FTS(3)

🌈 FTS(3) — traverse a file hierarchy 🌈

okay bestie, this is FTS(3) from the Library Functions Manual, and we're about to make file-hierarchy traversal make sense 💖✨

✨ NAME ✨

fts — traverse a file hierarchy 😌✨

📚 LIBRARY 📚

Standard C Library (libc, -lc) 💖

🧾 SYNOPSIS 🧾

#include <fts.h>

FTS *
fts_open(char * const *path_argv, int options,
    int (*compar)(const FTSENT * const *, const FTSENT * const *));

FTS *
fts_open_b(char * const *path_argv, int options,
    int (^compar)(const FTSENT * const *, const FTSENT * const *));

FTSENT *
fts_read(FTS *ftsp);

FTSENT *
fts_children(FTS *ftsp, int options);

int
fts_set(FTS *ftsp, FTSENT *f, int options);

void
fts_set_clientptr(FTS *ftsp, void *clientdata);

void *
fts_get_clientptr(FTS *ftsp);

FTS *
fts_get_stream(FTSENT *f);

int
fts_close(FTS *ftsp);

🦋 DESCRIPTION 🦋

okay so the fts functions are literally your tour guides for traversing Unix file hierarchies 🗺️✨ the simple overview is that fts_open() and fts_open_b() return a “handle” on a file hierarchy, and then you supply that handle to the other fts functions. fts_read() returns a pointer to a structure describing one of the files in the file hierarchy. fts_children() returns a pointer to a linked list of structures, and each one describes one of the files contained in a directory in the hierarchy. in general, directories are visited two distinguishable times: in pre-order (before any of their descendants are visited) and in post-order (after all of their descendants have been visited). files are visited once. it is possible to walk the hierarchy “logically” (ignoring symbolic links) or physically (visiting symbolic links), order the walk of the hierarchy or prune and/or re-visit portions of the hierarchy 👑

two structures are defined (and typedef'd) in the include file <fts.h>. the first is FTS, the structure that represents the file hierarchy itself. the second 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 FTS structure contains space for a single pointer, which may be used to store application data or per-hierarchy state. the fts_set_clientptr() and fts_get_clientptr() functions may be used to set and retrieve this pointer. this is likely to be useful only when accessed from the sort comparison function, which can determine the original FTS stream of its arguments using the fts_get_stream() function. the two get functions are also available as macros of the same name 🫶

the FTSENT structure contains at least the following fields, which are described in greater detail below:

typedef struct _ftsent {
        int fts_info;                   /* status for FTSENT structure */
        char *fts_accpath;              /* access path */
        char *fts_path;                 /* root path */
        size_t fts_pathlen;             /* strlen(fts_path) */
        char *fts_name;                 /* file name */
        size_t fts_namelen;             /* strlen(fts_name) */
        long fts_level;                 /* depth (-1 to N) */
        int fts_errno;                  /* file errno */
        long 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 pre-order.
  • 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 immediately follows FTS_D, in place of FTS_DP, when the directory could not be entered, or could be entered but not 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 file name to fts_open() or fts_open_b() (see FTS_SEEDOT).
  • FTS_DP — a directory being visited in post-order. the contents of the FTSENT structure will be unchanged from when the directory was visited in pre-order, except for the fts_info field.
  • 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 non-existent 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() or fts_open_b() as a prefix.
• fts_pathlen — the length of the string referenced by fts_path.
• 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 FTS_ROOTPARENTLEVEL (-1), and the FTSENT structure for the root itself is numbered FTS_ROOTLEVEL (0).
• fts_errno — upon return of a FTSENT structure from the fts_children() or fts_read() functions, with its fts_info field set to FTS_DNR, FTS_ERR or FTS_NS, the fts_errno field contains the value of the external variable errno 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, i.e., 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 NUL-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 NUL-terminated 🫡💖

🧵 Thread Safety 🧵

the fts functions can safely be used in multi-threaded programs provided no two threads access the same FTS or FTSENT structure simultaneously. however, unless the FTS_NOCHDIR flag was passed to fts_open() or fts_open_b(), calls to fts_read() and fts_children() may change the current working directory, which will affect all threads. conversely, changing the current working directory either during or between calls to fts_read() or fts_children() (even in a single-thread program) may cause fts to malfunction unless the FTS_NOCHDIR flag was passed to fts_open() or fts_open_b() and all paths in path_argv were absolute 😭⚠️

🗂️ 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 or'ing the following values:

• FTS_COMFOLLOW — this one makes any symbolic link specified as a root path get followed immediately, whether or not FTS_LOGICAL is also specified 💅
• FTS_COMFOLLOWDIR — same energy as FTS_COMFOLLOW, but only follows symbolic links to directories.
• FTS_LOGICAL — this makes the fts routines 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 non-existent files. Either FTS_LOGICAL or FTS_PHYSICAL must be provided to the fts_open() function. ⭐
• FTS_NOCHDIR — okay so, to allow descending to arbitrary depths (independent of {PATH_MAX}) and improve performance, the fts functions change directories as they walk the file hierarchy. The side-effect is that an application cannot rely on being in any particular directory during the traversal. FTS_NOCHDIR turns off this feature, 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_NOSTAT_TYPE — similar to FTS_NOSTAT, but it attempts to populate fts_info based on information from the d_type field of struct dirent. 🦋
• FTS_PHYSICAL — this makes the fts routines return FTSENT structures for the 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 one prevents fts from descending into directories that have a different device number than the file from which the descent began. 🛑

Okay, now the compar argument 👑 — it points to 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.

