int ftw (const char *dir, int (*fn)(const char *file, const struct stat *sb, int flag), int depth);
int nftw (const char *dir, int (*fn)(const char *file, const struct stat *sb, int flag, struct FTW *s), int depth, int flags);
If the item is a symbolic link and stat failed, XPG4v2 states that it is undefined whether FTW_NS or FTW_SL is used.
ftw() recursively calls itself for traversing found directories, handling a directory before its files or subdirectories. To avoid using up all a program's file descriptors, the depth specifies the number of simultaneous open directories. When the depth is exceeded, ftw() will become slower because directories have to be closed and reopened. ftw() uses at most one file descriptor for each level in the file hierarchy.
To stop the tree walk, fn() returns a non-zero value; this value will become the return value of ftw(). Otherwise, ftw() will continue until it has traversed the entire tree, in which case it will return zero, or until it hits an error other than EACCES (such as a malloc(3) failure), in which case it will return -1.
Because ftw() uses dynamic data structures, the only safe way to exit out of a tree walk is to return a non-zero value. To handle interrupts, for example, mark that the interrupt occurred and return a non-zero value---don't use longjmp(3) unless the program is going to terminate.
The function nftw() does precisely the same as ftw(), except that it has one additional argument flags (and calls the supplied function with one more argument). This flags argument is an OR of zero or more of the following flags:
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.
The function fn() is called with four arguments: the pathname of the reported entry, a pointer to a struct stat for this entry, an integer describing its type, and a pointer to a struct FTW. The type will be one of the following: FTW_F, FTW_D, FTW_DNR, FTW_SL, FTW_NS (with meaning as above; FTW_SL occurs only with FTW_PHYS set) or
The struct FTW pointed at by the fourth argument to fn() has at least the fields base, the offset of the item's filename in the pathname given as first argument of fn(), and level, the depth of the item relative to the starting point (which has depth 0).