601 lines
12 KiB
HTML
601 lines
12 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 ACCESS</TITLE>
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</HEAD><BODY>
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<H1>ACCESS</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2016-03-15<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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access, faccessat - check user's permissions for 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/unistd.h">unistd.h</A>></B>
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<B>int access(const char *</B><I>pathname</I><B>, int </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/unistd.h">unistd.h</A>></B>
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<B>int faccessat(int </B><I>dirfd</I><B>, const char *</B><I>pathname</I><B>, int </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>faccessat</B>():
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<DL COMPACT><DT id="1"><DD>
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<DL COMPACT>
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<DT id="2">Since glibc 2.10:<DD>
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_POSIX_C_SOURCE >= 200809L
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<DT id="3">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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<B>access</B>()
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checks whether the calling process can access the file
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<I>pathname</I>.
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If
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<I>pathname</I>
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is a symbolic link, it is dereferenced.
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<P>
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The
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<I>mode</I>
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specifies the accessibility check(s) to be performed,
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and is either the value
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<B>F_OK</B>,
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or a mask consisting of the bitwise OR of one or more of
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<B>R_OK</B>, <B>W_OK</B>, and <B>X_OK</B>.
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<B>F_OK</B>
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tests for the existence of the file.
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<B>R_OK</B>, <B>W_OK</B>, and <B>X_OK</B>
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test whether the file exists and grants read, write, and
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execute permissions, respectively.
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<P>
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The check is done using the calling process's
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<I>real</I>
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UID and GID, rather than the effective IDs as is done when
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actually attempting an operation (e.g.,
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2))
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on the file.
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Similarly, for the root user, the check uses the set of
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permitted capabilities rather than the set of effective
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capabilities; and for non-root users, the check uses an empty set
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of capabilities.
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<P>
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This allows set-user-ID programs and capability-endowed programs
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to easily determine the invoking user's authority.
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In other words,
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<B>access</B>()
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does not answer the "can I read/write/execute this file?" question.
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It answers a slightly different question:
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"(assuming I'm a setuid binary) can
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<I>the user who invoked me</I>
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read/write/execute this file?",
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which gives set-user-ID programs the possibility to
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prevent malicious users from causing them to read files
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which users shouldn't be able to read.
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<P>
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If the calling process is privileged (i.e., its real UID is zero),
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then an
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<B>X_OK</B>
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check is successful for a regular file if execute permission
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is enabled for any of the file owner, group, or other.
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<A NAME="lbAE"> </A>
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<H3>faccessat()</H3>
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The
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<B>faccessat</B>()
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system call operates in exactly the same way as
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<B>access</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>access</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>access</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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is constructed by ORing together zero or more of the following values:
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<DL COMPACT>
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<DT id="4"><B>AT_EACCESS</B>
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<DD>
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Perform access checks using the effective user and group IDs.
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By default,
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<B>faccessat</B>()
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uses the real IDs (like
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<B>access</B>()).
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<DT id="5"><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 return information about the link itself.
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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>faccessat</B>().
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<A NAME="lbAF"> </A>
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<H2>RETURN VALUE</H2>
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On success (all requested permissions granted, or
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<I>mode</I>
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is
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<B>F_OK</B>
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and the file exists), zero is returned.
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On error (at least one bit in
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<I>mode</I>
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asked for a permission that is denied, or
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<I>mode</I>
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is
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<B>F_OK</B>
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and the file does not exist, or some other error occurred),
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-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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<B>access</B>()
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and
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<B>faccessat</B>()
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shall fail if:
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<DL COMPACT>
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<DT id="6"><B>EACCES</B>
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<DD>
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The requested access would be denied to the file, or search permission
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is denied for one of the directories in the path prefix of
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<I>pathname</I>.
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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="7"><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="8"><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="9"><B>ENOENT</B>
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<DD>
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A component of
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<I>pathname</I>
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does not exist or is a dangling symbolic link.
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<DT id="10"><B>ENOTDIR</B>
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<DD>
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A component used as a directory in
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<I>pathname</I>
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is not, in fact, a directory.
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<DT id="11"><B>EROFS</B>
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<DD>
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Write permission was requested for a file on a read-only filesystem.
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</DL>
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<P>
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<B>access</B>()
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and
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<B>faccessat</B>()
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may fail if:
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<DL COMPACT>
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<DT id="12"><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="13"><B>EINVAL</B>
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<DD>
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<I>mode</I>
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was incorrectly specified.
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<DT id="14"><B>EIO</B>
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<DD>
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An I/O error occurred.
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<DT id="15"><B>ENOMEM</B>
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<DD>
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Insufficient kernel memory was available.
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<DT id="16"><B>ETXTBSY</B>
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<DD>
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Write access was requested to an executable which is being
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executed.
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</DL>
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<P>
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The following additional errors can occur for
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<B>faccessat</B>():
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<DL COMPACT>
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<DT id="17"><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="18"><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="19"><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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</DL>
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<A NAME="lbAH"> </A>
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<H2>VERSIONS</H2>
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<B>faccessat</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>access</B>():
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SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
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<P>
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<B>faccessat</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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<P>
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<B>Warning</B>:
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Using these calls to check if a user is authorized to, for example,
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open a file before actually doing so using
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2)
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creates a security hole, because the user might exploit the short time
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interval between checking and opening the file to manipulate it.
