2004-11-03 13:51:07 +00:00
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.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
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2007-06-20 22:33:04 +00:00
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.TH "CHMOD" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
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2004-11-03 13:51:07 +00:00
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.\" chmod
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2007-09-20 06:03:25 +00:00
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.SH PROLOG
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This manual page is part of the POSIX Programmer's Manual.
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The Linux implementation of this interface may differ (consult
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the corresponding Linux manual page for details of Linux behavior),
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or the interface may not be implemented on Linux.
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2004-11-03 13:51:07 +00:00
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.SH NAME
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chmod \- change mode of a file
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.SH SYNOPSIS
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.LP
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\fB#include <sys/stat.h>
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.br
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.sp
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int chmod(const char *\fP\fIpath\fP\fB, mode_t\fP \fImode\fP\fB);
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.br
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\fP
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.SH DESCRIPTION
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.LP
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The \fIchmod\fP() function shall change S_ISUID, S_ISGID, \ S_ISVTX,
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and the file permission bits of the file named by the pathname pointed
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to by the \fIpath\fP argument to the
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corresponding bits in the \fImode\fP argument. The application shall
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ensure that the effective user ID of the process matches the
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owner of the file or the process has appropriate privileges in order
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to do this.
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.LP
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S_ISUID, S_ISGID, \ S_ISVTX, and the file permission
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bits are described in \fI<sys/stat.h>\fP.
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.LP
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If the calling process does not have appropriate privileges, and if
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the group ID of the file does not match the effective group
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ID or one of the supplementary group IDs and if the file is a regular
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file, bit S_ISGID (set-group-ID on execution) in the file's
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mode shall be cleared upon successful return from \fIchmod\fP().
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.LP
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Additional implementation-defined restrictions may cause the S_ISUID
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and S_ISGID bits in \fImode\fP to be ignored.
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.LP
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The effect on file descriptors for files open at the time of a call
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to \fIchmod\fP() is implementation-defined.
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.LP
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Upon successful completion, \fIchmod\fP() shall mark for update the
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\fIst_ctime\fP field of the file.
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.SH RETURN VALUE
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.LP
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Upon successful completion, 0 shall be returned; otherwise, -1 shall
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be returned and \fIerrno\fP set to indicate the error. If
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-1 is returned, no change to the file mode occurs.
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.SH ERRORS
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.LP
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The \fIchmod\fP() function shall fail if:
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.TP 7
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.B EACCES
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Search permission is denied on a component of the path prefix.
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.TP 7
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.B ELOOP
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A loop exists in symbolic links encountered during resolution of the
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\fIpath\fP argument.
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.TP 7
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.B ENAMETOOLONG
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The length of the \fIpath\fP argument exceeds {PATH_MAX} or a pathname
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component is longer than {NAME_MAX}.
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.TP 7
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.B ENOTDIR
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A component of the path prefix is not a directory.
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.TP 7
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.B ENOENT
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A component of \fIpath\fP does not name an existing file or \fIpath\fP
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is an empty string.
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.TP 7
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.B EPERM
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The effective user ID does not match the owner of the file and the
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process does not have appropriate privileges.
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.TP 7
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.B EROFS
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The named file resides on a read-only file system.
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.sp
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.LP
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The \fIchmod\fP() function may fail if:
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.TP 7
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.B EINTR
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A signal was caught during execution of the function.
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.TP 7
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.B EINVAL
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The value of the \fImode\fP argument is invalid.
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.TP 7
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.B ELOOP
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More than {SYMLOOP_MAX} symbolic links were encountered during resolution
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of the \fIpath\fP argument.
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.TP 7
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.B ENAMETOOLONG
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As a result of encountering a symbolic link in resolution of the \fIpath\fP
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argument, the length of the substituted pathname
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strings exceeded {PATH_MAX}.
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.sp
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.LP
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\fIThe following sections are informative.\fP
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.SH EXAMPLES
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.SS Setting Read Permissions for User, Group, and Others
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.LP
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The following example sets read permissions for the owner, group,
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and others.
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.sp
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.RS
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.nf
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\fB#include <sys/stat.h>
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.sp
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const char *path;
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\&...
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chmod(path, S_IRUSR|S_IRGRP|S_IROTH);
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\fP
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.fi
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.RE
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.SS Setting Read, Write, and Execute Permissions for the Owner Only
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.LP
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The following example sets read, write, and execute permissions for
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the owner, and no permissions for group and others.
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.sp
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.RS
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.nf
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\fB#include <sys/stat.h>
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.sp
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const char *path;
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\&...
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chmod(path, S_IRWXU);
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\fP
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.fi
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.RE
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.SS Setting Different Permissions for Owner, Group, and Other
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.LP
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The following example sets owner permissions for CHANGEFILE to read,
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write, and execute, group permissions to read and execute,
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and other permissions to read.
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.sp
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.RS
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.nf
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\fB#include <sys/stat.h>
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.sp
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#define CHANGEFILE "/etc/myfile"
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\&...
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chmod(CHANGEFILE, S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH);
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\fP
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.fi
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.RE
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.SS Setting and Checking File Permissions
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.LP
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The following example sets the file permission bits for a file named
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\fB/home/cnd/mod1\fP, then calls the \fIstat\fP() function to verify
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the permissions.
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.sp
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.RS
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.nf
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\fB#include <sys/types.h>
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#include <sys/stat.h>
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.sp
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int status;
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struct stat buffer
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\&...
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chmod("home/cnd/mod1", S_IRWXU|S_IRWXG|S_IROTH|S_IWOTH);
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status = stat("home/cnd/mod1", &buffer;);
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\fP
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.fi
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.RE
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.SH APPLICATION USAGE
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.LP
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In order to ensure that the S_ISUID and S_ISGID bits are set, an application
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requiring this should use \fIstat\fP() after a successful \fIchmod\fP()
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to verify this.
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.LP
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Any file descriptors currently open by any process on the file could
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possibly become invalid if the mode of the file is changed
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to a value which would deny access to that process. One situation
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where this could occur is on a stateless file system. This
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behavior will not occur in a conforming environment.
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.SH RATIONALE
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.LP
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This volume of IEEE\ Std\ 1003.1-2001 specifies that the S_ISGID bit
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is cleared by \fIchmod\fP() on a regular file
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under certain conditions. This is specified on the assumption that
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regular files may be executed, and the system should prevent
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users from making executable \fIsetgid\fP() files perform with privileges
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that the caller
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does not have. On implementations that support execution of other
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file types, the S_ISGID bit should be cleared for those file
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types under the same circumstances.
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.LP
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Implementations that use the S_ISUID bit to indicate some other function
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(for example, mandatory record locking) on
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non-executable files need not clear this bit on writing. They should
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clear the bit for executable files and any other cases where
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the bit grants special powers to processes that change the file contents.
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Similar comments apply to the S_ISGID bit.
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.SH FUTURE DIRECTIONS
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.LP
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None.
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.SH SEE ALSO
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.LP
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2007-09-20 06:12:53 +00:00
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\fIchown\fP(), \fImkdir\fP(), \fImkfifo\fP(),
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2007-09-20 06:11:55 +00:00
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\fIopen\fP(), \fIstat\fP(), \fIstatvfs\fP(), the
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2004-11-03 13:51:07 +00:00
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Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI<sys/stat.h>\fP,
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\fI<sys/types.h>\fP
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.SH COPYRIGHT
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Portions of this text are reprinted and reproduced in electronic form
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from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
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-- Portable Operating System Interface (POSIX), The Open Group Base
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Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
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Electrical and Electronics Engineers, Inc and The Open Group. In the
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event of any discrepancy between this version and the original IEEE and
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The Open Group Standard, the original IEEE and The Open Group Standard
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is the referee document. The original Standard can be obtained online at
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http://www.opengroup.org/unix/online.html .
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