[PATCH v4] manual: Add Descriptor-Relative Access section
Florian Weimer
fweimer@redhat.com
Fri Dec 4 13:25:54 GMT 2020
From: Florian Weimer <fw@deneb.enyo.de>
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.)
---
v4: Use “file name” instead of “pathname”. Some small fixes.
manual/filesys.texi | 154 ++++++++++++++++++++++++++++++++++++++++++++++++++--
manual/llio.texi | 28 ++++++++++
manual/startup.texi | 7 ++-
3 files changed, 182 insertions(+), 7 deletions(-)
diff --git a/manual/filesys.texi b/manual/filesys.texi
index 73e630842e..9ee0e17abd 100644
--- a/manual/filesys.texi
+++ b/manual/filesys.texi
@@ -15,6 +15,7 @@ access permissions and modification times.
@menu
* Working Directory:: This is used to resolve relative
file names.
+* Descriptor-Relative Access:: Ways to control file name lookup.
* Accessing Directories:: Finding out what files a directory
contains.
* Working with Directory Trees:: Apply actions to all files or a selectable
@@ -206,6 +207,118 @@ An I/O error occurred.
@end table
@end deftypefun
+@node Descriptor-Relative Access
+@section Descriptor-Relative Access
+@cindex file name resolution based on descriptors
+@cindex descriptor-based file name resolution
+@cindex @code{@dots{}at} functions
+
+Many functions that accept file names have @code{@dots{}at} variants
+which accept a file descriptor and file name argument instead of just a
+file name argument. For example, @code{fstatat} is the descriptor-based
+variant of the @code{fstat} function. Most of such functions also
+accept an additional flags argument which changes the behavior of the
+file name lookup based on the passed @code{AT_@dots{}} flags.
+
+The file descriptor used by these @code{@dots{}at} functions has the
+following uses:
+
+@itemize @bullet
+@item
+It can be a file descriptor referring to a directory. Such a descriptor
+can be created explicitly using the @code{open} function and the
+@code{O_RDONLY} file access mode, with or without the @code{O_DIRECTORY}
+flag. @xref{Opening and Closing Files}. Or it can be created implicity
+by @code{opendir} and retrieved using the @code{dirfd} function.
+@xref{Opening a Directory}.
+
+If a directory descriptor is used with one of the @code{@dots{}at}
+functions, a relative file name argument is resolved relatively to that
+directory, just as if the directory were the current working directory.
+Absolute file name arguments (starting with @samp{/}) are resolved
+against the file system root, and the descriptor argument is effectively
+ignored.
+
+This means that file name lookup is not constrained to the directory of
+the descriptor. For example, it is possible to access a file
+@file{example} in the descriptor's parent directory using a file name
+argument @code{"../example"}, or in the root directory using
+@code{"/example"}.
+
+If the file descriptor refers to a directory, the empty string @code{""}
+is not a valid file name argument.
+
+@item
+@vindex @code{AT_FDCWD}
+The special value @code{AT_FDCWD}. This means that the current working
+directory is used for the lookup if the file name is a relative. For
+@code{@dots{}at} functions with an @code{AT_@dots{}} flags argument,
+this provides a shortcut to use those flags with regular (not
+descriptor-based) file name lookups.
+
+If @code{AT_FDCWD} is used, the empty string @code{""} is not a valid
+file name argument.
+
+@item
+An arbitrary file descriptor, along with an empty string @code{""} as
+the file name argument, and the @code{AT_EMPTY_PATH} flag. In this
+case, the operation uses the file descriptor directly, without further
+file name resolution. On Linux, this allows operations on descriptors
+opened with the @code{O_PATH} flag. For regular descriptors (opened
+without @code{O_PATH}), the same functionality is also available through
+the plain descriptor-based functions (for example, @code{fstat} instead
+of @code{fstatat}).
+
+This is a GNU extension.
+@end itemize
+
+@cindex file name resolution flags
+@cindex @code{AT_*} file name resolution flags
+The flags argument in @code{@dots{}at} functions can be a combination of
+the following flags, defined in @file{fcntl.h}. Not all such functions
+support all flags, and some (such as @code{openat}) do not accept a
+flags argument at all.
+
+In the flag descriptions below, the @dfn{effective final path component}
+refers to the final component (basename) of the full path constructed
+from the descriptor and file name arguments, using file name lookup, as
+described above.
+
+@vtable @code
+@item AT_EMPTY_PATH
+This flag is used with an empty file name @code{""} and a descriptor
+which does not necessarily refer to a directory. It is most useful with
+@code{O_PATH} descriptors, as described above. This flag is a GNU
+extension.
+
+@item AT_NO_AUTOMOUNT
+If the effective final path component refers to a potential file system
+mount point controlled by an auto-mounting service, the operation does
+not trigger auto-mounting and refers to the unmounted mount point
+instead. @xref{Mount-Unmount-Remount}. If a file system has already
+been mounted at the effective final path component, the operation
+applies to the file or directory in the mounted file system, not the
+underlying file system that was mounted over. This flag is a GNU
+extension.
+
+@item AT_SYMLINK_FOLLOW
+If the effective final path component is a symbolic link, the
+operation follows the symbolic link and operates on its target. (For
+most functions, this is the default behavior.)