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<B>For this reason, the use of this system call should be avoided</B>.
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(In the example just described,
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a safer alternative would be to temporarily switch the process's
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effective user ID to the real ID and then call
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2).)
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<P>
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<B>access</B>()
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always dereferences symbolic links.
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If you need to check the permissions on a symbolic link, use
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<B>faccessat</B>()
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with the flag
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<B>AT_SYMLINK_NOFOLLOW</B>.
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<P>
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These calls return an error if any of the access types in
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<I>mode</I>
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is denied, even if some of the other access types in
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<I>mode</I>
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are permitted.
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<P>
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If the calling process has appropriate privileges (i.e., is superuser),
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POSIX.1-2001 permits an implementation to indicate success for an
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<B>X_OK</B>
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check even if none of the execute file permission bits are set.
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Linux does not do this.
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<P>
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A file is accessible only if the permissions on each of the
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directories in the path prefix of
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<I>pathname</I>
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grant search (i.e., execute) access.
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If any directory is inaccessible, then the
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<B>access</B>()
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call fails, regardless of the permissions on the file itself.
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<P>
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Only access bits are checked, not the file type or contents.
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Therefore, if a directory is found to be writable,
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it probably means that files can be created in the directory,
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and not that the directory can be written as a file.
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Similarly, a DOS file may be found to be "executable," but the
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<B><A HREF="/cgi-bin/man/man2html?2+execve">execve</A></B>(2)
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call will still fail.
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<P>
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These calls
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may not work correctly on NFSv2 filesystems with UID mapping enabled,
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because UID mapping is done on the server and hidden from the client,
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which checks permissions. (NFS versions 3 and higher perform the check on
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the server.)
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Similar problems can occur to FUSE mounts.
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<A NAME="lbAK"> </A>
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<H3>C library/kernel differences</H3>
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The raw
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<B>faccessat</B>()
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system call takes only the first three arguments.
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The
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<B>AT_EACCESS</B>
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and
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<B>AT_SYMLINK_NOFOLLOW</B>
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flags are actually implemented within the glibc wrapper function for
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<B>faccessat</B>().
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If either of these flags is specified, then the wrapper function employs
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<B><A HREF="/cgi-bin/man/man2html?2+fstatat">fstatat</A></B>(2)
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to determine access permissions.
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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>faccessat</B>()
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is unavailable (and when the
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<B>AT_EACCESS</B>
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and
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<B>AT_SYMLINK_NOFOLLOW</B>
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flags are not specified),
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the glibc wrapper function falls back to the use of
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<B>access</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>BUGS</H2>
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In kernel 2.4 (and earlier) there is some strangeness in the handling of
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<B>X_OK</B>
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tests for superuser.
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If all categories of execute permission are disabled
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for a nondirectory file, then the only
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<B>access</B>()
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test that returns -1 is when
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<I>mode</I>
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is specified as just
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<B>X_OK</B>;
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if
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<B>R_OK</B>
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or
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<B>W_OK</B>
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is also specified in
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<I>mode</I>,
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then
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<B>access</B>()
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returns 0 for such files.
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Early 2.6 kernels (up to and including 2.6.3)
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also behaved in the same way as kernel 2.4.
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<P>
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In kernels before 2.6.20,
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these calls ignored the effect of the
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<B>MS_NOEXEC</B>
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flag if it was used to
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<B><A HREF="/cgi-bin/man/man2html?2+mount">mount</A></B>(2)
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the underlying filesystem.
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Since kernel 2.6.20, the
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<B>MS_NOEXEC</B>
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flag is honored.
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<A NAME="lbAN"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?2+chmod">chmod</A></B>(2),
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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+open">open</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+setgid">setgid</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+setuid">setuid</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?3+euidaccess">euidaccess</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?7+credentials">credentials</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="lbAO"> </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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|
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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="20"><A HREF="#lbAB">NAME</A><DD>
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<DT id="21"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="22"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DL>
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<DT id="23"><A HREF="#lbAE">faccessat()</A><DD>
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</DL>
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<DT id="24"><A HREF="#lbAF">RETURN VALUE</A><DD>
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<DT id="25"><A HREF="#lbAG">ERRORS</A><DD>
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<DT id="26"><A HREF="#lbAH">VERSIONS</A><DD>
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<DT id="27"><A HREF="#lbAI">CONFORMING TO</A><DD>
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<DT id="28"><A HREF="#lbAJ">NOTES</A><DD>
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<DL>
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<DT id="29"><A HREF="#lbAK">C library/kernel differences</A><DD>
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<DT id="30"><A HREF="#lbAL">Glibc notes</A><DD>
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</DL>
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<DT id="31"><A HREF="#lbAM">BUGS</A><DD>
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<DT id="32"><A HREF="#lbAN">SEE ALSO</A><DD>
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<DT id="33"><A HREF="#lbAO">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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