574 lines
11 KiB
HTML
574 lines
11 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of CHMOD</TITLE>
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</HEAD><BODY>
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<H1>CHMOD</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2017-09-15<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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chmod, fchmod, fchmodat - change permissions of a file
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<PRE>
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<B>#include <<A HREF="file:///usr/include/sys/stat.h">sys/stat.h</A>></B>
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<B>int chmod(const char *</B><I>pathname</I><B>, mode_t </B><I>mode</I><B>);</B>
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<BR>
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<B>int fchmod(int </B><I>fd</I><B>, mode_t </B><I>mode</I><B>);</B>
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<B>#include <<A HREF="file:///usr/include/fcntl.h">fcntl.h</A>></B> /* Definition of AT_* constants */
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<B>#include <<A HREF="file:///usr/include/sys/stat.h">sys/stat.h</A>></B>
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<B>int fchmodat(int </B><I>dirfd</I><B>, const char *</B><I>pathname</I><B>, mode_t </B><I>mode</I><B>, int </B><I>flags</I><B>);</B>
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</PRE>
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<P>
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Feature Test Macro Requirements for glibc (see
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<B><A HREF="/cgi-bin/man/man2html?7+feature_test_macros">feature_test_macros</A></B>(7)):
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<P>
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<B>fchmod</B>():
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<DL COMPACT><DT id="1"><DD>
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Since glibc 2.24:
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<BR> _POSIX_C_SOURCE >= 199309L
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<P>
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Glibc 2.19 to 2.23
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<BR> _POSIX_C_SOURCE
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<P>
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Glibc 2.16 to 2.19:
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<BR> _BSD_SOURCE || _POSIX_C_SOURCE
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<P>
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Glibc 2.12 to 2.16:
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<BR> _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
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<BR> _POSIX_C_SOURCE >= 200809L
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<P>
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Glibc 2.11 and earlier:
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<BR> _BSD_SOURCE || _XOPEN_SOURCE >= 500
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</DL>
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<P>
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<B>fchmodat</B>():
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<DL COMPACT><DT id="2"><DD>
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<DL COMPACT>
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<DT id="3">Since glibc 2.10:<DD>
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_POSIX_C_SOURCE >= 200809L
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<DT id="4">Before glibc 2.10:<DD>
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_ATFILE_SOURCE
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</DL>
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</DL>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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The
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<B>chmod</B>()
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and
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<B>fchmod</B>()
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system calls change a files mode bits.
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(The file mode consists of the file permission bits plus the set-user-ID,
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set-group-ID, and sticky bits.)
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These system calls differ only in how the file is specified:
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<DL COMPACT>
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<DT id="5">*<DD>
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<B>chmod</B>()
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changes the mode of the file specified whose pathname is given in
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<I>pathname</I>,
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which is dereferenced if it is a symbolic link.
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<DT id="6">*<DD>
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<B>fchmod</B>()
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changes the mode of the file referred to by the open file descriptor
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<I>fd</I>.
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</DL>
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<P>
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The new file mode is specified in
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<I>mode</I>,
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which is a bit mask created by ORing together zero or
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more of the following:
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<DL COMPACT>
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<DT id="7"><B>S_ISUID</B> (04000)
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<DD>
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set-user-ID (set process effective user ID on
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<B><A HREF="/cgi-bin/man/man2html?2+execve">execve</A></B>(2))
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<DT id="8"><B>S_ISGID</B> (02000)
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<DD>
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set-group-ID (set process effective group ID on
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<B><A HREF="/cgi-bin/man/man2html?2+execve">execve</A></B>(2);
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mandatory locking, as described in
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<B><A HREF="/cgi-bin/man/man2html?2+fcntl">fcntl</A></B>(2);
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take a new file's group from parent directory, as described in
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<B><A HREF="/cgi-bin/man/man2html?2+chown">chown</A></B>(2)
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and
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<B><A HREF="/cgi-bin/man/man2html?2+mkdir">mkdir</A></B>(2))
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<DT id="9"><B>S_ISVTX</B> (01000)
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<DD>
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sticky bit (restricted deletion flag, as described in
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<B><A HREF="/cgi-bin/man/man2html?2+unlink">unlink</A></B>(2))
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<DT id="10"><B>S_IRUSR</B> (00400)
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<DD>
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read by owner
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<DT id="11"><B>S_IWUSR</B> (00200)
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<DD>
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write by owner
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<DT id="12"><B>S_IXUSR</B> (00100)
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<DD>
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execute/search by owner ("search" applies for directories,
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and means that entries within the directory can be accessed)
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<DT id="13"><B>S_IRGRP</B> (00040)
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<DD>
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read by group
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<DT id="14"><B>S_IWGRP</B> (00020)
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<DD>
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write by group
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<DT id="15"><B>S_IXGRP</B> (00010)
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<DD>
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execute/search by group
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<DT id="16"><B>S_IROTH</B> (00004)
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<DD>
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read by others
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<DT id="17"><B>S_IWOTH</B> (00002)
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<DD>
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write by others
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<DT id="18"><B>S_IXOTH</B> (00001)
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<DD>
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execute/search by others
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</DL>
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<P>
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The effective UID of the calling process must match the owner of the file,
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or the process must be privileged (Linux: it must have the
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<B>CAP_FOWNER</B>
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capability).
