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 relative to
       directory file descriptor

SYNOPSIS         top

       #include <unistd.h>

       int access(const char *path, int amode);
       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, 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

       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         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 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.

              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);


       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.1‐2008 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.



SEE ALSO         top

       chmod(3p), fstatat(3p)

       The Base Definitions volume of POSIX.1‐2008, Section 4.4, 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, 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 .

       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                 2013                          ACCESS(3P)

Pages that refer to this page: unistd.h(0p)ex(1p)chmod(3p)faccessat(3p)fstatat(3p)