While historically there have been several different interfaces for
this, the only one that really matters these days the the Posix.1
standard `<dirent.h>' functions.
The function opendir() opens a specified directory;
readdir() reads directory entries from it in a standardised
format; closedir() does the obvious. Also provided are
rewinddir(), telldir() and seekdir() which should
also be obvious.
If you are looking to expand a wildcard filename, then most systems have
the glob() function; also check out fnmatch() to match
filenames against a wildcard, or ftw() to traverse entire
directory trees.