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diff --git a/manual/filesys.texi b/manual/filesys.texi index 5ddd8a2..7e8a1a1 100644 --- a/manual/filesys.texi +++ b/manual/filesys.texi @@ -17,6 +17,8 @@ access permissions and modification times. file names. * Accessing Directories:: Finding out what files a directory contains. +* Working on Directory Trees:: Apply actions to all files or a selectable + subset of a directory hierachy. * Hard Links:: Adding alternate names to a file. * Symbolic Links:: A file that ``points to'' a file name. * Deleting Files:: How to delete a file, and what that means. @@ -500,6 +502,210 @@ Please note the simple selector function for this example. Since we want to see all directory entries we always return @code{1}. +@node Working on Directory Trees +@section Working on Directory Trees +@cindex directory hierachy +@cindex hierachy, directory +@cindex tree, directory + +The functions to handle files in directories described so far allowed to +retrieve all the information in small pieces or process all files in a +directory (see @code{scandir}). Sometimes it is useful to process whole +hierachies of directories and the contained files. The X/Open +specification define two functions to do this. The simpler form is +derived from an early definition in @w{System V} systems and therefore +this function is available on SVID derived systems. The prototypes and +required definitions can be found in the @file{ftw.h} header. + +Both functions of this @code{ftw} family take as one of the arguments a +reference to a callback function. The functions must be of these types. + +@deftp {Data Type} __ftw_func_t + +@smallexample +int (*) (const char *, const struct stat *, int) +@end smallexample + +Type for callback functions given to the @code{ftw} function. The first +parameter will contain a pointer to the filename, the second parameter +will point to an object of type @code{struct stat} which will be filled +for the file named by the first parameter. + +@noindent +The last parameter is a flag given more information about the current +file. It can have the following values: + +@vindex FTW_F +@vindex FTW_D +@vindex FTW_NS +@vindex FTW_DNR +@vindex FTW_SL +@table @code +@item FTW_F +The current item is a normal file or files which do not fit into one of +the following categories. This means especially special files, sockets +etc. +@item FTW_D +The current item is a directory. +@item FTW_NS +The @code{stat} call to fill the object pointed to by the second +parameter failed and so the information is invalid. +@item FTW_DNR +The item is a directory which cannot be read. +@item FTW_SL +The item is a symbolic link. Since symbolic links are normally followed +seeing this value in a @code{ftw} callback function means the referenced +file does not exist. The situation for @code{nftw} is different. + +This value is only available if the program is compiled with +@code{_BSD_SOURCE} or @code{_XOPEN_EXTENDED} defined before including +the first header. The original SVID systems do not have symbolic links. +@end table +@end deftp + +@deftp {Data Type} __nftw_func_t + +@smallexample +int (*) (const char *, const struct stat *, int, struct FTW *) +@end smallexample + +@vindex FTW_DP +@vindex FTW_SLN +The first three arguments have the same as for the @code{__ftw_func_t} +type. A difference is that for the third argument some additional +values are defined to allow finer differentiation: +@table @code +@item FTW_DP +The current item is a directory and all subdirectories have already been +visited and reported. This flag is returned instead of @code{FTW_D} if +the @code{FTW_DEPTH} flag is given to @code{nftw} (see below). +@item FTW_SLN +The current item is a stale symbolic link. The file it points to does +not exist. +@end table + +The last parameter of the callback function is a pointer to a structure +with some extra information as described below. +@end deftp + +@deftp {Data Type} {struct FTW} +The contained information helps to interpret the name parameter and +gives some information about current state of the traversal of the +directory hierachy. + +@table @code +@item int base +The value specifies which part of the filename argument given in the +first parameter to the callback function is the name of the file. The +rest of the string is the path to locate the file. This information is +especially important if the @code{FTW_CHDIR} flag for @code{nftw} was +set since then the current directory is the one the current item is +found in. +@item int level +While processing the directory the functions tracks how many directories +have been examine to find the current item. This nesting level is +@math{0} for the item given starting item (file or directory) and is +incremented by one for each entered directory. +@end table +@end deftp + + +@comment ftw.h +@comment SVID +@deftypefun int ftw (const char *@var{filename}, __ftw_func_t @var{func}, int @var{descriptors}) +The @code{ftw} function calls the callback function given in the +parameter @var{func} for every item which is found in the directory +specified by @var{filename} and all directories below. The function +follows symbolic links if necessary but does not process an item twice. +If @var{filename} names no directory this item is the only object +reported by calling the callback function. + +The filename given to the callback function is constructed by taking the +@var{filename} parameter and appending the names of all passed +directories and then the local file name. So the callback function can +use this parameter to access the file. Before the callback function is +called @code{ftw} calls @code{stat} for this file and passes the +information up to the callback function. If this @code{stat} call was +not successful the failure is indicated by setting the falg argument of +the callback function to @code{FTW_NS}. Otherwise the flag is set +according to the description given in the description of +@code{__ftw_func_t} above. + +The callback function is expected to return @math{0} to indicate that no +error occurred and the processing should be continued. If an error +occurred in the callback function or the call to @code{ftw} shall return +immediately the callback function can return a value other than +@math{0}. This is the only correct way to stop the function. The +program must not use @code{setjmp} or similar techniques to continue the +program in another place. This would leave the resources allocated in +the @code{ftw} function allocated. + +The @var{descriptors} parameter to the @code{ftw} function specifies how +many file descriptors the @code{ftw} function is allowed to consume. +The more descriptors can be used the faster the function can run. For +each level of directories at most one descriptor is used so that for +very deep directory hierachies the limit on open file descriptors for +the process or the system can be exceeded. Beside this the limit on +file descriptors is counted together for all threads in a multi-threaded +program and therefore it is always good too limit the maximal number of +open descriptors to a reasonable number. + +The return value of the @code{ftw} function is @math{0} if all callback +function calls returned @math{0} and all actions performed by the +@code{ftw} succeeded. If some function call failed (other than calling +@code{stat} on an item) the function return @math{-1}. If a callback +function returns a value other than @math{0} this value is returned as +the return value of @code{ftw}. +@end deftypefun + +@comment ftw.h +@comment XPG4.2 +@deftypefun int nftw (const char *@var{filename}, __nftw_func_t @var{func}, int @var{descriptors}, int @var{flag}) +The @code{nftw} functions works like the @code{ftw} functions. It calls +the callback function @var{func} for all items it finds in the directory +@var{filename} and below. At most @var{descriptors} file descriptors +are consumed during the @code{nftw} call. + +The differences are that for one the callback function is of a different +type. It is of type @w{@code{struct FTW *}} and provides the callback +functions the information described above. + +The second difference is that @code{nftw} takes an additional fourth +argument which is @math{0} or a combination of any of the following +values, combined using bitwise OR. + +@table @code +@item FTW_PHYS +While traversing the directory symbolic links are not followed. I.e., +if this flag is given symbolic links are reported using the +@code{FTW_SL} value for the type parameter to the callback function. +Please note that if this flag is used the appearence of @code{FTW_SL} in +a callback function does not mean the referenced file does not exist. +To indicate this the extra value @code{FTW_SLN} exists. +@item FTW_MOUNT +The callback function is only called for items which are on the same +mounted filesystem as the directory given as the @var{filename} +parameter to @code{nftw}. +@item FTW_CHDIR +If this flag is given the current working directory is changed to the +directory containing the reported object before the callback function is +called. +@item FTW_DEPTH +If this option is given the function visits first all files and +subdirectories before the callback function is called for the directory +itself (depth-first processing). This also means the type flag given to +the callback function is @code{FTW_DP} and not @code{FTW_D}. +@end table + +The return value is computed in the same way as for @code{ftw}. +@code{nftw} return @math{0} if no failure occurred in @code{nftw} and +all callback function call return values are also @math{0}. For +internal errors such as memory problems @math{-1} is returned and +@var{errno} is set accordingly. If the return value of a callback +invocation is nonzero this very same value is returned. +@end deftypefun + + @node Hard Links @section Hard Links @cindex hard link |