man3/ftw.3

.\" Copyright (c) 1993 Michael Haardt (michael@moria.de)
.\" and copyright (c) 1999 Andries Brouwer (aeb@cwi.nl)
.\" and copyright (c) 2006 Justin Pryzby <justinpryzby@users.sf.net>
.\" and copyright (c) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
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.\" Modified Sun Jul 25 11:02:22 1993 by Rik Faith (faith@cs.unc.edu)
.\" 2006-05-24, Justin Pryzby <justinpryzby@users.sf.net>
.\"  	document FTW_ACTIONRETVAL; include .SH RETURN VALUE;
.\" 2006-05-24, Justin Pryzby <justinpryzby@users.sf.net> and
.\"	Michael Kerrisk <mtk.manpages@gmail.com>
.\" 	reorganized and rewrote much of the page
.\" 2006-05-24, Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Added an example program.
.\"
.TH FTW 3 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
ftw, nftw \- file tree walk
.SH SYNOPSIS
.nf
.B #include <ftw.h>
.PP
.BI "int nftw(const char *" dirpath ,
.BI "        int (*" fn ")(const char *" fpath ", const struct stat *" sb ,
.BI "                  int " typeflag ", struct FTW *" ftwbuf ),
.BI "        int " nopenfd ", int " flags );
.PP
.BI "int ftw(const char *" dirpath ,
.BI "        int (*" fn ")(const char *" fpath ", const struct stat *" sb ,
.BI "                  int " typeflag ),
.BI "        int " nopenfd );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR nftw ():
.nf
    _XOPEN_SOURCE >= 500
.fi
.SH DESCRIPTION
.BR nftw ()
walks through the directory tree that is
located under the directory \fIdirpath\fP,
and calls \fIfn\fP() once for each entry in the tree.
By default, directories are handled before the files and
subdirectories they contain (preorder traversal).
.PP
To avoid using up all of the calling process's file descriptors,
\fInopenfd\fP specifies the maximum number of directories that
.BR nftw ()
will hold open simultaneously.
When
the search depth exceeds this,
.BR nftw ()
will become slower because
directories have to be closed and reopened.
.BR nftw ()
uses at most
one file descriptor for each level in the directory tree.
.PP
For each entry found in the tree,
.BR nftw ()
calls
\fIfn\fP() with four arguments:
.IR fpath ,
.IR sb ,
.IR typeflag ,
and
.IR ftwbuf .
.I fpath
is the pathname of the entry,
and is expressed either as a pathname relative to the calling process's
current working directory at the time of the call to
.BR nftw (),
if
.IR dirpath
was expressed as a relative pathname,
or as an absolute pathname, if
.I dirpath
was expressed as an absolute pathname.
.I sb
is a pointer to the
.I stat
structure returned by a call to
.BR stat (2)
for
.IR fpath .
.PP
The
.I typeflag
argument passed to
.IR fn ()
is an integer that has one of the following values:
.TP
.B FTW_F
.I fpath
is a regular file.
.TP
.B FTW_D
.I fpath
is a directory.
.TP
.B FTW_DNR
.I fpath
is a directory which can't be read.
.TP
.B FTW_DP
.I fpath
is a directory, and \fBFTW_DEPTH\fP was specified in \fIflags\fP.
(If
.B FTW_DEPTH
was not specified in
.IR flags ,
then directories will always be visited with
.I typeflag
set to
.BR FTW_D .)
All of the files
and subdirectories within \fIfpath\fP have been processed.
.TP
.B FTW_NS
The
.BR stat (2)
call failed on
.IR fpath ,
which is not a symbolic link.
The probable cause for this is that the caller had read permission
on the parent directory, so that the filename
.I fpath
could be seen,
but did not have execute permission,
so that the file could not be reached for
.BR stat (2).
The contents of the buffer pointed to by
.I sb
are undefined.
.TP
.B FTW_SL
.I fpath
is a symbolic link, and \fBFTW_PHYS\fP was set in \fIflags\fP.
.\" To obtain the definition of this constant from
.\" .IR <ftw.h> ,
.\" either
.\" .B _BSD_SOURCE
.\" must be defined, or
.\" .BR _XOPEN_SOURCE
.\" must be defined with a value of 500 or more.
.TP
.B FTW_SLN
.I fpath
is a symbolic link pointing to a nonexistent file.
(This occurs only if \fBFTW_PHYS\fP is not set.)
In this case the
.I sb
argument passed to
.IR fn ()
contains information returned by performing
.BR lstat (2)
on the "dangling" symbolic link.
(But see BUGS.)
.PP
The fourth argument
.RI ( ftwbuf )
that
.BR nftw ()
supplies when calling
\fIfn\fP()
is a pointer to a structure of type \fIFTW\fP:
.PP
.in +4n
.EX
struct FTW {
    int base;
    int level;
};
.EE
.in
.PP
.I base
is the offset of the filename (i.e., basename component)
in the pathname given in
.IR fpath .
.I level
is the depth of
.I fpath
in the directory tree, relative to the root of the tree
.RI ( dirpath ,
which has depth 0).
.PP
To stop the tree walk, \fIfn\fP() returns a nonzero value; this
value will become the return value of
.BR nftw ().
As long as \fIfn\fP() returns 0,
.BR nftw ()
will continue either until it has traversed the entire tree,
in which case it will return zero,
or until it encounters an error (such as a
.BR malloc (3)
failure), in which case it will return \-1.
.PP
Because
.BR nftw ()
uses dynamic data structures, the only safe way to
exit out of a tree walk is to return a nonzero value from \fIfn\fP().
To allow a signal to terminate the walk without causing a memory leak,
have the handler set a global flag that is checked by \fIfn\fP().
\fIDon't\fP use
.BR longjmp (3)
unless the program is going to terminate.
.PP
The \fIflags\fP argument of
.BR nftw ()
is formed by ORing zero or more of the
following flags:
.TP
.BR FTW_ACTIONRETVAL " (since glibc 2.3.3)"
If this glibc-specific flag is set, then
.BR nftw ()
handles the return value from
.IR fn ()
differently.
