mirror of
https://sourceware.org/git/glibc.git
synced 2024-11-21 12:30:06 +00:00
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>
This commit is contained in:
Florian Weimer
parent
ca90758b2a
commit
3de73f974f
@ -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 +207,151 @@ 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 a 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 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.
|
||||
|
||||
There are several reasons to use descriptor-relative access:
|
||||
|
||||
@itemize @bullet
|
||||
@item
|
||||
The working directory is a process-wide resource, so individual threads
|
||||
cannot change it without affecting other threads in the process.
|
||||
Explicitly specifying the directory against which relative paths are
|
||||
resolved can be a thread-safe alternative to changing the working
|
||||
directory.
|
||||
|
||||
@item
|
||||
If a program wishes to access a directory tree which is being modified
|
||||
concurrently, perhaps even by a different user on the system, the
|
||||
program must avoid looking up file names with multiple components, in
|
||||
order to detect symbolic links, using the @code{O_NOFOLLOW} flag
|
||||
(@pxref{Open-time Flags}) or the @code{AT_SYMLINK_FOLLOW} flag
|
||||
(described below). Without directory-relative access, it is necessary
|
||||
to use the @code{fchdir} function to change the working directory
|
||||
(@pxref{Working Directory}), which is not thread-safe.
|
||||
|
||||
@item
|
||||
Listing directory contents using the @code{readdir} or @code{readdir64}
|
||||
functions (@pxref{Reading/Closing Directory}) does not provide full file
|
||||
name paths. Using @code{@dots{}at} functions, it is possible to use the
|
||||
file names directly, without having to construct such full paths.
|
||||
|
||||
@item
|
||||
Additional flags available with some of the @code{@dots{}at} functions
|
||||
provide access to functionality which is not available otherwise.
|
||||
@end itemize
|
||||
|
||||
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
|
||||
implicitly 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 relative to
|
||||
directory referred to by the file descriptor, just as if that 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. It is possible to use @code{"."} to
|
||||
refer to the directory itself. Also see @code{AT_EMPTY_PATH} below.
|
||||
|
||||
@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 component of the file name are still followed.
|
||||
@end vtable
|
||||
|
||||
@strong{Note:} 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}). @xref{Auxiliary Vector}.
|
||||
|
||||
@node Accessing Directories
|
||||
@section Accessing Directories
|
||||
@ -1250,10 +1396,11 @@ 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 value @code{AT_FDCWD} denotes the
|
||||
current directory.
|
||||
|
||||
The @var{flags} argument is a combination of the following flags:
|
||||
|
||||
@ -2091,9 +2238,44 @@ function is available under the name @code{fstat} and so transparently
|
||||
replaces the interface for small files on 32-bit machines.
|
||||
@end deftypefun
|
||||
|
||||
@c fstatat will call alloca and snprintf if the syscall is not
|
||||
@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
|
||||
|
||||
When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} this
|
||||
function is in fact @code{fstatat64} since the LFS interface transparently
|
||||
replaces the normal implementation.
|
||||
@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}.
|
||||
|
||||
When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} this
|
||||
function is available under the name @code{fstatat} and so transparently
|
||||
replaces the interface for small files on 32-bit machines.
|
||||
@end deftypefun
|
||||
|
||||
@deftypefun int lstat (const char *@var{filename}, struct stat *@var{buf})
|
||||
@standards{BSD, sys/stat.h}
|
||||
|
||||
@ -181,6 +181,43 @@ 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
|
||||
|
||||
When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} this
|
||||
function is in fact @code{openat64} since the LFS interface transparently
|
||||
replaces the normal implementation.
|
||||
@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}.
|
||||
|
||||
When the sources are translated with @code{_FILE_OFFSET_BITS == 64} this
|
||||
function is actually available under the name @code{openat}. I.e., the
|
||||
new, extended API using 64 bit file sizes and offsets transparently
|
||||
replaces the old API.
|
||||
@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{}}}
|
||||
@ -3807,7 +3844,9 @@ contains it is still needed), and permissions are checked when the
|
||||
descriptor is used later on.
|
||||
|
||||
For example, such descriptors can be used with the @code{fexecve}
|
||||
function (@pxref{Executing a File}).
|
||||
function (@pxref{Executing a File}). Other applications involve the
|
||||
@samp{*at} function variants, along with the @code{AT_EMPTY_PATH} flag.
|
||||
@xref{Descriptor-Relative Access}.
|
||||
|
||||
This access mode is specific to Linux. On @gnuhurdsystems{}, it is
|
||||
possible to use @code{O_EXEC} explicitly, or specify no access modes
|
||||
|
||||
@ -664,8 +664,12 @@ basis there may be information that is not available any other way.
|
||||
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}.
|
||||
returned; if the entry is not found, zero is returned and @code{errno}
|
||||
is set to @code{ENOENT}.
|
||||
|
||||
@strong{Note:} There is no relationship between the @samp{AT_} contants
|
||||
defined in @file{elf.h} and the file name lookup flags in
|
||||
@file{fcntl.h}. @xref{Descriptor-Relative Access}.
|
||||
@end deftypefun
|
||||
|
||||
For some platforms, the key @code{AT_HWCAP} is the easiest way to inquire
|
||||
|
||||
Loading…
Reference in New Issue
Block a user