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        access - check user’s permissions for a file


        #include <unistd.h>
        int access(const char *pathname, int mode);


        access()  checks whether the process would be allowed to read, write or
        test for existence of the file (or other file system object) whose name
        is  pathname.   If  pathname is a symbolic link permissions of the file
        referred to by this symbolic link are tested.
        mode is a mask consisting of one or more of R_OK, W_OK, X_OK and  F_OK.
        R_OK,  W_OK  and  X_OK request checking whether the file exists and has
        read, write and execute permissions, respectively.  F_OK just  requests
        checking for the existence of the file.
        The tests depend on the permissions of the directories occurring in the
        path to the file, as given in  pathname,  and  on  the  permissions  of
        directories  and files referred to by symbolic links encountered on the
        The check is done with the process’s real UID and GID, rather than with
        the  effective  IDs  as  is done when actually attempting an operation.
        This is to allow set-user-ID programs to easily determine the  invoking
        user’s authority.
        Only  access  bits  are checked, not the file type or contents.  There‐
        fore, if a directory is found to be "writable," it probably means  that
        files  can  be created in the directory, and not that the directory can
        be written as a file.  Similarly, a DOS file may be found to  be  "exe‐
execve(2) call will still fail.
        If  the process has appropriate privileges, an implementation may indi‐
        cate success for X_OK even if none of the execute file permission  bits
        are set.
        On  success  (all requested permissions granted), zero is returned.  On
        error (at least one bit in mode asked for a permission that is  denied,
        or  some other error occurred), -1 is returned, and errno is set appro‐


        access() shall fail if:
        EACCES The requested access would be denied to the file or search  per‐
               mission  is denied for one of the directories in the path prefix
        ELOOP  Too many symbolic links were encountered in resolving  pathname.
               pathname is too long.
        ENOENT A directory component in pathname would have been accessible but
               does not exist or was a dangling symbolic link.
               A component used as a directory in pathname is not, in  fact,  a
        EROFS  Write  permission  was  requested  for  a  file  on  a read-only
        access() may fail if:
        EFAULT pathname points outside your accessible address space.
        EINVAL mode was incorrectly specified.
        EIO    An I/O error occurred.
        ENOMEM Insufficient kernel memory was available.
               Write access was requested to an executable which is being  exe‐


        access()  returns  an error if any of the access types in the requested
        call fails, even if other types might be successful.
        access() may not work correctly on NFS file systems  with  UID  mapping
        enabled,  because UID mapping is done on the server and hidden from the
        client, which checks permissions.
        Using access() to check if a user is authorized to  e.g.  open  a  file
open(2) creates a security hole, because
        the user might exploit the short time  interval  between  checking  and
        opening the file to manipulate it.
        SVr4, POSIX.1-2001, 4.3BSD
path_resolution(2), set     


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