ACCESS(3)                  POSIX Programmer's Manual                 ACCESS(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       access, faccessat -- determine accessibility of a file descriptor

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);

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().

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 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.

EXAMPLES
   Testing for the Existence of aa 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
       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().

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.

FUTURE DIRECTIONS
       These functions may be formally deprecated (for example, by shading
       them OB) in a future version of this standard.

SEE ALSO
       chmod(), fstatat()

       The Base Definitions volume of POSIX.1-2017, Section 4.5, File Access
       Permissions, <fcntl.h>, <unistd.h>

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 .

       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                  2017                            ACCESS(3)
