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ACCESS(2)                  Linux Programmer's Manual                 ACCESS(2)

NAME
       access - check real user's permissions for a file

SYNOPSIS
       #include <unistd.h>

       int access(const char *pathname, int mode);

DESCRIPTION
       access()  checks  whether the calling process can access the file path-
       name.  If pathname is a symbolic link, it is dereferenced.

       The mode specifies the accessibility check(s) to be performed,  and  is
       either the value F_OK, or a mask consisting of the bitwise OR of one or
       more of R_OK, W_OK, and X_OK.  F_OK tests  for  the  existence  of  the
       file.   R_OK,  W_OK,  and  X_OK test whether the file exists and grants
       read, write, and execute permissions, respectively.

       The check is done using the calling process's real UID and GID,  rather
       than the effective IDs as is done when actually attempting an operation
       (e.g., open(2)) on the file.  This allows set-user-ID programs to  eas-
       ily determine the invoking user's authority.

       If the calling process is privileged (i.e., its real UID is zero), then
       an X_OK check is successful for a regular file if execute permission is
       enabled for any of the file owner, group, or other.

RETURN VALUE
       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-
       priately.

ERRORS
       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
              of pathname.  (See also path_resolution(7).)

       ELOOP  Too many symbolic links were encountered in resolving  pathname.

       ENAMETOOLONG
              pathname is too long.

       ENOENT A component of pathname does not exist or is a dangling symbolic
              link.

       ENOTDIR
              A component used as a directory in pathname is not, in  fact,  a
              directory.

       EROFS  Write  permission  was  requested for a file on a read-only file
              system.

       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.

       ETXTBSY
              Write access was requested to an executable which is being  exe-
              cuted.

CONFORMING TO
       SVr4, 4.3BSD, POSIX.1-2001.

NOTES
       Warning:  Using access() to check if a user is authorized to, for exam-
       ple, open a file before actually doing so using open(2) creates a secu-
       rity  hole,  because  the  user  might  exploit the short time interval
       between checking and opening the file to manipulate it.  For this  rea-
       son, the use of this system call should be avoided.

       access() returns an error if any of the access types in mode is denied,
       even if some of the other access types in mode are permitted.

       If the calling process has appropriate privileges (i.e., is superuser),
       POSIX.1-2001  permits  implementation  to  indicate success for an X_OK
       check even if none of the execute file permission bits are set.   Linux
       does not do this.

       A file is only accessible if the permissions on each of the directories
       in the path prefix of pathname grant search (i.e., execute) access.  If
       any  directory  is  inaccessible,  then  the  access()  call will fail,
       regardless of the permissions on the file itself.

       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-
       cutable," but the execve(2) call will still fail.

       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.

BUGS
       In kernel 2.4 (and earlier) there is some strangeness in  the  handling
       of  X_OK  tests for superuser.  If all categories of execute permission
       are disabled for a nondirectory file, then the only access() test  that
       returns  -1  is when mode is specified as just X_OK; if R_OK or W_OK is
       also specified in mode, then access() returns 0 for such files.   Early
       2.6 kernels (up to and including 2.6.3) also behaved in the same way as
       kernel 2.4.

       In kernels before 2.6.20, access() ignored the effect of the  MS_NOEXEC
       flag if it was used to mount(2) the underlying file system.  Since ker-
       nel 2.6.20, access() honors this flag.

SEE ALSO
       chmod(2),  chown(2),  faccessat(2),  open(2),   setgid(2),   setuid(2),
       stat(2), euidaccess(3), credentials(7), path_resolution(7)

COLOPHON
       This  page  is  part of release 3.25 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                             2007-07-10                         ACCESS(2)
 

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