The fts functions are provided for traversing file hierarchies.
A simple overview is that the fts_open() function returns a
"handle" (of type FTS *) that refers to a file hierarchy
"stream". This handle is then supplied to the other fts
functions. The function fts_read() returns a pointer to a
structure describing one of the files in the file hierarchy. The
function fts_children() returns a pointer to a linked list of
structures, each of which describes one of the files contained in
a directory in the hierarchy.
In general, directories are visited two distinguishable times; in
preorder (before any of their descendants are visited) and in
postorder (after all of their descendants have been visited).
Files are visited once. It is possible to walk the hierarchy
"logically" (visiting the files that symbolic links point to) or
physically (visiting the symbolic links themselves), order the
walk of the hierarchy or prune and/or revisit portions of the
hierarchy.
Two structures (and associated types) are defined in the include
file <fts.h>. The first type is FTS, the structure that
represents the file hierarchy itself. The second type is FTSENT,
the structure that represents a file in the file hierarchy.
Normally, an FTSENT structure is returned for every file in the
file hierarchy. In this manual page, "file" and "FTSENT
structure" are generally interchangeable.
The FTSENT structure contains fields describing a file. The
structure contains at least the following fields (there are
additional fields that should be considered private to the
implementation):
typedef struct _ftsent {
unsigned short fts_info; /* flags for FTSENT structure */
char *fts_accpath; /* access path */
char *fts_path; /* root path */
short fts_pathlen; /* strlen(fts_path) +
strlen(fts_name) */
char *fts_name; /* filename */
short fts_namelen; /* strlen(fts_name) */
short fts_level; /* depth (-1 to N) */
int fts_errno; /* file errno */
long fts_number; /* local numeric value */
void *fts_pointer; /* local address value */
struct _ftsent *fts_parent; /* parent directory */
struct _ftsent *fts_link; /* next file structure */
struct _ftsent *fts_cycle; /* cycle structure */
struct stat *fts_statp; /* stat(2) information */
} FTSENT;
These fields are defined as follows:
fts_info
One of the following values describing the returned FTSENT
structure and the file it represents. With the exception
of directories without errors (FTS_D), all of these
entries are terminal, that is, they will not be revisited,
nor will any of their descendants be visited.
FTS_D A directory being visited in preorder.
FTS_DC A directory that causes a cycle in the tree. (The
fts_cycle field of the FTSENT structure will be
filled in as well.)
FTS_DEFAULT
Any FTSENT structure that represents a file type
not explicitly described by one of the other
fts_info values.
FTS_DNR
A directory which cannot be read. This is an error
return, and the fts_errno field will be set to
indicate what caused the error.
FTS_DOT
A file named "." or ".." which was not specified
as a filename to fts_open() (see FTS_SEEDOT).
FTS_DP A directory being visited in postorder. The
contents of the FTSENT structure will be unchanged
from when it was returned in preorder, that is,
with the fts_info field set to FTS_D.
FTS_ERR
This is an error return, and the fts_errno field
will be set to indicate what caused the error.
FTS_F A regular file.
FTS_NS A file for which no stat(2) information was
available. The contents of the fts_statp field are
undefined. This is an error return, and the
fts_errno field will be set to indicate what caused
the error.
FTS_NSOK
A file for which no stat(2) information was
requested. The contents of the fts_statp field are
undefined.
FTS_SL A symbolic link.
FTS_SLNONE
A symbolic link with a nonexistent target. The
contents of the fts_statp field reference the file
characteristic information for the symbolic link
itself.
fts_accpath
A path for accessing the file from the current directory.
fts_path
The path for the file relative to the root of the
traversal. This path contains the path specified to
fts_open() as a prefix.
fts_pathlen
The sum of the lengths of the strings referenced by
fts_path and fts_name.
fts_name
The name of the file.
fts_namelen
The length of the string referenced by fts_name.
fts_level
The depth of the traversal, numbered from -1 to N, where
this file was found. The FTSENT structure representing
the parent of the starting point (or root) of the
traversal is numbered -1, and the FTSENT structure for the
root itself is numbered 0.
fts_errno
If fts_children() or fts_read() returns an FTSENT
structure whose fts_info field is set to FTS_DNR, FTS_ERR,
or FTS_NS, the fts_errno field contains the error number
(i.e., the errno value) specifying the cause of the error.
