Manpages - ftw.3

ftw, nftw - file tree walk

  #include <ftw.h>

  int nftw(const char *dirpath,
   int (*fn)(const char *fpath, const struct stat *sb,
   int typeflag, struct FTW *ftwbuf),
   int nopenfd, int flags);

  int ftw(const char *dirpath,
   int (*fn)(const char *fpath, const struct stat *sb,
   int typeflag),
   int nopenfd);

Feature Test Macro Requirements for glibc (see *feature_test_macros*(7)):

*nftw*():

      _XOPEN_SOURCE >= 500

*nftw*() walks through the directory tree that is located under the directory dirpath, and calls /fn/() once for each entry in the tree. By default, directories are handled before the files and subdirectories they contain (preorder traversal).

To avoid using up all of the calling process’s file descriptors, nopenfd specifies the maximum number of directories that *nftw*() will hold open simultaneously. When the search depth exceeds this, *nftw*() will become slower because directories have to be closed and reopened. *nftw*() uses at most one file descriptor for each level in the directory tree.

For each entry found in the tree, *nftw*() calls fn/() with four arguments: /fpath, sb, typeflag, and ftwbuf. 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 *nftw*(), if dirpath was expressed as a relative pathname, or as an absolute pathname, if dirpath was expressed as an absolute pathname. sb is a pointer to the stat structure returned by a call to *stat*(2) for fpath.

The typeflag argument passed to /fn/() is an integer that has one of the following values:

FTW_F

fpath is a regular file.

FTW_D

fpath is a directory.

FTW_DNR

fpath is a directory which can’t be read.

FTW_DP

fpath is a directory, and FTW_DEPTH was specified in flags. (If FTW_DEPTH was not specified in flags, then directories will always be visited with typeflag set to FTW_D.) All of the files and subdirectories within fpath have been processed.

FTW_NS

The *stat*(2) call failed on 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 fpath could be seen, but did not have execute permission, so that the file could not be reached for *stat*(2). The contents of the buffer pointed to by sb are undefined.

FTW_SL

fpath is a symbolic link, and FTW_PHYS was set in flags.

FTW_SLN

fpath is a symbolic link pointing to a nonexistent file. (This occurs only if FTW_PHYS is not set.) In this case the sb argument passed to /fn/() contains information returned by performing *lstat*(2) on the “dangling” symbolic link. (But see BUGS.)

The fourth argument (ftwbuf) that *nftw*() supplies when calling fn/() is a pointer to a structure of type /FTW:

  struct FTW {
      int base;
      int level;
  };

base is the offset of the filename (i.e., basename component) in the pathname given in fpath. level is the depth of fpath in the directory tree, relative to the root of the tree (dirpath, which has depth 0).

To stop the tree walk, /fn/() returns a nonzero value; this value will become the return value of *nftw*(). As long as /fn/() returns 0, *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 *malloc*(3) failure), in which case it will return -1.

Because *nftw*() uses dynamic data structures, the only safe way to exit out of a tree walk is to return a nonzero value from fn/(). To allow a signal to terminate the walk without causing a memory leak, have the handler set a global flag that is checked by /fn/(). /Don’t use *longjmp*(3) unless the program is going to terminate.

The flags argument of *nftw*() is formed by ORing zero or more of the following flags:

FTW_ACTIONRETVAL (since glibc 2.3.3)

If this glibc-specific flag is set, then *nftw*() handles the return value from /fn/() differently. /fn/() should return one of the following values:

FTW_CONTINUE

Instructs *nftw*() to continue normally.

FTW_SKIP_SIBLINGS

If /fn/() returns this value, then siblings of the current entry will be skipped, and processing continues in the parent.

FTW_SKIP_SUBTREE

If fn/() is called with an entry that is a directory (/typeflag is FTW_D), this return value will prevent objects within that directory from being passed as arguments to /fn/(). *nftw*() continues processing with the next sibling of the directory.

FTW_STOP

Causes nftw*() to return immediately with the return value *FTW_STOP.

Other return values could be associated with new actions in the future; /fn/() should not return values other than those listed above.

The feature test macro _GNU_SOURCE must be defined (before including any header files) in order to obtain the definition of FTW_ACTIONRETVAL from <ftw.h>.

FTW_CHDIR

If set, do a *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 fpath resides. (Specifying this flag has no effect on the pathname that is passed in the fpath argument of fn.)

FTW_DEPTH

If set, do a post-order traversal, that is, call fn/() for the directory itself /after handling the contents of the directory and its subdirectories. (By default, each directory is handled before its contents.)

FTW_MOUNT

If set, stay within the same filesystem (i.e., do not cross mount points).

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.

If FTW_PHYS is not set, but FTW_DEPTH is set, then the function /fn/() is never called for a directory that would be a descendant of itself.

These functions return 0 on success, and -1 if an error occurs.

If /fn/() returns nonzero, then the tree walk is terminated and the value returned by /fn/() is returned as the result of *ftw*() or *nftw*().

If nftw*() is called with the *FTW_ACTIONRETVAL flag, then the only nonzero value that should be used by /fn/() to terminate the tree walk is FTW_STOP, and that value is returned as the result of *nftw*().

*nftw*() is available under glibc since version 2.1.

For an explanation of the terms used in this section, see *attributes*(7).

POSIX.1-2001, POSIX.1-2008, SVr4, SUSv1. POSIX.1-2008 marks *ftw*() as obsolete.

POSIX.1-2008 notes that the results are unspecified if fn does not preserve the current working directory.

The function nftw*() and the use of *FTW_SL with *ftw*() were introduced in SUSv1.

In some implementations (e.g., glibc), ftw*() will never use *FTW_SL, on other systems FTW_SL occurs only for symbolic links that do not point to an existing file, and again on other systems ftw*() will use *FTW_SL for each symbolic link. If fpath is a symbolic link and stat*(2) failed, POSIX.1-2008 states that it is undefined whether *FTW_NS or FTW_SL is passed in typeflag. For predictable results, use *nftw*().

According to POSIX.1-2008, when the typeflag argument passed to fn/() contains FTW_SLN, the buffer pointed to by /sb should contain information about the dangling symbolic link (obtained by calling 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 sb were undefined when *FTW_SLN is passed in typeflag. (More precisely, the contents of the buffer were left unchanged in this case.) This regression was eventually fixed in glibc 2.30, so that the glibc implementation (once more) follows the POSIX specification.

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 flags argument when calling *nftw*().

*stat*(2), *fts*(3), *readdir*(3)

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