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ACCESS(3P) POSIX Programmer's Manual ACCESS(3P)
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.
access, faccessat — determine accessibility of a file relative to
directory file descriptor
#include <unistd.h>
int access(const char *path, int amode);
int faccessat(int fd, const char *path, int amode, int flag);
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
individually, as described in the Base Definitions volume of
POSIX.1‐2008, Section 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 permission 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 relative to the directory associated with the file descriptor
fd instead of the current working directory. If the file descriptor
was opened without 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
parameter, 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
effective user and group IDs instead of the real user and
group ID as required in a call to access().
Upon successful completion, these functions shall return 0.
Otherwise, these functions shall return −1 and set errno to indicate
the error.
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
resolution 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.
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);
Additional values of amode other than the set defined in the
description may be valid; for example, if a system has extended
access controls.
The use of the AT_EACCESS value for flag enables functionality not
available in access().
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
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.
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
implementations 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
working 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.
None.
chmod(3p), fstatat(3p)
The Base Definitions volume of POSIX.1‐2008, Section 4.4, File Access
Permissions, fcntl.h(0p), unistd.h(0p)
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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 ACCESS(3P)
Pages that refer to this page: unistd.h(0p), ex(1p), chmod(3p), faccessat(3p), fstatat(3p)
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