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<P>
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If the calling process is not privileged (Linux: does not have the
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<B>CAP_FSETID</B>
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capability), and the group of the file does not match
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the effective group ID of the process or one of its
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supplementary group IDs, the
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<B>S_ISGID</B>
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bit will be turned off,
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but this will not cause an error to be returned.
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<P>
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As a security measure, depending on the filesystem,
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the set-user-ID and set-group-ID execution bits
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may be turned off if a file is written.
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(On Linux, this occurs if the writing process does not have the
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<B>CAP_FSETID</B>
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capability.)
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On some filesystems, only the superuser can set the sticky bit,
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which may have a special meaning.
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For the sticky bit, and for set-user-ID and set-group-ID bits on
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directories, see
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<B><A HREF="/cgi-bin/man/man2html?7+inode">inode</A></B>(7).
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<P>
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On NFS filesystems, restricting the permissions will immediately influence
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already open files, because the access control is done on the server, but
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open files are maintained by the client.
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Widening the permissions may be
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delayed for other clients if attribute caching is enabled on them.
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<A NAME="lbAE"> </A>
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<H3>fchmodat()</H3>
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The
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<B>fchmodat</B>()
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system call operates in exactly the same way as
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<B>chmod</B>(),
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except for the differences described here.
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<P>
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If the pathname given in
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<I>pathname</I>
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is relative, then it is interpreted relative to the directory
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referred to by the file descriptor
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<I>dirfd</I>
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(rather than relative to the current working directory of
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the calling process, as is done by
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<B>chmod</B>()
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for a relative pathname).
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<P>
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If
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<I>pathname</I>
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is relative and
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<I>dirfd</I>
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is the special value
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<B>AT_FDCWD</B>,
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then
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<I>pathname</I>
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is interpreted relative to the current working
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directory of the calling process (like
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<B>chmod</B>()).
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<P>
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If
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<I>pathname</I>
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is absolute, then
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<I>dirfd</I>
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is ignored.
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<P>
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<I>flags</I>
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can either be 0, or include the following flag:
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<DL COMPACT>
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<DT id="19"><B>AT_SYMLINK_NOFOLLOW</B>
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<DD>
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If
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<I>pathname</I>
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is a symbolic link, do not dereference it:
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instead operate on the link itself.
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This flag is not currently implemented.
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</DL>
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<P>
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See
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<B><A HREF="/cgi-bin/man/man2html?2+openat">openat</A></B>(2)
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for an explanation of the need for
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<B>fchmodat</B>().
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<A NAME="lbAF"> </A>
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<H2>RETURN VALUE</H2>
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On success, zero is returned.
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On error, -1 is returned, and
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<I>errno</I>
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is set appropriately.
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<A NAME="lbAG"> </A>
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<H2>ERRORS</H2>
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Depending on the filesystem,
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errors other than those listed below can be returned.
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<P>
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The more general errors for
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<B>chmod</B>()
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are listed below:
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<DL COMPACT>
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<DT id="20"><B>EACCES</B>
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<DD>
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Search permission is denied on a component of the path prefix.
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(See also
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<B><A HREF="/cgi-bin/man/man2html?7+path_resolution">path_resolution</A></B>(7).)
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<DT id="21"><B>EFAULT</B>
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<DD>
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<I>pathname</I>
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points outside your accessible address space.
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<DT id="22"><B>EIO</B>
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<DD>
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An I/O error occurred.
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<DT id="23"><B>ELOOP</B>
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<DD>
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Too many symbolic links were encountered in resolving
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<I>pathname</I>.
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<DT id="24"><B>ENAMETOOLONG</B>
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<DD>
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<I>pathname</I>
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is too long.
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<DT id="25"><B>ENOENT</B>
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<DD>
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The file does not exist.
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<DT id="26"><B>ENOMEM</B>
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<DD>
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Insufficient kernel memory was available.
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<DT id="27"><B>ENOTDIR</B>
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<DD>
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A component of the path prefix is not a directory.
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<DT id="28"><B>EPERM</B>
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<DD>
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The effective UID does not match the owner of the file,
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and the process is not privileged (Linux: it does not have the
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<B>CAP_FOWNER</B>
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capability).