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🦋 fts_open_b() 🦋

The fts_open_b() function is identical to fts_open() except that it takes a block pointer instead of a function pointer. The block is copied before fts_open_b() returns, so the original can safely go out of scope or be released. 🫶

🌸 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 pre-order and once in post-order. 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 the external variable errno to 0. If an error unrelated to a file in the hierarchy occurs, fts_read() returns NULL and sets errno appropriately. 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). Note that fts_read() will not set errno to 0 if called again with the same ftsp argument after the FTS_STOP flag has been set or the end of the stream has been reached. 💀

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 fts_read() function in post-order. 🎀

💅 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 recreate 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() or fts_open_b(), i.e., the arguments specified to fts_open() or fts_open_b(). Otherwise, if the FTSENT structure most recently returned by fts_read() is not a directory being visited in pre-order, 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 appropriately. 🍓

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.

Option may be set to 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 fts_set() function 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. Option must be set to one of the following values:

• FTS_AGAIN — re-visit the file; any file type may be re-visited. 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 post-order directory visits, where it causes the directory to be re-visited (in both pre and post-order) 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 pre-order return, followed by the return of all of its descendants, followed by a post-order 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_set_clientptr(), fts_get_clientptr() 🧚

The fts_set_clientptr() function sets the client data pointer for the stream ftsp to clientdata. The fts_get_clientptr() function returns the client data pointer associated with ftsp. This can be used to pass per-stream data to the comparison function. 🌈

For performance reasons, fts_get_clientptr() may be shadowed by a preprocessor macro. 👀

⭐ fts_get_stream() ⭐

The fts_get_stream() function returns the fts stream associated with the file entry f. A typical use for this would be for a comparison function to first call fts_get_stream() on one of its arguments, then call fts_get_clientptr() to obtain the client data pointer, which in turn points to information necessary to correctly order the two entries. 💫

For performance reasons, fts_get_stream() may be shadowed by a preprocessor macro. 👑

🌈 fts_close() 🌈

The fts_close() function closes a file hierarchy stream ftsp and restores the current directory to the directory from which fts_open() or fts_open_b() was called to open ftsp. And yes, it literally puts your cwd back where it started 🫶💖

💖 RETURN VALUES 💖

Okay bestie, here's the receipts on what each function hands back to you:

• fts_open() and fts_open_b() — slay on success: they give you a pointer to the shiny new fts stream. On failure? NULL. That's it, that's the vibe. 😭
• fts_read() — returns a pointer to the next file entry on success, or if an error happens that's specifically about that file entry. When you hit the end of the file hierarchy, it returns NULL and sets the external variable errno to 0. On actual failure it returns NULL and sets errno to an appropriate non-zero value. And lowkey important: if you call it again after the FTS_STOP flag has been set, or after you already reached the end of the stream, fts_read() returns NULL and leaves errno completely untouched. No gaslighting your errno, periodt. ✨
• fts_children() — returns a pointer to a linked list of file entries on success. When you reach the end of the file hierarchy it returns NULL and sets errno to 0. On failure it returns NULL and sets errno to an appropriate non-zero value. 👑
• fts_set() — returns 0 on success, and -1 if its instr argument is invalid. Invalid instruction? Not today. 💅
• fts_get_clientptr() — returns the client data pointer associated with its argument, or NULL if none has been set.
• fts_get_stream() — returns a pointer to the fts stream associated with its argument.
• fts_close() — returns 0 on success, and -1 if an error occurs.

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🫶 ERRORS 🫶

Yeah so sometimes things go sideways, and errno is gonna tell you all about it.

• fts_open() and fts_open_b() may fail and set errno for any of the errors specified for the library functions open(2) and malloc(3).
• fts_open_b() may also fail and set errno to ENOSYS if the blocks runtime is missing. 😬
• fts_close() may fail and set errno for any of the errors specified for chdir(2) and close(2).
• fts_read() and fts_children() may fail and set errno for any of the errors specified for chdir(2), malloc(3), opendir(3), readdir(3) and stat(2).

And in addition, fts_children(), fts_open(), and fts_set() may fail and set errno as follows:

• [EINVAL] — The options were invalid, or the list was empty. 🚩

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🦋 SEE ALSO 🦋

Go read these next, bestie, they're literally the whole friend group:

find(1), chdir(2), stat(2), ftw(3), qsort(3)

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🌸 HISTORY 🌸

The fts interface first showed up in 4.4BSD — vintage icon behavior. 🎀

The fts_get_clientptr(), fts_get_stream(), and fts_set_clientptr() functions were introduced in FreeBSD 5.0, principally to provide for alternative interfaces to the fts functionality using different data structures. (iykyk — it's giving "we made it flexible.")

Blocks support and the FTS_COMFOLLOWDIR and FTS_NOSTAT options were added in FreeBSD 15.0 based on similar functionality in macOS. Newer era, same energy. ✨

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💀 BUGS 💀

Heads up, this one's a little sneaky:

The fts_open() function will automatically set the FTS_NOCHDIR option if the FTS_LOGICAL option is provided, or if it cannot open(2) the current directory.

So like… you didn't ask for FTS_NOCHDIR, but it might just show up uninvited. Not a bug you fix, just a bug you know about. 🙈

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💜 That's the whole page, bestie! You just read FTS(3) from the Debian crowd, dated October 6, 2025 — go forth and traverse some file hierarchies, you absolute main character. 🧚‍♀️🌈🫶