access(3p) — Linux manual page


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

PROLOG         top

       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         top

       access, faccessat — determine accessibility of a file descriptor

SYNOPSIS         top

       #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         top

       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

       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

       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         top

       Upon successful completion, these functions shall return 0.
       Otherwise, these functions shall return -1 and set errno to
       indicate the error.

ERRORS         top

       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.

              The length of a component of a pathname is longer than

       ENOENT A component of path does not name an existing file or path
              is an empty string.

              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

       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.

              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.

              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}.

              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         top

   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);


       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

       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         top

       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

       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.


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

SEE ALSO         top

       chmod(3p), fstatat(3p)

       The Base Definitions volume of POSIX.1‐2017, Section 4.5, File
       Access Permissions, fcntl.h(0p), unistd.h(0p)

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group               2017                        ACCESS(3P)

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