fts

fts is a set of functions in the C standard library used to traverse file hierarchies in a controlled, portable way. Originating in BSD Unix and adopted in POSIX, the interface provides a depth-first walk of directories and their contents, returning information about each filesystem entry through a structured type. It is commonly used by utilities and programs that need to process many files recursively, with fine-grained control over how symlinks, directories, and device boundaries are handled.

The core API consists of a few functions. fts_open takes an array of pathnames, a set of option flags, and an optional comparison function, and returns a handle to an internal traversal state. fts_read returns the next entry in the traversal as an FTSENT object, which contains details about the current path, its depth in the tree, and the type or status of the entry. fts_children can be used to obtain the list of children for a directory node, and fts_close releases resources when traversal is finished. A companion function, fts_options, allows querying or modifying traversal behavior.

FTS and FTSENT provide fields that describe each entry, including the path, depth level, and traversal information. The traversal returns entries with an fts_info value that encodes the kind of entry, such as a directory, a regular file, or a symbolic link, as well as error states or special conditions. This information enables callers to implement custom logic for processing, skipping, or deleting files.

Options control traversal behavior. For example, FTS_PHYSICAL performs a non-following of symbolic links, while FTS_LOGICAL follows them. FTS_XDEV prevents descending onto other mounted filesystems. FTS_SEEDOT includes dot entries (like . and ..) in the walk, and FTS_NOCHDIR prevents changing the current working directory during traversal. These controls provide robust, portable file-system operations across Unix-like systems.

On systems without a native fts implementation, similar functionality may be provided by libraries or alternatives such as nftw or ftw, or via libbsd on Linux.