEE Std 1003.1-2001 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.
None.
chown() ,
mkdir() ,
mkfifo() ,
open() ,
stat() ,
statvfs() , the Base Definitions volume of
IEEE Std 1003.1-2001,
<sys/stat.h>,
<sys/types.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 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
.
