Why does Linux use getdents() on directories instead of read()?

Question

I was skimming through K&R C and I noticed that to read the entries in a directories, they used:

while (read(dp->fd, (char *) &dirbuf, sizeof(dirbuf))         


        

Answer 1

Your suspicion is correct: It would make more sense to have the read system call work on directories and return some standardized data, rather than have the separate getdents system call. getdents is superfluous and reduces the uniformity of the interface. The other answers assert that "read" as an interface would be inferior in some way to "getdents". They are incorrect. As you can observe, the arguments and return value of "read" and "getdents" are identical; just "read" only works on non-directories and "getdents" only works on directories. "getdents" could easily be folded into "read" to get a single uniform syscall.

The reason this is not the case is historical. Originally, "read" worked on directories, but returned the actual raw directory entry in the filesystem. This was complex to parse, so the getdirents call was added in addition to read, to provide a filesystem-independent view of directory entries. Eventually, "read" on directories was turned off. "read" on directories just as well could have been made to behave identically to getdirents instead of being turned off. It just wasn't, possibly because it seemed duplicative.

In Linux, in particular, "read" has returned an error when reading directories for so long that it's almost certain that some program is relying on this behavior. So, backwards compatibility demands that "read" on Linux will never work on directories.