Otherwise, the contents of the fts_errno field are
undefined.
fts_number
This field is provided for the use of the application
program and is not modified by the fts functions. It is
initialized to 0.
fts_pointer
This field is provided for the use of the application
program and is not modified by the fts functions. It is
initialized to NULL.
fts_parent
A pointer to the FTSENT structure referencing the file in
the hierarchy immediately above the current file, that is,
the directory of which this file is a member. A parent
structure for the initial entry point is provided as well,
however, only the fts_level, fts_number, and fts_pointer
fields are guaranteed to be initialized.
fts_link
Upon return from the fts_children() function, the fts_link
field points to the next structure in the NULL-terminated
linked list of directory members. Otherwise, the contents
of the fts_link field are undefined.
fts_cycle
If a directory causes a cycle in the hierarchy (see
FTS_DC), either because of a hard link between two
directories, or a symbolic link pointing to a directory,
the fts_cycle field of the structure will point to the
FTSENT structure in the hierarchy that references the same
file as the current FTSENT structure. Otherwise, the
contents of the fts_cycle field are undefined.
fts_statp
A pointer to stat(2) information for the file.
A single buffer is used for all of the paths of all of the files
in the file hierarchy. Therefore, the fts_path and fts_accpath
fields are guaranteed to be null-terminated only for the file
most recently returned by fts_read(). To use these fields to
reference any files represented by other FTSENT structures will
require that the path buffer be modified using the information
contained in that FTSENT structure's fts_pathlen field. Any such
modifications should be undone before further calls to fts_read()
are attempted. The fts_name field is always null-terminated.
fts_open()
The fts_open() function takes a pointer to an array of character
pointers naming one or more paths which make up a logical file
hierarchy to be traversed. The array must be terminated by a
null pointer.
There are a number of options, at least one of which (either
FTS_LOGICAL or FTS_PHYSICAL) must be specified. The options are
selected by ORing the following values:
FTS_COMFOLLOW
This option causes any symbolic link specified as a root
path to be followed immediately whether or not FTS_LOGICAL
is also specified.
FTS_LOGICAL
This option causes the fts routines to return FTSENT
structures for the targets of symbolic links instead of
the symbolic links themselves. If this option is set, the
only symbolic links for which FTSENT structures are
returned to the application are those referencing
nonexistent files. Either FTS_LOGICAL or FTS_PHYSICAL
must be provided to the fts_open() function.
FTS_NOCHDIR
As a performance optimization, the fts functions change
directories as they walk the file hierarchy. This has the
side-effect that an application cannot rely on being in
any particular directory during the traversal. The
FTS_NOCHDIR option turns off this optimization, and the
fts functions will not change the current directory. Note
that applications should not themselves change their
current directory and try to access files unless
FTS_NOCHDIR is specified and absolute pathnames were
provided as arguments to fts_open().
FTS_NOSTAT
By default, returned FTSENT structures reference file
characteristic information (the statp field) for each file
visited. This option relaxes that requirement as a
performance optimization, allowing the fts functions to
set the fts_info field to FTS_NSOK and leave the contents
of the statp field undefined.
FTS_PHYSICAL
This option causes the fts routines to return FTSENT
structures for symbolic links themselves instead of the
target files they point to. If this option is set, FTSENT
structures for all symbolic links in the hierarchy are
returned to the application. Either FTS_LOGICAL or
FTS_PHYSICAL must be provided to the fts_open() function.
FTS_SEEDOT
By default, unless they are specified as path arguments to
fts_open(), any files named "." or ".." encountered in
the file hierarchy are ignored. This option causes the
fts routines to return FTSENT structures for them.
FTS_XDEV
This option prevents fts from descending into directories
that have a different device number than the file from
which the descent began.
