The functions described so far for handling the files in a directory
have allowed you to either retrieve the information bit by bit, or to
process all the files as a group (see scandir). Sometimes it is
useful to process whole hierarchies of directories and their contained
files. The X/Open specification defines two functions to do this. The
simpler form is derived from an early definition in System V systems
and therefore this function is available on SVID-derived systems. The
prototypes and required definitions can be found in the ftw.h
header.
There are four functions in this family: ftw, nftw and
their 64-bit counterparts ftw64 and nftw64. These
functions take as one of their arguments a pointer to a callback
function of the appropriate type.
— Data Type: __ftw_func_t
int (*) (const char *, const struct stat *, int)
The type of callback functions given to the ftw function. The
first parameter points to the file name, the second parameter to an
object of type struct stat which is filled in for the file named
in the first parameter.
The last parameter is a flag giving more information about the current
file. It can have the following values:
FTW_F
The item is either a normal file or a file which does not fit into one
of the following categories. This could be special files, sockets etc.
FTW_D
The item is a directory.
FTW_NS
The stat call failed and so the information pointed to by the
second paramater is invalid.
FTW_DNR
The item is a directory which cannot be read.
FTW_SL
The item is a symbolic link. Since symbolic links are normally followed
seeing this value in a ftw callback function means the referenced
file does not exist. The situation for nftw is different.
This value is only available if the program is compiled with
_BSD_SOURCE or _XOPEN_EXTENDED defined before including
the first header. The original SVID systems do not have symbolic links.
If the sources are compiled with _FILE_OFFSET_BITS == 64 this
type is in fact __ftw64_func_t since this mode changes
struct stat to be struct stat64.
For the LFS interface and for use in the function ftw64, the
header ftw.h defines another function type.
— Data Type: __ftw64_func_t
int (*) (const char *, const struct stat64 *, int)
This type is used just like __ftw_func_t for the callback
function, but this time is called from ftw64. The second
parameter to the function is a pointer to a variable of type
struct stat64 which is able to represent the larger values.
— Data Type: __nftw_func_t
int (*) (const char *, const struct stat *, int, struct FTW *)
The first three arguments are the same as for the __ftw_func_t
type. However for the third argument some additional values are defined
to allow finer differentiation:
FTW_DP
The current item is a directory and all subdirectories have already been
visited and reported. This flag is returned instead of FTW_D if
the FTW_DEPTH flag is passed to nftw (see below).
FTW_SLN
The current item is a stale symbolic link. The file it points to does
not exist.
The last parameter of the callback function is a pointer to a structure
with some extra information as described below.
If the sources are compiled with _FILE_OFFSET_BITS == 64 this
type is in fact __nftw64_func_t since this mode changes
struct stat to be struct stat64.
For the LFS interface there is also a variant of this data type
available which has to be used with the nftw64 function.
This type is used just like __nftw_func_t for the callback
function, but this time is called from nftw64. The second
parameter to the function is this time a pointer to a variable of type
struct stat64 which is able to represent the larger values.
— Data Type: struct FTW
The information contained in this structure helps in interpreting the
name parameter and gives some information about the current state of the
traversal of the directory hierarchy.
int base
The value is the offset into the string passed in the first parameter to
the callback function of the beginning of the file name. The rest of
the string is the path of the file. This information is especially
important if the FTW_CHDIR flag was set in calling nftw
since then the current directory is the one the current item is found
in.
int level
Whilst processing, the code tracks how many directories down it has gone
to find the current file. This nesting level starts at 0 for
files in the initial directory (or is zero for the initial file if a
file was passed).
— Function: int ftw (const char *filename, __ftw_func_t func, int descriptors)
The ftw function calls the callback function given in the
parameter func for every item which is found in the directory
specified by filename and all directories below. The function
follows symbolic links if necessary but does not process an item twice.
If filename is not a directory then it itself is the only object
returned to the callback function.
The file name passed to the callback function is constructed by taking
the 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. ftw also calls
stat for the file and passes that information on to the callback
function. If this stat call was not successful the failure is
indicated by setting the third argument of the callback function to
FTW_NS. Otherwise it is set according to the description given
in the account of __ftw_func_t above.
The callback function is expected to return 0 to indicate that no
error occurred and that processing should continue. If an error
occurred in the callback function or it wants ftw to return
immediately, the callback function can return a value other than
0. This is the only correct way to stop the function. The
program must not use setjmp or similar techniques to continue
from another place. This would leave resources allocated by the
ftw function unfreed.
The descriptors parameter to ftw specifies how many file
descriptors it is allowed to consume. The function runs faster the more
descriptors it can use. For each level in the directory hierarchy at
most one descriptor is used, but for very deep ones any limit on open
file descriptors for the process or the system may be exceeded.
Moreover, file descriptor limits in a multi-threaded program apply to
all the threads as a group, and therefore it is a good idea to supply a
reasonable limit to the number of open descriptors.
The return value of the ftw function is 0 if all callback
function calls returned 0 and all actions performed by the
ftw succeeded. If a function call failed (other than calling
stat on an item) the function returns -1. If a callback
function returns a value other than 0 this value is returned as
the return value of ftw.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32-bit system this function is in fact ftw64, i.e. the LFS
interface transparently replaces the old interface.
— Function: int ftw64 (const char *filename, __ftw64_func_t func, int descriptors)
This function is similar to ftw but it can work on filesystems
with large files. File information is reported using a variable of type
struct stat64 which is passed by reference to the callback
function.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32-bit system this function is available under the name ftw and
transparently replaces the old implementation.
— Function: int nftw (const char *filename, __nftw_func_t func, int descriptors, int flag)
The nftw function works like the ftw functions. They call
the callback function func for all items found in the directory
filename and below. At most descriptors file descriptors
are consumed during the nftw call.
One difference is that the callback function is of a different type. It
is of type struct FTW * and provides the callback function
with the extra information described above.
A second difference is that nftw takes a fourth argument, which
is 0 or a bitwise-OR combination of any of the following values.
FTW_PHYS
While traversing the directory symbolic links are not followed. Instead
symbolic links are reported using the FTW_SL value for the type
parameter to the callback function. If the file referenced by a
symbolic link does not exist FTW_SLN is returned instead.
FTW_MOUNT
The callback function is only called for items which are on the same
mounted filesystem as the directory given by the filename
parameter to nftw.
FTW_CHDIR
If this flag is given the current working directory is changed to the
directory of the reported object before the callback function is called.
When ntfw finally returns the current directory is restored to
its original value.
FTW_DEPTH
If this option is specified then all subdirectories and files within
them are processed before processing the top directory itself
(depth-first processing). This also means the type flag given to the
callback function is FTW_DP and not FTW_D.
The return value is computed in the same way as for ftw.
nftw returns 0 if no failures occurred and all callback
functions returned 0. In case of internal errors, such as memory
problems, the return value is -1 and errno is set
accordingly. If the return value of a callback invocation was non-zero
then that value is returned.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32-bit system this function is in fact nftw64, i.e. the LFS
interface transparently replaces the old interface.
— Function: int nftw64 (const char *filename, __nftw64_func_t func, int descriptors, int flag)
This function is similar to nftw but it can work on filesystems
with large files. File information is reported using a variable of type
struct stat64 which is passed by reference to the callback
function.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a
32-bit system this function is available under the name nftw and
transparently replaces the old implementation.
Published under the terms of the GNU General Public License
