Efficiently Traverse Directory Tree with opendir(), readdir() and closedir()
The C routines opendir(), readdir() and closedir() provide a way for me to traverse a directory structure. However, each dirent structure returned by readdir() does not seem to provide a useful way for me to obtain the set of pointers to DIR that I would need to recurse into the directory subdirectories.
Of course, they give me the name of the files, so I could either append that name to the directory path and stat() and opendir() them, or I could change the current working directory of the process via chdir() and roll it back via chdir("..").
The problem with the first approach is that if the length of the directory path is great enough, then the cost to pass a string containing it to opendir() will overweight the cost of opening a directory. If you are a bit more theoretical, you could say your complexity could increase beyond linear time (in the total character count of the (relative) filenames in the directory tree).
Also, the second approach has a problem. Since each process has a single current working directory, all but one thread will have to block in a multithreaded application. Also, I don't know if the current working directory is just a mere convenience (i.e., the relative path will be appended to it prior to a filesystem query). If it is, this approach will be inefficient too.
I am accepting alternatives to these functions. So how is it one can traverse a UNIX directory tree efficiently (linear time in the total character count of the files under it)?
Answer
Have you tried ftw() aka File Tree Walk ?
Snippit from man 3 ftw:
int ftw(const char *dir, int (*fn)(const char *file, const struct stat *sb, int flag), int nopenfd);
ftw() walks through the directory tree starting from the indicated directory dir. For each found entry in the tree, it calls fn() with the full pathname of the entry, a pointer to the stat(2) structure for the entry and an int flag