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Fairly substantial rewrites, and a few additions.
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Michael Kerrisk
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man2/access.2
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man2/access.2
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.\"
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.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
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.\" 1993 Michael Haardt, Ian Jackson.
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.\" and Copyright (C) 2007 Michael Kerrisk <mtk-manpages@gmx.net>
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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@ -37,10 +38,11 @@
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.\" Modified 2001-10-16 by aeb
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.\" Modified 2002-04-23 by Roger Luethi <rl@hellgate.ch>
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.\" Modified 2004-06-23 by Michael Kerrisk
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.\" 2007-06-10, mtk, various parts rewritten, and added BUGS section.
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.\"
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.TH ACCESS 2 2004-06-23 "Linux" "Linux Programmer's Manual"
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.TH ACCESS 2 2007-07-10 "Linux" "Linux Programmer's Manual"
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.SH NAME
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access \- check user's permissions for a file
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access \- check real user's permissions for a file
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.SH SYNOPSIS
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.nf
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.B #include <unistd.h>
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@ -49,53 +51,43 @@ access \- check user's permissions for a file
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.fi
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.SH DESCRIPTION
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.BR access ()
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checks whether the calling process would be allowed to read,
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write or test for existence of the file (or other file system
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object) whose name is
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checks whether the calling process can access the file
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.IR pathname .
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If
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.I pathname
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is a symbolic link permissions of the file referred to by this
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symbolic link are tested.
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is a symbolic link, it is dereferenced.
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The
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.I mode
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is either the value
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.BR F_OK
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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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.BR F_OK ,
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or a mask consisting of the bitwise OR of one or more of
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.BR R_OK ", " W_OK " and " X_OK .
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.BR R_OK ", " W_OK " and " X_OK
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request checking whether the file exists and has read, write and
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execute permissions, respectively.
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.BR R_OK ", " W_OK ", and " X_OK .
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.\" FIXME . the kernel currently allows F_OK to be specified
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.\" in conjunction with any of the other flags.
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.\" Test whether other implementations permit this and if not
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.\" file a bug report.
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.B F_OK
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just requests checking for the existence of the file.
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tests for the existence of the file.
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.BR R_OK ", " W_OK ", and " X_OK
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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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The tests depend on the permissions of the directories
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occurring in the path to the file, as given in
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.IR pathname ,
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and on the permissions of directories and files referred to by symbolic
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links encountered on the way.
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The check is done with the calling process's
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The check is done using the calling process's
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.I real
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UID and GID, rather than with the effective IDs as is done when
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actually attempting an operation.
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This is to allow set-user-ID programs to
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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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.BR open (2))
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on the file.
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This allows set-user-ID programs to
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easily determine the invoking user's authority.
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Only access bits are checked, not the file type or contents.
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Therefore, if
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a directory is found to be "writable," it probably means that files can be
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created in the directory, and not that the directory can be written as a
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file.
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Similarly, a DOS file may be found to be "executable," but the
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.BR execve (2)
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call will still fail.
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If the calling process has appropriate privileges, an implementation may
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indicate success for
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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
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even if none of the execute file permission bits are set.
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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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.SH "RETURN VALUE"
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On success (all requested permissions granted), zero is returned.
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On error (at least one bit in
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@ -159,14 +151,6 @@ executed.
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.SH "CONFORMING TO"
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SVr4, 4.3BSD, POSIX.1-2001.
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.SH NOTES
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.BR access ()
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returns an error if any of the access types in the requested call
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fails, even if other types might be successful.
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.PP
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.BR access ()
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may not work correctly on NFS file systems 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.
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.PP
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.BR Warning:
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Using
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@ -176,7 +160,65 @@ open a file before actually doing so using
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.BR open (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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.SS Linux Notes
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.BR "For this reason, the use of this system call should be avoided" .
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.PP
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.BR access ()
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returns an error if any of the access types in
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.I mode
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is denied, even if some of the other access types in
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.I mode
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are permitted.
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.PP
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If the calling process has appropriate privileges (i.e., is superuser),
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POSIX.1-2001 permits implementation to indicate success for an
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.B X_OK
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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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.PP
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A file is only accessible if the permissions on each of the
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directories in the path prefix of
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.I pathname
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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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.BR access ()
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call will fail, regardless of the permissions on the file itself.
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.PP
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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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.BR execve (2)
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call will still fail.
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.PP
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.BR access ()
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may not work correctly on NFS file systems 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.
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.SH BUGS
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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
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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 non-directory file, then the only
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.BR access ()
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test that returns \-1 is when
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.I mode
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is specified as just
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.BR X_OK ;
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if
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.B R_OK
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or
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.B W_OK
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is also specified in
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.IR mode ,
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then
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.BR access ()
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returns 0 for such files.
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.\" This behavior appears to have been an implementation accident.
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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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In kernels before 2.6.20,
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.BR access ()
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ignored the effect of the
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