path: root/man-pages-posix-2017/man3p/access.3p

'\" et
.TH ACCESS "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
access, faccessat
\(em determine accessibility of a file
descriptor
.SH SYNOPSIS
.LP
.nf
#include <unistd.h>
.P
int access(const char *\fIpath\fP, int \fIamode\fP);
.P
#include <fcntl.h>
.P
int faccessat(int \fIfd\fP, const char *\fIpath\fP, int \fIamode\fP, int \fIflag\fP);
.fi
.SH DESCRIPTION
The
\fIaccess\fR()
function shall check the file named by the pathname pointed to by the
.IR path
argument for accessibility according to the bit pattern contained in
.IR amode .
The checks for accessibility (including directory permissions
checked during pathname resolution) shall be performed using the
real user ID in place of the effective user ID and the real group
ID in place of the effective group ID.
.P
The value of
.IR amode
is either the bitwise-inclusive OR of the access permissions to be
checked (R_OK, W_OK, X_OK) or the existence test (F_OK).
.P
If any access permissions are checked, each shall be checked
individually, as described in the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 4.5" ", " "File Access Permissions",
except that where that description refers to execute permission for
a process with appropriate privileges, an implementation may indicate
success for X_OK even if execute permission is not granted to any user.
.P
The
\fIfaccessat\fR()
function, when called with a
.IR flag
value of zero, shall be equivalent to the
\fIaccess\fR()
function, except in the case where
.IR path
specifies a relative path. In this case the file whose accessibility is
to be determined shall be located relative to the directory associated
with the file descriptor
.IR fd
instead of the current working directory.
If the access mode of the open file description associated with
the file descriptor is not O_SEARCH, the function shall check
whether directory searches are permitted using the current
permissions of the directory underlying the file descriptor.
If the access mode is O_SEARCH, the function shall not perform the check.
.P
If
\fIfaccessat\fR()
is passed the special value AT_FDCWD in the
.IR fd
parameter, the current working directory shall be used and, if flag is
zero, the behavior shall be identical to a call to
\fIaccess\fR().
.P
Values for
.IR flag
are constructed by a bitwise-inclusive OR of flags from the following
list, defined in
.IR <fcntl.h> :
.IP AT_EACCESS 12
The checks for accessibility (including directory permissions
checked during pathname resolution) shall be performed using the
effective user ID and group ID instead of the real user ID and group ID
as required in a call to
\fIaccess\fR().
.SH "RETURN VALUE"
Upon successful completion, these functions shall return 0. Otherwise,
these functions shall return \-1 and set
.IR errno
to indicate the error.
.SH ERRORS
These functions shall fail if:
.TP
.BR EACCES
Permission bits of the file mode do not permit the requested access, or
search permission is denied on a component of the path prefix.
.TP
.BR ELOOP
A loop exists in symbolic links encountered during resolution of the
.IR path
argument.
.TP
.BR ENAMETOOLONG
.br
The length of a component of a pathname is longer than
{NAME_MAX}.
.TP
.BR ENOENT
A component of
.IR path
does not name an existing file or
.IR path
is an empty string.
.TP
.BR ENOTDIR
A component of the path prefix names an existing file that is neither
a directory nor a symbolic link to a directory, or the
.IR path
argument contains at least one non-\c
<slash>
character and ends with one or more trailing
<slash>
characters and the last pathname component names an existing file
that is neither a directory nor a symbolic link to a directory.
.TP
.BR EROFS
Write access is requested for a file on a read-only file system.
.P
The
\fIfaccessat\fR()
function shall fail if:
.TP
.BR EACCES
The access mode of the open file description associated with
.IR fd
is not O_SEARCH and the permissions of the directory underlying
.IR fd
do not permit directory searches.
.TP
.BR EBADF
The
.IR path
argument does not specify an absolute path and the
.IR fd
argument is neither AT_FDCWD nor a valid file descriptor open
for reading or searching.
.TP
.BR ENOTDIR
The
.IR path
argument is not an absolute path and
.IR fd
is a file descriptor associated with a non-directory file.
.P
These functions may fail if:
.TP
.BR EINVAL
The value of the \fIamode\fP argument is invalid.
.TP
.BR ELOOP
More than
{SYMLOOP_MAX}
symbolic links were encountered during resolution of the
.IR path
argument.
.TP
.BR ENAMETOOLONG
.br
The length of a pathname exceeds
{PATH_MAX},
or pathname resolution of a symbolic link produced an intermediate
result with a length that exceeds
{PATH_MAX}.
.TP
.BR ETXTBSY
Write access is requested for a pure procedure (shared text) file that
is being executed.
.P
The
\fIfaccessat\fR()
function may fail if:
.TP
.BR EINVAL
The value of the
.IR flag
argument is not valid.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Testing for the Existence of a File"
.P
The following example tests whether a file named
.BR myfile
exists in the
.BR /tmp
directory.
