определить доступность файлового дескриптора (determine accessibility of a file descriptor)
Пролог (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.
Имя (Name)
access, faccessat — determine accessibility of a file descriptor
Синопсис (Synopsis)
#include <unistd.h>
int access(const char *path, int amode);
#include <fcntl.h>
int faccessat(int fd, const char *path, int amode, int flag);
Описание (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. 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.
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.1‐2017, 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.
The faccessat() function, when called with a flag value of zero,
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 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.
If faccessat() is passed the special value AT_FDCWD in the fd
parameter, the current working directory shall be used and, if
flag is zero, 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 (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 access().
Возвращаемое значение (Return value)
Upon successful completion, these functions shall return 0.
Otherwise, these functions shall return -1 and set errno to
indicate the error.
Ошибки (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 The access mode of the open file description associated
with fd is not 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.
Примеры (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);
Использование в приложениях (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 [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 access() or faccessat() without
AT_EACCES would have been used.)
Historically, one of the uses of access() 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
access() 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 access(), the
change of IDs has no effect on the privileges and therefore they
are taken into account in the accessibility checks. Thus,
access() (and faccessat() 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.
If a relative pathname is used, no account is taken of whether
the current directory (or the directory associated with the file
descriptor fd) is accessible via any absolute pathname.
Applications using access(), or faccessat() 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 fd) in the
file hierarchy.
If access() or faccessat() 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.1‐2017, Section 4.3, Directory
Protection).
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().
Обоснование (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.1‐2017 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.1‐2017.
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.
Будущие направления (Future directions)
These functions may be formally deprecated (for example, by
shading them OB) in a future version of this standard.
Смотри также (See also)
chmod(3p), fstatat(3p)
The Base Definitions volume of POSIX.1‐2017, Section 4.5, File
Access Permissions, fcntl.h(0p), unistd.h(0p)
