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manual: Add Descriptor-Relative Access section
Reference this new section from the O_PATH documentation. And document the functions openat, openat64, fstatat, fstatat64. (The safety assessment for fstatat was already obsolete because current glibc assumes kernel support for the underlying system call.) Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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Florian Weimer
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@ -15,6 +15,7 @@ access permissions and modification times.
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@menu
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* Working Directory:: This is used to resolve relative
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file names.
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* Descriptor-Relative Access:: Ways to control file name lookup.
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* Accessing Directories:: Finding out what files a directory
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contains.
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* Working with Directory Trees:: Apply actions to all files or a selectable
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@ -206,6 +207,151 @@ An I/O error occurred.
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@end table
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@end deftypefun
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@node Descriptor-Relative Access
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@section Descriptor-Relative Access
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@cindex file name resolution based on descriptors
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@cindex descriptor-based file name resolution
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@cindex @code{@dots{}at} functions
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Many functions that accept file names have @code{@dots{}at} variants
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which accept a file descriptor and a file name argument instead of just
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a file name argument. For example, @code{fstatat} is the
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descriptor-based variant of the @code{fstat} function. Most such
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functions also accept an additional flags argument which changes the
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behavior of the file name lookup based on the passed @code{AT_@dots{}}
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flags.
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There are several reasons to use descriptor-relative access:
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@itemize @bullet
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@item
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The working directory is a process-wide resource, so individual threads
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cannot change it without affecting other threads in the process.
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Explicitly specifying the directory against which relative paths are
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resolved can be a thread-safe alternative to changing the working
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directory.
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@item
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If a program wishes to access a directory tree which is being modified
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concurrently, perhaps even by a different user on the system, the
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program must avoid looking up file names with multiple components, in
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order to detect symbolic links, using the @code{O_NOFOLLOW} flag
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(@pxref{Open-time Flags}) or the @code{AT_SYMLINK_FOLLOW} flag
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(described below). Without directory-relative access, it is necessary
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to use the @code{fchdir} function to change the working directory
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(@pxref{Working Directory}), which is not thread-safe.
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@item
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Listing directory contents using the @code{readdir} or @code{readdir64}
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functions (@pxref{Reading/Closing Directory}) does not provide full file
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name paths. Using @code{@dots{}at} functions, it is possible to use the
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file names directly, without having to construct such full paths.
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@item
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Additional flags available with some of the @code{@dots{}at} functions
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provide access to functionality which is not available otherwise.
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@end itemize
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The file descriptor used by these @code{@dots{}at} functions has the
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following uses:
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@itemize @bullet
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@item
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It can be a file descriptor referring to a directory. Such a descriptor
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can be created explicitly using the @code{open} function and the
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@code{O_RDONLY} file access mode, with or without the @code{O_DIRECTORY}
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flag. @xref{Opening and Closing Files}. Or it can be created
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implicitly by @code{opendir} and retrieved using the @code{dirfd}
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function. @xref{Opening a Directory}.
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If a directory descriptor is used with one of the @code{@dots{}at}
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functions, a relative file name argument is resolved relative to
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directory referred to by the file descriptor, just as if that directory
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were the current working directory. Absolute file name arguments
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(starting with @samp{/}) are resolved against the file system root, and
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the descriptor argument is effectively ignored.
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This means that file name lookup is not constrained to the directory of
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the descriptor. For example, it is possible to access a file
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@file{example} in the descriptor's parent directory using a file name
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argument @code{"../example"}, or in the root directory using
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@code{"/example"}.
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If the file descriptor refers to a directory, the empty string @code{""}
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is not a valid file name argument. It is possible to use @code{"."} to
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refer to the directory itself. Also see @code{AT_EMPTY_PATH} below.
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@item
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@vindex @code{AT_FDCWD}
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The special value @code{AT_FDCWD}. This means that the current working
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directory is used for the lookup if the file name is a relative. For
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@code{@dots{}at} functions with an @code{AT_@dots{}} flags argument,
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this provides a shortcut to use those flags with regular (not
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descriptor-based) file name lookups.
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If @code{AT_FDCWD} is used, the empty string @code{""} is not a valid
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file name argument.