.sp
.RS 4
.nf

#include <unistd.h>
\&...
int result;
const char *pathname = "/tmp/myfile";
.P
result = access (pathname, F_OK);
.fi
.P
.RE
.SH "APPLICATION USAGE"
Use of these functions is discouraged since by the time the
returned information is acted upon, it is out-of-date. (That is,
acting upon the information always leads to a time-of-check-to-time-of-use
race condition.) An application should instead attempt the action
itself and handle the
.BR [EACCES] 
error that occurs if the file is not accessible (with a change of
effective user and group IDs beforehand, and perhaps a change back
afterwards, in the case where
\fIaccess\fR()
or
\fIfaccessat\fR()
without AT_EACCES would have been used.)
.P
Historically, one of the uses of
\fIaccess\fR()
was in set-user-ID root programs to check whether the user running
the program had access to a file. This relied on ``super-user''
privileges which were granted based on the effective user ID being
zero, so that when
\fIaccess\fR()
used the real user ID to check accessibility those privileges
were not taken into account. On newer systems where privileges
can be assigned which have no association with user or group IDs,
if a program with such privileges calls
\fIaccess\fR(),
the change of IDs has no effect on the privileges and therefore
they are taken into account in the accessibility checks. Thus,
\fIaccess\fR()
(and
\fIfaccessat\fR()
with flag zero) cannot be used for this historical purpose in
such programs. Likewise, if a system provides any additional or
alternate file access control mechanisms that are not user ID-based,
they will still be taken into account.
.P
If a relative pathname is used, no account is taken of whether
the current directory (or the directory associated with the
file descriptor
.IR fd )
is accessible via any absolute pathname. Applications using
\fIaccess\fR(),
or
\fIfaccessat\fR()
without AT_EACCES, may consequently act as if the file would be
accessible to a user with the real user ID and group ID of the
process when such a user would not in practice be able to access the file
because access would be denied at some point above the current directory
(or the directory associated with the file descriptor
.IR fd )
in the file hierarchy.
.P
If
\fIaccess\fR()
or
\fIfaccessat\fR()
is used with W_OK to check for write access to a directory which has
the S_ISVTX bit set, a return value indicating the directory is
writable can be misleading since some operations on files in the
directory would not be permitted based on the ownership of those files
(see the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 4.3" ", " "Directory Protection").
.P
Additional values of
.IR amode
other than the set defined in the description may be valid; for
example, if a system has extended access controls.
.P
The use of the AT_EACCESS value for
.IR flag
enables functionality not available in
\fIaccess\fR().
.SH RATIONALE
In early proposals, some inadequacies in the
\fIaccess\fR()
function led to the creation of an
\fIeaccess\fR()
function because:
.IP " 1." 4
Historical implementations of
\fIaccess\fR()
do not test file access correctly when the process'
real user ID is
superuser. In particular, they always return zero when testing execute
permissions without regard to whether the file is executable.
.IP " 2." 4
The superuser has complete access to all files on a system. As a
consequence, programs started by the superuser and switched to the
effective user ID
with lesser privileges cannot use
\fIaccess\fR()
to test their file access permissions.
.P
However, the historical model of
\fIeaccess\fR()
does not resolve problem (1), so this volume of POSIX.1\(hy2017 now allows
\fIaccess\fR()
to behave in the desired way because several implementations have
corrected the problem. It was also argued that problem (2) is more
easily solved by using
\fIopen\fR(),
\fIchdir\fR(),
or one of the
.IR exec
functions as appropriate and responding to the error, rather than
creating a new function that would not be as reliable. Therefore,
\fIeaccess\fR()
is not included in this volume of POSIX.1\(hy2017.
.P
The sentence concerning appropriate privileges and execute permission
bits
reflects the two possibilities implemented by historical
implementations when checking superuser access for X_OK.
.P
New implementations are discouraged from returning X_OK unless at least
one execution permission bit is set.
.P
The purpose of the
\fIfaccessat\fR()
function is to enable the checking of the accessibility of files in
directories other than the current working directory without exposure
to race conditions. Any part of the path of a file could be changed in
parallel to a call to
\fIaccess\fR(),
resulting in unspecified behavior. By opening a file descriptor for
the target directory and using the
\fIfaccessat\fR()
function it can be guaranteed that the file tested for accessibility is
located relative to the desired directory.
.SH "FUTURE DIRECTIONS"
These functions may be formally deprecated (for example, by shading
them OB) in a future version of this standard.
.SH "SEE ALSO"
.IR "\fIchmod\fR\^(\|)",
.IR "\fIfstatat\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 4.5" ", " "File Access Permissions",
.IR "\fB<fcntl.h>\fP",
.IR "\fB<unistd.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .