chmod.2

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.TH CHMOD 2 "Jan 26, 2022"
.SH NAME
chmod, fchmod, fchmodat \- change access permission mode of file
.SH SYNOPSIS
.nf
#include <sys/types.h>
#include <sys/stat.h>

\fBint\fR \fBchmod\fR(\fBconst char *\fR\fIpath\fR, \fBmode_t\fR \fImode\fR);
.fi

.LP
.nf
\fBint\fR \fBfchmod\fR(\fBint\fR \fIfildes\fR, \fBmode_t\fR \fImode\fR);
.fi

.LP
.nf
\fBint\fR \fBfchmodat\fR(\fBint\fR \fIfildes\fR, \fBconst char *\fR\fIpath\fR, \fBmode_t\fR \fImode\fR, \fBint\fR \fIflag\fR);
.fi

.SH DESCRIPTION
The \fBchmod()\fR, \fBfchmod()\fR, and \fBfchmodat()\fR functions set the access
permission portion of the mode of the file whose name is given by \fIpath\fR or
referenced by the open file descriptor \fIfildes\fR to the bit pattern contained
in \fImode\fR. Access permission bits are interpreted as follows:
.sp

.sp
.TS
l l l
l l l .
\fBS_ISUID\fR	04000	Set user ID on execution.
\fBS_ISGID\fR	020#0	T{
Set group ID on execution if # is \fB7\fR, \fB5\fR, \fB3\fR, or \fB1\fR. Enable mandatory file/record locking if # is \fB6\fR, \fB4\fR, \fB2\fR, or \fB0\fR.
T}
\fBS_ISVTX\fR	01000	Sticky bit.
\fBS_IRWXU\fR	00700	Read, write, execute by owner.
\fBS_IRUSR\fR	00400	Read by owner.
\fBS_IWUSR\fR	00200	Write by owner.
\fBS_IXUSR\fR	00100	T{
Execute (search if a directory) by owner.
T}
\fBS_IRWXG\fR	00070	Read, write, execute by group.
\fBS_IRGRP\fR	00040	Read by group.
\fBS_IWGRP\fR	00020	Write by group.
\fBS_IXGRP\fR	00010	Execute by group.
\fBS_IRWXO\fR	00007	Read, write, execute (search) by others.
\fBS_IROTH\fR	00004	Read by others.
\fBS_IWOTH\fR	00002	Write by others.
\fBS_IXOTH\fR	00001	Execute by others.
.TE

.sp
.LP
Modes are constructed by the bitwise OR operation of the access permission
bits.
.sp
.LP
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.
.sp
.LP
If the process is not a privileged process and the file is not a directory,
mode bit 01000 (save text image on execution) is cleared.
.sp
.LP
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.
.sp
.LP
If a directory is writable and has \fBS_ISVTX\fR (the sticky bit) set, files
within that directory can be removed or renamed only if one or more of the
following is true (see \fBunlink\fR(2) and \fBrename\fR(2)):
.RS +4
.TP
.ie t \(bu
.el o
the user owns the file
.RE
.RS +4
.TP
.ie t \(bu
.el o
the user owns the directory
.RE
.RS +4
.TP
.ie t \(bu
.el o
the file is writable by the user
.RE
.RS +4
.TP
.ie t \(bu
.el o
the user is a privileged user
.RE
.sp
.LP
If a regular file is not executable and has \fBS_ISVTX\fR 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 \fBS_ISVTX\fR bit is set on any other
file, the results are unspecified.
.sp
.LP
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.
.sp
.LP
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 \fBopen\fR(2),
\fBcreat\fR(2), \fBread\fR(2), and \fBwrite\fR(2) on this file.
.sp
.LP
If \fIfildes\fR references a shared memory object, \fBfchmod()\fR need only
affect the \fBS_IRUSR\fR, \fBS_IRGRP\fR, \fBS_IROTH\fR, \fBS_IWUSR\fR,
\fBS_IWGRP\fR, \fBS_IWOTH\fR, \fBS_IXUSR\fR, \fBS_IXGRP\fR, and \fBS_IXOTH\fR
file permission bits.
.sp
.LP
If \fIfildes\fR refers to a socket, \fBfchmod()\fR does not fail but no action
is taken.
.sp
.LP
If \fIfildes\fR refers to a stream that is attached to an object in the file
system name space with \fBfattach\fR(3C), the \fBfchmod()\fR call performs no
action and returns successfully.
.sp
.LP
The \fBfchmodat()\fR function behaves similarly to \fBchmod()\fR, except when
\fIpath\fR is a relative path, it is resolved relative to the directory
specified by \fIfiledes\fR. If \fBfiledes\fR has the value \fBAT_FDCWD\fR, then
\fBpath\fR will be resolved relative to the current working directory. The
argument \fIflag\fR should be zero, but may include the value
\fBAT_SYMLINK_NOFOLLOW\fR, which indicates that if \fIpath\fR refers to a
symbolic link, then permissions should be changed on the symbolic link itself.
However, changing permissions of symbolic links is not supported on illumos, and
will result in an error.
.sp
.LP
Upon successful completion, \fBchmod()\fR, \fBfchmod()\fR, \fBfchmodat()\fR mark
for update the \fBst_ctime\fR field of the file.
.SH RETURN VALUES
Upon successful completion, \fB0\fR is returned. Otherwise, \fB\(mi1\fR is
returned, the file mode is unchanged, and \fBerrno\fR is set to indicate the
error.
.SH ERRORS
The \fBchmod()\fR, \fBfchmod()\fR, and \fBfchmodat()\fR functions will fail if:
.sp
.ne 2
.na
\fB\fBEIO\fR\fR
.ad
.RS 9n
An I/O error occurred while reading from or writing to the file system.
.RE

.sp
.ne 2
.na
\fB\fBEPERM\fR\fR
.ad
.RS 9n
The effective user ID does not match the owner of the file and the process does
not have appropriate privilege.
.sp
The {\fBPRIV_FILE_OWNER\fR} privilege overrides constraints on ownership when
changing permissions on a file.
.sp
The {\fBPRIV_FILE_SETID\fR} 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 \fBprivileges\fR(5).
.RE

.sp
.LP
The \fBchmod()\fR and \fBfchmodat()\fR functions will fail if:
.sp
.ne 2
.na
\fB\fBEACCES\fR\fR
.ad
.RS 16n
Search permission is denied on a component of the path prefix of \fIpath\fR and
for \fBfchmodat()\fR, \fBfiledes\fR was not opened with \fBO_SEARCH\fR requested.
The privilege {\fBFILE_DAC_SEARCH\fR} overrides file permissions restrictions
in that case.
.RE

.sp
.ne 2
.na
\fB\fBEFAULT\fR\fR
.ad
.RS 16n
The \fIpath\fR argument points to an illegal address.
.RE

.sp
.ne 2
.na
\fB\fBELOOP\fR\fR
.ad
.RS 16n
A loop exists in symbolic links encountered during the resolution of the
\fIpath\fR argument.
.RE

.sp
.ne 2
.na
\fB\fBENAMETOOLONG\fR\fR
.ad
.RS 16n
The length of the \fIpath\fR argument exceeds \fBPATH_MAX\fR, or the length of
a \fIpath\fR component exceeds \fBNAME_MAX\fR while \fB_POSIX_NO_TRUNC\fR is in
effect.
.RE

.sp
.ne 2
.na
\fB\fBENOENT\fR\fR
.ad
.RS 16n
Either a component of the path prefix or the file referred to by \fIpath\fR
does not exist or is a null pathname.
.RE

.sp
.ne 2
.na
\fB\fBENOLINK\fR\fR
.ad
.RS 16n
The \fIfildes\fR argument points to a remote machine and the link to that
machine is no longer active.
.RE

.sp
.ne 2
.na
\fB\fBENOTDIR\fR\fR
.ad
.RS 16n
A component of the prefix of \fIpath\fR is not a directory.
.RE

.sp
.ne 2
.na
\fB\fBEROFS\fR\fR
.ad
.RS 16n
The file referred to by \fIpath\fR resides on a read-only file system.
.RE

.sp
.LP
The \fBfchmod()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEBADF\fR\fR
.ad
.RS 11n
The \fIfildes\fR argument is not an open file descriptor
.RE

.sp
.ne 2
.na
\fB\fBENOLINK\fR\fR
.ad
.RS 11n
The \fIpath\fR argument points to a remote machine and the link to that machine
is no longer active.
.RE

.sp
.ne 2
.na
\fB\fBEROFS\fR\fR
.ad
.RS 11n
The file referred to by \fIfildes\fR resides on a read-only file system.
.RE

.sp
.LP
The \fBchmod()\fR and \fBfchmod()\fR functions may fail if:
.sp
.ne 2
.na
\fB\fBEINTR\fR\fR
.ad
.RS 10n
A signal was caught during execution of the function.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The value of the \fImode\fR argument is invalid.
.RE

.sp
.LP
The \fBfchmodat()\fR will fail if:
.sp
.ne 2
.na
.B EBADF
.ad
.RS 16n
The argument \fIpath\fR is a relative path and \fIfiledes\fR is not an open file
descriptor or the value \fBAT_FDCWD\fR.
.RE

.sp
.ne 2
.na
.B EINVAL
.ad
.RS 16n
The argument \fIflags\fR has a non-zero value other than
\fBAT_SYMLINK_NOFOLLOW\fR.
.RE

.sp
.ne 2
.na
.B ENOTDIR
.ad
.RS 16n
The argument \fIpath\fR is a relative path and \fIfiledes\fR is a valid file
descriptor which does not refer to a file.
.RE

.sp
.ne 2
.na
.B EOPNOTSUPP
.ad
.RS 16n
The \fBAT_SYMLINK_NOFOLLOW\fR bit is set in the \fIflags\fR argument and
\fIpath\fR refers to a symbolic link.
.RE

.sp
.LP
The \fBchmod()\fR and \fBfchmodat()\fR functions may fail if:
.sp
.ne 2
.na
\fB\fBELOOP\fR\fR
.ad
.RS 16n
More than {\fBSYMLOOP_MAX\fR} symbolic links were encountered during the
resolution of the \fIpath\fR argument.
.RE

.sp
.ne 2
.na
\fB\fBENAMETOOLONG\fR\fR
.ad
.RS 16n
As a result of encountering a symbolic link in resolution of the \fIpath\fR
argument, the length of the substituted pathname strings exceeds
{\fBPATH_MAX\fR}.
.RE

.sp
.LP
The \fBfchmod()\fR function may fail if:
.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The \fIfildes\fR argument refers to a pipe and the system disallows execution
of this function on a pipe.
.RE

.SH EXAMPLES
\fBExample 1 \fRSet Read Permissions for User, Group, and Others
.sp
.LP
The following example sets read permissions for the owner, group, and others.

.sp
.in +2
.nf
#include <sys/stat.h>
const char *path;
\&...
chmod(path, S_IRUSR|S_IRGRP|S_IROTH);
.fi
.in -2

.LP
\fBExample 2 \fRSet Read, Write, and Execute Permissions for the Owner Only
.sp
.LP
The following example sets read, write, and execute permissions for the owner,
and no permissions for group and others.

.sp
.in +2
.nf
#include <sys/stat.h>
const char *path;
\&...
chmod(path, S_IRWXU);
.fi
.in -2

.LP
\fBExample 3 \fRSet Different Permissions for Owner, Group, and Other
.sp
.LP
The following example sets owner permissions for CHANGEFILE to read, write, and
execute, group permissions to read and execute, and other permissions to read.

.sp
.in +2
.nf
#include <sys/stat.h>
#define CHANGEFILE "/etc/myfile"
\&...
chmod(CHANGEFILE, S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH);
.fi
.in -2

.LP
\fBExample 4 \fRSet and Checking File Permissions
.sp
.LP
The following example sets the file permission bits for a file named
\fB/home/cnd/mod1\fR, then calls the \fBstat\fR(2) function to verify the
permissions.

.sp
.in +2
.nf
#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;);
.fi
.in -2

.SH USAGE
If \fBchmod()\fR or \fBfchmod()\fR 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 \fBmknod\fR(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.
.SH ATTRIBUTES
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT-Level	Async-Signal-Safe
.TE

.SH SEE ALSO
\fBchmod\fR(1), \fBchown\fR(2), \fBcreat\fR(2), \fBfcntl\fR(2), \fBmknod\fR(2),
\fBopen\fR(2), \fBread\fR(2), \fBrename\fR(2), \fBstat\fR(2), \fBwrite\fR(2),
\fBfattach\fR(3C), \fBmkfifo\fR(3C), \fBstat.h\fR(3HEAD), \fBattributes\fR(5),
\fBprivileges\fR(5), \fBstandards\fR(5)
.sp
.LP
\fIProgramming Interfaces Guide\fR