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@item
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An arbitrary file descriptor, along with an empty string @code{""} as
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the file name argument, and the @code{AT_EMPTY_PATH} flag. In this
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case, the operation uses the file descriptor directly, without further
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file name resolution. On Linux, this allows operations on descriptors
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opened with the @code{O_PATH} flag. For regular descriptors (opened
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without @code{O_PATH}), the same functionality is also available through
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the plain descriptor-based functions (for example, @code{fstat} instead
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of @code{fstatat}).
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This is a GNU extension.
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@end itemize
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@cindex file name resolution flags
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@cindex @code{AT_*} file name resolution flags
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The flags argument in @code{@dots{}at} functions can be a combination of
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the following flags, defined in @file{fcntl.h}. Not all such functions
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support all flags, and some (such as @code{openat}) do not accept a
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flags argument at all.
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In the flag descriptions below, the @dfn{effective final path component}
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refers to the final component (basename) of the full path constructed
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from the descriptor and file name arguments, using file name lookup, as
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described above.
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@vtable @code
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@item AT_EMPTY_PATH
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This flag is used with an empty file name @code{""} and a descriptor
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which does not necessarily refer to a directory. It is most useful with
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@code{O_PATH} descriptors, as described above. This flag is a GNU
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extension.
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@item AT_NO_AUTOMOUNT
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If the effective final path component refers to a potential file system
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mount point controlled by an auto-mounting service, the operation does
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not trigger auto-mounting and refers to the unmounted mount point
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instead. @xref{Mount-Unmount-Remount}. If a file system has already
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been mounted at the effective final path component, the operation
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applies to the file or directory in the mounted file system, not the
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underlying file system that was mounted over. This flag is a GNU
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extension.
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@item AT_SYMLINK_FOLLOW
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If the effective final path component is a symbolic link, the
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operation follows the symbolic link and operates on its target. (For
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most functions, this is the default behavior.)
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@item AT_SYMLINK_NOFOLLOW
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If the effective final path component is a symbolic link, the
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operation operates on the symbolic link, without following it. The
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difference in behavior enabled by this flag is similar to the difference
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between the @code{lstat} and @code{stat} functions, or the behavior
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activated by the @code{O_NOFOLLOW} argument to the @code{open} function.
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Even with the @code{AT_SYMLINK_NOFOLLOW} flag present, symbolic links in
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a non-final component of the file name are still followed.
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@end vtable
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@strong{Note:} There is no relationship between these flags and the type
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argument to the @code{getauxval} function (with @code{AT_@dots{}}
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constants defined in @file{elf.h}). @xref{Auxiliary Vector}.
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@node Accessing Directories
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@section Accessing Directories
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@ -1250,10 +1396,11 @@ A hardware error occurred while trying to read or write the to filesystem.
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The @code{linkat} function is analogous to the @code{link} function,
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except that it identifies its source and target using a combination of a
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file descriptor (referring to a directory) and a pathname. If a
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pathnames is not absolute, it is resolved relative to the corresponding
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file descriptor. The special file descriptor @code{AT_FDCWD} denotes
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the current directory.
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file descriptor (referring to a directory) and a file name.
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@xref{Descriptor-Relative Access}. For @code{linkat}, if a file name is
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not absolute, it is resolved relative to the corresponding file
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descriptor. As usual, the special value @code{AT_FDCWD} denotes the
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current directory.
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The @var{flags} argument is a combination of the following flags:
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@ -2091,9 +2238,44 @@ function is available under the name @code{fstat} and so transparently
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replaces the interface for small files on 32-bit machines.
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@end deftypefun
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@c fstatat will call alloca and snprintf if the syscall is not
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@c available.
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@c @safety{@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
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@deftypefun int fstatat (int @var{filedes}, const char *@var{filename}, struct stat *@var{buf}, int @var{flags})
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@standards{POSIX.1, sys/stat.h}
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
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This function is a descriptor-relative version of the @code{fstat}
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function above. @xref{Descriptor-Relative Access}. The @var{flags}
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argument can contain a combination of the flags @code{AT_EMPTY_PATH},
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@code{AT_NO_AUTOMOUNT}, @code{AT_SYMLINK_NOFOLLOW}.