The argument compar() specifies a user-defined function which may
be used to order the traversal of the hierarchy. It takes two
pointers to pointers to FTSENT structures as arguments and should
return a negative value, zero, or a positive value to indicate if
the file referenced by its first argument comes before, in any
order with respect to, or after, the file referenced by its
second argument. The fts_accpath, fts_path, and fts_pathlen
fields of the FTSENT structures may never be used in this
comparison. If the fts_info field is set to FTS_NS or FTS_NSOK,
the fts_statp field may not either. If the compar() argument is
NULL, the directory traversal order is in the order listed in
path_argv for the root paths, and in the order listed in the
directory for everything else.
fts_read()
The fts_read() function returns a pointer to an FTSENT structure
describing a file in the hierarchy. Directories (that are
readable and do not cause cycles) are visited at least twice,
once in preorder and once in postorder. All other files are
visited at least once. (Hard links between directories that do
not cause cycles or symbolic links to symbolic links may cause
files to be visited more than once, or directories more than
twice.)
If all the members of the hierarchy have been returned,
fts_read() returns NULL and sets errno to 0. If an error
unrelated to a file in the hierarchy occurs, fts_read() returns
NULL and sets errno to indicate the error. If an error related
to a returned file occurs, a pointer to an FTSENT structure is
returned, and errno may or may not have been set (see fts_info).
The FTSENT structures returned by fts_read() may be overwritten
after a call to fts_close() on the same file hierarchy stream,
or, after a call to fts_read() on the same file hierarchy stream
unless they represent a file of type directory, in which case
they will not be overwritten until after a call to fts_read()
after the FTSENT structure has been returned by the function
fts_read() in postorder.
fts_children()
The fts_children() function returns a pointer to an FTSENT
structure describing the first entry in a NULL-terminated linked
list of the files in the directory represented by the FTSENT
structure most recently returned by fts_read(). The list is
linked through the fts_link field of the FTSENT structure, and is
ordered by the user-specified comparison function, if any.
Repeated calls to fts_children() will re-create this linked list.
As a special case, if fts_read() has not yet been called for a
hierarchy, fts_children() will return a pointer to the files in
the logical directory specified to fts_open(), that is, the
arguments specified to fts_open(). Otherwise, if the FTSENT
structure most recently returned by fts_read() is not a directory
being visited in preorder, or the directory does not contain any
files, fts_children() returns NULL and sets errno to zero. If an
error occurs, fts_children() returns NULL and sets errno to
indicate the error.
The FTSENT structures returned by fts_children() may be
overwritten after a call to fts_children(), fts_close(), or
fts_read() on the same file hierarchy stream.
The instr argument is either zero or the following value:
FTS_NAMEONLY
Only the names of the files are needed. The contents of
all the fields in the returned linked list of structures
are undefined with the exception of the fts_name and
fts_namelen fields.
fts_set()
The function fts_set() allows the user application to determine
further processing for the file f of the stream ftsp. The
fts_set() function returns 0 on success, and -1 if an error
occurs.
The instr argument is either 0 (meaning "do nothing") or one of
the following values:
FTS_AGAIN
Revisit the file; any file type may be revisited. The
next call to fts_read() will return the referenced file.
The fts_stat and fts_info fields of the structure will be
reinitialized at that time, but no other fields will have
been changed. This option is meaningful only for the most
recently returned file from fts_read(). Normal use is for
postorder directory visits, where it causes the directory
to be revisited (in both preorder and postorder) as well
as all of its descendants.
FTS_FOLLOW
The referenced file must be a symbolic link. If the
referenced file is the one most recently returned by
fts_read(), the next call to fts_read() returns the file
with the fts_info and fts_statp fields reinitialized to
reflect the target of the symbolic link instead of the
symbolic link itself. If the file is one of those most
recently returned by fts_children(), the fts_info and
fts_statp fields of the structure, when returned by
fts_read(), will reflect the target of the symbolic link
instead of the symbolic link itself. In either case, if
the target of the symbolic link does not exist, the fields
of the returned structure will be unchanged and the
fts_info field will be set to FTS_SLNONE.
If the target of the link is a directory, the preorder
return, followed by the return of all of its descendants,
followed by a postorder return, is done.
FTS_SKIP
No descendants of this file are visited. The file may be
one of those most recently returned by either
fts_children() or fts_read().
fts_close()
The fts_close() function closes the file hierarchy stream
referred to by ftsp and restores the current directory to the
directory from which fts_open() was called to open ftsp. The
fts_close() function returns 0 on success, and -1 if an error
occurs.