.IR fn ()
should return one of the following values:
.RS
.TP
.B FTW_CONTINUE
Instructs
.BR nftw ()
to continue normally.
.TP
.B FTW_SKIP_SIBLINGS
If \fIfn\fP() returns this value, then
siblings of the current entry will be skipped,
and processing continues in the parent.
.\" If \fBFTW_DEPTH\fP
.\" is set, the entry's parent directory is processed next (with
.\" \fIflag\fP set to \fBFTW_DP\fP).
.TP
.B FTW_SKIP_SUBTREE
If \fIfn\fP() is called with an entry that is a directory
(\fItypeflag\fP is \fBFTW_D\fP), this return
value will prevent objects within that directory from being passed as
arguments to \fIfn\fP().
.BR nftw ()
continues processing with the next sibling of the directory.
.TP
.B FTW_STOP
Causes
.BR nftw ()
to return immediately with the return value
\fBFTW_STOP\fP.
.PP
Other return values could be associated with new actions in the future;
\fIfn\fP() should not return values other than those listed above.
.PP
The feature test macro
.B _GNU_SOURCE
must be defined
(before including
.I any
header files)
in order to
obtain the definition of \fBFTW_ACTIONRETVAL\fP from \fI<ftw.h>\fP.
.RE
.TP
.B FTW_CHDIR
If set, do a
.BR chdir (2)
to each directory before handling its contents.
This is useful if the program needs to perform some action
in the directory in which \fIfpath\fP resides.
(Specifying this flag has no effect on the pathname that is passed in the
.I fpath
argument of
.IR fn .)
.TP
.B FTW_DEPTH
If set, do a post-order traversal, that is, call \fIfn\fP() for
the directory itself \fIafter\fP handling the contents of the directory
and its subdirectories.
(By default, each directory is handled \fIbefore\fP its contents.)
.TP
.B FTW_MOUNT
If set, stay within the same filesystem
(i.e., do not cross mount points).
.TP
.B FTW_PHYS
If set, do not follow symbolic links.
(This is what you want.)
If not set, symbolic links are followed, but no file is reported twice.
.IP
If \fBFTW_PHYS\fP is not set, but \fBFTW_DEPTH\fP is set,
then the function
.IR fn ()
is never called for a directory that would be a descendant of itself.
.SS ftw()
.BR ftw ()
is an older function that offers a subset of the functionality of
.BR nftw ().
The notable differences are as follows:
.IP * 3
.BR ftw ()
has no
.IR flags
argument.
It behaves the same as when
.BR nftw ()
is called with
.I flags
specified as zero.
.IP *
The callback function,
.IR fn (),
is not supplied with a fourth argument.
.IP *
The range of values that is passed via the
.I typeflag
argument supplied to
.IR fn ()
is smaller: just
.BR FTW_F ,
.BR FTW_D ,
.BR FTW_DNR ,
.BR FTW_NS ,
and (possibly)
.BR FTW_SL .
.SH RETURN VALUE
These functions return 0 on success, and \-1 if an error occurs.
.PP
If \fIfn\fP() returns nonzero,
then the tree walk is terminated and the value returned by \fIfn\fP()
is returned as the result of
.BR ftw ()
or
.BR nftw ().
.PP
If
.BR nftw ()
is called with the \fBFTW_ACTIONRETVAL\fP flag,
then the only nonzero value that should be used by \fIfn\fP()
to terminate the tree walk is \fBFTW_STOP\fP,
and that value is returned as the result of
.BR nftw ().
.SH VERSIONS
.BR nftw ()
is available under glibc since version 2.1.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR nftw ()
T}	Thread safety	MT-Safe cwd
T{
.BR ftw ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4, SUSv1.
POSIX.1-2008 marks
.BR ftw ()
as obsolete.
.SH NOTES
POSIX.1-2008 notes that the results are unspecified if
.I fn
does not preserve the current working directory.
.PP
The function
.BR nftw ()
and the use of \fBFTW_SL\fP with
.BR ftw ()
were introduced in SUSv1.
.PP
In some implementations (e.g., glibc),
.BR ftw ()
will never use \fBFTW_SL\fP, on other systems \fBFTW_SL\fP occurs only
for symbolic links that do not point to an existing file,
and again on other systems
.BR ftw ()
will use \fBFTW_SL\fP for each symbolic link.
If
.I fpath
is a symbolic link and
.BR stat (2)
failed, POSIX.1-2008 states
that it is undefined whether \fBFTW_NS\fP or \fBFTW_SL\fP
is passed in
.IR typeflag .
For predictable results, use
.BR nftw ().
.SH BUGS
According to POSIX.1-2008, when the
.IR typeflag
argument passed to
.IR fn ()
contains
.BR FTW_SLN ,
the buffer pointed to by
.I sb
should contain information about the dangling symbolic link
(obtained by calling
.BR lstat (2)
on the link).
Early glibc versions correctly followed the POSIX specification on this point.
However, as a result of a regression introduced in glibc 2.4,
the contents of the buffer pointed to by
.I sb
were undefined when
.B FTW_SLN
is passed in
.IR typeflag .
(More precisely, the contents of the buffer were left unchanged in this case.)
This regression was eventually fixed in glibc 2.30,
.\" https://bugzilla.redhat.com/show_bug.cgi?id=1422736
.\" http://austingroupbugs.net/view.php?id=1121
.\" glibc commit 6ba205b2c35e3e024c8c12d2ee1b73363e84da87
.\" https://sourceware.org/bugzilla/show_bug.cgi?id=23501
so that the glibc implementation (once more) follows the POSIX specification.
.SH EXAMPLES
The following program traverses the directory tree under the path named
in its first command-line argument, or under the current directory
if no argument is supplied.
It displays various information about each file.
The second command-line argument can be used to specify characters that
control the value assigned to the \fIflags\fP
argument when calling
.BR nftw ().
.SS Program source
\&
.EX
#define _XOPEN_SOURCE 500
#include <ftw.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>