+
+@item AT_SYMLINK_NOFOLLOW
+If the effective final path component is a symbolic link, the
+operation operates on the symbolic link, without following it. The
+difference in behavior enabled by this flag is similar to the difference
+between the @code{lstat} and @code{stat} functions, or the behavior
+activated by the @code{O_NOFOLLOW} argument to the @code{open} function.
+Even with the @code{AT_SYMLINK_NOFOLLOW} flag present, symbolic links in
+a non-final position of the path are still followed.
+@end vtable
+
+There is no relationship between these flags and the type argument to
+the @code{getauxval} function (with @code{AT_@dots{}} constants defined
+in @file{elf.h}).
@node Accessing Directories
@section Accessing Directories
@@ -1250,10 +1363,11 @@ A hardware error occurred while trying to read or write the to filesystem.
The @code{linkat} function is analogous to the @code{link} function,
except that it identifies its source and target using a combination of a
-file descriptor (referring to a directory) and a pathname. If a
-pathnames is not absolute, it is resolved relative to the corresponding
-file descriptor. The special file descriptor @code{AT_FDCWD} denotes
-the current directory.
+file descriptor (referring to a directory) and a file name.
+@xref{Descriptor-Relative Access}. For @code{linkat}, if a file name is
+not absolute, it is resolved relative to the corresponding file
+descriptor. As usual, the special file descriptor @code{AT_FDCWD}
+denotes the current directory.
The @var{flags} argument is a combination of the following flags:
@@ -2095,6 +2209,38 @@ replaces the interface for small files on 32-bit machines.
@c available.
@c @safety{@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
+
+@deftypefun int fstatat (int @var{filedes}, const char *@var{filename}, struct stat *@var{buf}, int @var{flags})
+@standards{POSIX.1, sys/stat.h}
+@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
+This function is a descriptor-relative version of the @code{fstat}
+function above. @xref{Descriptor-Relative Access}. The @var{flags}
+argument can contain a combination of the flags @code{AT_EMPTY_PATH},
+@code{AT_NO_AUTOMOUNT}, @code{AT_SYMLINK_NOFOLLOW}.
+
+Compared to @code{fstat}, the following additional error conditions can
+occur:
+
+@table @code
+@item EBADF
+The @var{filedes} argument is not a valid file descriptor.
+
+@item EINVAL
+The @var{flags} argument is not valid for this function.
+
+@item ENOTDIR
+The descriptor @var{filedes} is not associated with a directory, and
+@var{filename} is a relative file name.
+@end table
+@end deftypefun
+
+@deftypefun int fstatat64 (int @var{filedes}, const char *@var{filename}, struct stat64 *@var{buf}, int @var{flags})
+@standards{GNU, sys/stat.h}
+@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
+This function is the large-file variant of @code{fstatat}, similar to
+how @code{fstat64} is the variant of @code{fstat}.
+@end deftypefun
+
@deftypefun int lstat (const char *@var{filename}, struct stat *@var{buf})
@standards{BSD, sys/stat.h}
@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
diff --git a/manual/llio.texi b/manual/llio.texi
index c0a53e1a6e..aae0340755 100644
--- a/manual/llio.texi
+++ b/manual/llio.texi
@@ -180,6 +180,34 @@ new, extended API using 64 bit file sizes and offsets transparently
replaces the old API.
@end deftypefun
+@deftypefun int openat (int @var{filedes}, const char *@var{filename}, int @var{flags}[, mode_t @var{mode}])
+@standards{POSIX.1, fcntl.h}
+@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{@acsfd{}}}
+This function is the descriptor-relative variant of the @code{open}
+function. @xref{Descriptor-Relative Access}.
+
+Note that the @var{flags} argument of @code{openat} does not accept
+@code{AT_@dots{}} flags, only the flags described for the @code{open}
+function above.
+
+The @code{openat} function can fail for additional reasons:
+
+@table @code
+@item EBADF
+The @var{filedes} argument is not a valid file descriptor.
+
+@item ENOTDIR
+The descriptor @var{filedes} is not associated with a directory, and
+@var{filename} is a relative file name.
+@end table
+@end deftypefun
+
+@deftypefun int openat64 (int @var{filedes}, const char *@var{filename}, int @var{flags}[, mode_t @var{mode}])
+@standards{GNU, fcntl.h}
+The large-file variant of the @code{openat}, similar to how
+@code{open64} is the large-file variant of @code{open}.
+@end deftypefun
+
@deftypefn {Obsolete function} int creat (const char *@var{filename}, mode_t @var{mode})
@standards{POSIX.1, fcntl.h}
@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{@acsfd{}}}
diff --git a/manual/startup.texi b/manual/startup.texi
index 9bf24123f5..daedd4ff79 100644
--- a/manual/startup.texi
+++ b/manual/startup.texi
@@ -664,9 +664,10 @@ basis there may be information that is not available any other way.
@c Reads from hwcap or iterates over constant auxv.
This function is used to inquire about the entries in the auxiliary
vector. The @var{type} argument should be one of the @samp{AT_} symbols
-defined in @file{elf.h}. If a matching entry is found, the value is
-returned; if the entry is not found, zero is returned and @code{errno} is
-set to @code{ENOENT}.
+defined in @file{elf.h}. (There is no relationship between these types
+and the file name lookup flags in @file{fcntl.h}.) If a matching entry
+is found, the value is returned; if the entry is not found, zero is
+returned and @code{errno} is set to @code{ENOENT}.
@end deftypefun
For some platforms, the key @code{AT_HWCAP} is the easiest way to inquire
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