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Compared to @code{fstat}, the following additional error conditions can
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occur:
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@table @code
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@item EBADF
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The @var{filedes} argument is not a valid file descriptor.
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@item EINVAL
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The @var{flags} argument is not valid for this function.
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@item ENOTDIR
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The descriptor @var{filedes} is not associated with a directory, and
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@var{filename} is a relative file name.
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@end table
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When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} this
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function is in fact @code{fstatat64} since the LFS interface transparently
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replaces the normal implementation.
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@end deftypefun
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@deftypefun int fstatat64 (int @var{filedes}, const char *@var{filename}, struct stat64 *@var{buf}, int @var{flags})
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@standards{GNU, sys/stat.h}
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
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This function is the large-file variant of @code{fstatat}, similar to
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how @code{fstat64} is the variant of @code{fstat}.
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When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} this
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function is available under the name @code{fstatat} and so transparently
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replaces the interface for small files on 32-bit machines.
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@end deftypefun
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@deftypefun int lstat (const char *@var{filename}, struct stat *@var{buf})
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@standards{BSD, sys/stat.h}
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@ -181,6 +181,43 @@ new, extended API using 64 bit file sizes and offsets transparently
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replaces the old API.
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@end deftypefun
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@deftypefun int openat (int @var{filedes}, const char *@var{filename}, int @var{flags}[, mode_t @var{mode}])
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@standards{POSIX.1, fcntl.h}
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{@acsfd{}}}
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This function is the descriptor-relative variant of the @code{open}
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function. @xref{Descriptor-Relative Access}.
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Note that the @var{flags} argument of @code{openat} does not accept
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@code{AT_@dots{}} flags, only the flags described for the @code{open}
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function above.
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The @code{openat} function can fail for additional reasons:
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@table @code
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@item EBADF
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The @var{filedes} argument is not a valid file descriptor.
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@item ENOTDIR
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The descriptor @var{filedes} is not associated with a directory, and
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@var{filename} is a relative file name.
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@end table
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When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} this
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function is in fact @code{openat64} since the LFS interface transparently
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replaces the normal implementation.
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@end deftypefun
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@deftypefun int openat64 (int @var{filedes}, const char *@var{filename}, int @var{flags}[, mode_t @var{mode}])
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@standards{GNU, fcntl.h}
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The large-file variant of the @code{openat}, similar to how
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@code{open64} is the large-file variant of @code{open}.
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When the sources are translated with @code{_FILE_OFFSET_BITS == 64} this
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function is actually available under the name @code{openat}. I.e., the
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new, extended API using 64 bit file sizes and offsets transparently
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replaces the old API.
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@end deftypefun
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@deftypefn {Obsolete function} int creat (const char *@var{filename}, mode_t @var{mode})
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@standards{POSIX.1, fcntl.h}
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{@acsfd{}}}
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@ -3807,7 +3844,9 @@ contains it is still needed), and permissions are checked when the
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descriptor is used later on.
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For example, such descriptors can be used with the @code{fexecve}
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function (@pxref{Executing a File}).
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function (@pxref{Executing a File}). Other applications involve the
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@samp{*at} function variants, along with the @code{AT_EMPTY_PATH} flag.
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@xref{Descriptor-Relative Access}.
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This access mode is specific to Linux. On @gnuhurdsystems{}, it is
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possible to use @code{O_EXEC} explicitly, or specify no access modes
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@ -664,8 +664,12 @@ basis there may be information that is not available any other way.
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This function is used to inquire about the entries in the auxiliary
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vector. The @var{type} argument should be one of the @samp{AT_} symbols
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defined in @file{elf.h}. If a matching entry is found, the value is
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returned; if the entry is not found, zero is returned and @code{errno} is
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set to @code{ENOENT}.
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returned; if the entry is not found, zero is returned and @code{errno}
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is set to @code{ENOENT}.
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@strong{Note:} There is no relationship between the @samp{AT_} contants
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defined in @file{elf.h} and the file name lookup flags in
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@file{fcntl.h}. @xref{Descriptor-Relative Access}.
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@end deftypefun
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For some platforms, the key @code{AT_HWCAP} is the easiest way to inquire
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