static int
display_info(const char *fpath, const struct stat *sb,
             int tflag, struct FTW *ftwbuf)
{
    printf("%\-3s %2d ",
            (tflag == FTW_D) ?   "d"   : (tflag == FTW_DNR) ? "dnr" :
            (tflag == FTW_DP) ?  "dp"  : (tflag == FTW_F) ?   "f" :
            (tflag == FTW_NS) ?  "ns"  : (tflag == FTW_SL) ?  "sl" :
            (tflag == FTW_SLN) ? "sln" : "???",
            ftwbuf\->level);

    if (tflag == FTW_NS)
        printf("\-\-\-\-\-\-\-");
    else
        printf("%7jd", (intmax_t) sb\->st_size);

    printf("   %\-40s %d %s\en",
            fpath, ftwbuf\->base, fpath + ftwbuf\->base);

    return 0;           /* To tell nftw() to continue */
}

int
main(int argc, char *argv[])
{
    int flags = 0;

    if (argc > 2 && strchr(argv[2], \(aqd\(aq) != NULL)
        flags |= FTW_DEPTH;
    if (argc > 2 && strchr(argv[2], \(aqp\(aq) != NULL)
        flags |= FTW_PHYS;

    if (nftw((argc < 2) ? "." : argv[1], display_info, 20, flags)
            == \-1) {
        perror("nftw");
        exit(EXIT_FAILURE);
    }

    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR stat (2),
.BR fts (3),
.BR readdir (3)