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This manual page is part of the POSIX Programmer's Manual. The Linux implementation of
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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 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().
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 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.
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 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().
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 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.
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 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
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