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<DT id="29"><B>EPERM</B>
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<DD>
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The file is marked immutable or append-only.
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(See
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<B><A HREF="/cgi-bin/man/man2html?2+ioctl_iflags">ioctl_iflags</A></B>(2).)
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<DT id="30"><B>EROFS</B>
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<DD>
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The named file resides on a read-only filesystem.
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</DL>
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<P>
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The general errors for
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<B>fchmod</B>()
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are listed below:
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<DL COMPACT>
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<DT id="31"><B>EBADF</B>
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<DD>
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The file descriptor
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<I>fd</I>
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is not valid.
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<DT id="32"><B>EIO</B>
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<DD>
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See above.
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<DT id="33"><B>EPERM</B>
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<DD>
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See above.
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<DT id="34"><B>EROFS</B>
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<DD>
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See above.
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</DL>
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<P>
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The same errors that occur for
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<B>chmod</B>()
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can also occur for
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<B>fchmodat</B>().
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The following additional errors can occur for
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<B>fchmodat</B>():
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<DL COMPACT>
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<DT id="35"><B>EBADF</B>
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<DD>
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<I>dirfd</I>
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is not a valid file descriptor.
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<DT id="36"><B>EINVAL</B>
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<DD>
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Invalid flag specified in
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<I>flags</I>.
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<DT id="37"><B>ENOTDIR</B>
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<DD>
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<I>pathname</I>
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is relative and
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<I>dirfd</I>
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is a file descriptor referring to a file other than a directory.
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<DT id="38"><B>ENOTSUP</B>
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<DD>
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<I>flags</I>
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specified
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<B>AT_SYMLINK_NOFOLLOW</B>,
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which is not supported.
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</DL>
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<A NAME="lbAH"> </A>
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<H2>VERSIONS</H2>
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<B>fchmodat</B>()
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was added to Linux in kernel 2.6.16;
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library support was added to glibc in version 2.4.
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<A NAME="lbAI"> </A>
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<H2>CONFORMING TO</H2>
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<B>chmod</B>(),
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<B>fchmod</B>():
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4.4BSD, SVr4, POSIX.1-2001i, POSIX.1-2008.
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<P>
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<B>fchmodat</B>():
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POSIX.1-2008.
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<A NAME="lbAJ"> </A>
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<H2>NOTES</H2>
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<A NAME="lbAK"> </A>
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<H3>C library/kernel differences</H3>
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The GNU C library
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<B>fchmodat</B>()
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wrapper function implements the POSIX-specified
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interface described in this page.
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This interface differs from the underlying Linux system call, which does
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<I>not</I>
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have a
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<I>flags</I>
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argument.
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<A NAME="lbAL"> </A>
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<H3>Glibc notes</H3>
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On older kernels where
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<B>fchmodat</B>()
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is unavailable, the glibc wrapper function falls back to the use of
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<B>chmod</B>().
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When
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<I>pathname</I>
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is a relative pathname,
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glibc constructs a pathname based on the symbolic link in
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<I>/proc/self/fd</I>
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that corresponds to the
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<I>dirfd</I>
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argument.
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<A NAME="lbAM"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?1+chmod">chmod</A></B>(1),
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<B><A HREF="/cgi-bin/man/man2html?2+chown">chown</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+execve">execve</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+stat">stat</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?7+inode">inode</A></B>(7),
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<B><A HREF="/cgi-bin/man/man2html?7+path_resolution">path_resolution</A></B>(7),
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<B><A HREF="/cgi-bin/man/man2html?7+symlink">symlink</A></B>(7)
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<A NAME="lbAN"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
|
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<I>man-pages</I>
|
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project.
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A description of the project,
|
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information about reporting bugs,
|
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and the latest version of this page,
|
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can be found at
|
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
|
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<HR>
|
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
|
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<DT id="39"><A HREF="#lbAB">NAME</A><DD>
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<DT id="40"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="41"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DL>
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<DT id="42"><A HREF="#lbAE">fchmodat()</A><DD>
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</DL>
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<DT id="43"><A HREF="#lbAF">RETURN VALUE</A><DD>
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<DT id="44"><A HREF="#lbAG">ERRORS</A><DD>
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<DT id="45"><A HREF="#lbAH">VERSIONS</A><DD>
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<DT id="46"><A HREF="#lbAI">CONFORMING TO</A><DD>
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<DT id="47"><A HREF="#lbAJ">NOTES</A><DD>
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<DL>
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<DT id="48"><A HREF="#lbAK">C library/kernel differences</A><DD>
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<DT id="49"><A HREF="#lbAL">Glibc notes</A><DD>
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</DL>
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<DT id="50"><A HREF="#lbAM">SEE ALSO</A><DD>
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<DT id="51"><A HREF="#lbAN">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:32 GMT, March 31, 2021
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