ACCESS(3P) POSIX Programmer's Manual ACCESS(3P)
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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.
NAME
access, faccessat -- determine accessibility of a file relative to
directory file descriptor
SYNOPSIS
#include <unistd.h>
int access(const char *path, int amode);
int faccessat(int fd, const char *path, int amode, int flag);
DESCRIPTION
The access() function shall check the file named by the pathname
pointed to by the path argument for accessibility according to the bit
pattern contained in amode, using the real user ID in place of the
effective user ID and the real group ID in place of the effective group
ID.
The value of 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).
If any access permissions are checked, each shall be checked individu-
ally, as described in the Base Definitions volume of POSIX.1-2008, Sec-
tion 4.4, 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 permis-
sion is not granted to any user.
The faccessat() function shall be equivalent to the access() function,
except in the case where path specifies a relative path. In this case
the file whose accessibility is to be determined shall be located rela-
tive to the directory associated with the file descriptor fd instead of
the current working directory. If the file descriptor was opened with-
out O_SEARCH, the function shall check whether directory searches are
permitted using the current permissions of the directory underlying the
file descriptor. If the file descriptor was opened with O_SEARCH, the
function shall not perform the check.
If faccessat() is passed the special value AT_FDCWD in the fd parame-
ter, the current working directory shall be used and the behavior shall
be identical to a call to access().
Values for flag are constructed by a bitwise-inclusive OR of flags from
the following list, defined in <fcntl.h>:
AT_EACCESS The checks for accessibility are performed using the effec-
tive user and group IDs instead of the real user and group
ID as required in a call to access().
RETURN VALUE
Upon successful completion, these functions shall return 0. Otherwise,
these functions shall return -1 and set errno to indicate the error.
ERRORS
These functions shall fail if:
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.
ELOOP A loop exists in symbolic links encountered during resolution of
the path argument.
ENAMETOOLONG
The length of a component of a pathname is longer than
{NAME_MAX}.
ENOENT A component of path does not name an existing file or path is an
empty string.
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
path argument contains at least one non-<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.
EROFS Write access is requested for a file on a read-only file system.
The faccessat() function shall fail if:
EACCES fd was not opened with O_SEARCH and the permissions of the
directory underlying fd do not permit directory searches.
EBADF The path argument does not specify an absolute path and the fd
argument is neither AT_FDCWD nor a valid file descriptor open
for reading or searching.
ENOTDIR
The path argument is not an absolute path and fd is a file
descriptor associated with a non-directory file.
These functions may fail if:
EINVAL The value of the amode argument is invalid.
ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
resolution of the path argument.
ENAMETOOLONG
The length of a pathname exceeds {PATH_MAX}, or pathname resolu-
tion of a symbolic link produced an intermediate result with a
length that exceeds {PATH_MAX}.
ETXTBSY
Write access is requested for a pure procedure (shared text)
file that is being executed.
The faccessat() function may fail if:
EINVAL The value of the flag argument is not valid.
The following sections are informative.
EXAMPLES
Testing for the Existence of a File
The following example tests whether a file named myfile exists in the
/tmp directory.
#include <unistd.h>
...
int result;
const char *pathname = "/tmp/myfile";
result = access (pathname, F_OK);
APPLICATION USAGE
Additional values of amode other than the set defined in the descrip-
tion may be valid; for example, if a system has extended access con-
trols.
The use of the AT_EACCESS value for flag enables functionality not
available in access().
RATIONALE
In early proposals, some inadequacies in the access() function led to
the creation of an eaccess() function because:
1. Historical implementations of access() do not test file access cor-
rectly 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.
2. 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 access() to
test their file access permissions.
However, the historical model of eaccess() does not resolve problem
(1), so this volume of POSIX.1-2008 now allows access() 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 open(), chdir(), or one of the exec functions as appropriate and
responding to the error, rather than creating a new function that would
not be as reliable. Therefore, eaccess() is not included in this volume
of POSIX.1-2008.
The sentence concerning appropriate privileges and execute permission
bits reflects the two possibilities implemented by historical implemen-
tations when checking superuser access for X_OK.
New implementations are discouraged from returning X_OK unless at least
one execution permission bit is set.
The purpose of the faccessat() function is to enable the checking of
the accessibility of files in directories other than the current work-
ing directory without exposure to race conditions. Any part of the path
of a file could be changed in parallel to a call to access(), resulting
in unspecified behavior. By opening a file descriptor for the target
directory and using the faccessat() function it can be guaranteed that
the file tested for accessibility is located relative to the desired
directory.
FUTURE DIRECTIONS
None.
SEE ALSO
chmod(), fstatat()
The Base Definitions volume of POSIX.1-2008, Section 4.4, File Access
Permissions, <fcntl.h>, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
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.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 ACCESS(3P)
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