.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "CHMOD" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" chmod 
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.SH NAME
chmod \- change mode of a file
.SH SYNOPSIS
.LP
\fB#include <sys/stat.h>
.br
.sp
int chmod(const char *\fP\fIpath\fP\fB, mode_t\fP \fImode\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
The \fIchmod\fP() 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 \fIpath\fP argument to the
corresponding bits in the \fImode\fP 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.
.LP
S_ISUID, S_ISGID,  S_ISVTX,  and the file permission
bits are described in \fI<sys/stat.h>\fP.
.LP
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 \fIchmod\fP().
.LP
Additional implementation-defined restrictions may cause the S_ISUID
and S_ISGID bits in \fImode\fP to be ignored.
.LP
The effect on file descriptors for files open at the time of a call
to \fIchmod\fP() is implementation-defined.
.LP
Upon successful completion, \fIchmod\fP() shall mark for update the
\fIst_ctime\fP field of the file.
.SH RETURN VALUE
.LP
Upon successful completion, 0 shall be returned; otherwise, -1 shall
be returned and \fIerrno\fP set to indicate the error. If
-1 is returned, no change to the file mode occurs.
.SH ERRORS
.LP
The \fIchmod\fP() function shall fail if:
.TP 7
.B EACCES
Search permission is denied on a component of the path prefix.
.TP 7
.B ELOOP
A loop exists in symbolic links encountered during resolution of the
\fIpath\fP argument.
.TP 7
.B ENAMETOOLONG
The length of the \fIpath\fP argument exceeds {PATH_MAX} or a pathname
component is longer than {NAME_MAX}.
.TP 7
.B ENOTDIR
A component of the path prefix is not a directory.
.TP 7
.B ENOENT
A component of \fIpath\fP does not name an existing file or \fIpath\fP
is an empty string.
.TP 7
.B EPERM
The effective user ID does not match the owner of the file and the
process does not have appropriate privileges.
.TP 7
.B EROFS
The named file resides on a read-only file system.
.sp
.LP
The \fIchmod\fP() function may fail if:
.TP 7
.B EINTR
A signal was caught during execution of the function.
.TP 7
.B EINVAL
The value of the \fImode\fP argument is invalid.
.TP 7
.B ELOOP
More than {SYMLOOP_MAX} symbolic links were encountered during resolution
of the \fIpath\fP argument.
.TP 7
.B ENAMETOOLONG
As a result of encountering a symbolic link in resolution of the \fIpath\fP
argument, the length of the substituted pathname
strings exceeded {PATH_MAX}.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.SS Setting Read Permissions for User, Group, and Others
.LP
The following example sets read permissions for the owner, group,
and others.
.sp
.RS
.nf

\fB#include <sys/stat.h>
.sp

const char *path;
\&...
chmod(path, S_IRUSR|S_IRGRP|S_IROTH);
\fP
.fi
.RE
.SS Setting Read, Write, and Execute Permissions for the Owner Only
.LP
The following example sets read, write, and execute permissions for
the owner, and no permissions for group and others.
.sp
.RS
.nf

\fB#include <sys/stat.h>
.sp

const char *path;
\&...
chmod(path, S_IRWXU);
\fP
.fi
.RE
.SS Setting Different Permissions for Owner, Group, and Other
.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
.RS
.nf

\fB#include <sys/stat.h>
.sp

#define CHANGEFILE "/etc/myfile"
\&...
chmod(CHANGEFILE, S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH);
\fP
.fi
.RE
.SS Setting and Checking File Permissions
.LP
The following example sets the file permission bits for a file named
\fB/home/cnd/mod1\fP, then calls the \fIstat\fP() function to verify
the permissions.
.sp
.RS
.nf

\fB#include <sys/types.h>
#include <sys/stat.h>
.sp

int status;
struct stat buffer
\&...
chmod("home/cnd/mod1", S_IRWXU|S_IRWXG|S_IROTH|S_IWOTH);
status = stat("home/cnd/mod1", &buffer;);
\fP
.fi
.RE
.SH APPLICATION USAGE
.LP
In order to ensure that the S_ISUID and S_ISGID bits are set, an application
requiring this should use \fIstat\fP() after a successful \fIchmod\fP()
to verify this.
.LP
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.
.SH RATIONALE
.LP
This volume of IEEE\ Std\ 1003.1-2001 specifies that the S_ISGID bit
is cleared by \fIchmod\fP() 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 \fIsetgid\fP() 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.
.LP
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.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIchown\fP(), \fImkdir\fP(), \fImkfifo\fP(),
\fIopen\fP(), \fIstat\fP(), \fIstatvfs\fP(), the
Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI<sys/stat.h>\fP,
\fI<sys/types.h>\fP
.SH COPYRIGHT
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 .