Clarifications of exercise

Directory entries: you are required to use the first and second files to work on. Note that these files are the 3rd and 4rth directory entries, respectively. That is because "." and ".." are the first 2 directory entries.

Useful information for exercise

Getting directory information

You can use the documentation on the "getdents" system call. However, some of the documentation is a bit confusing, and there are several versions depending on the system version, so below is some clarification of the confusing part. You still need to refer to the docs.

The arguments to the system call are:

fd of directory (which you get by using the "open" system call).
address of a buffer to which the system call should transfer the data.
size of buffer (how many bytes of directory information will be provided, at most).

Returned value is how many bytes of information were transfered - less or equal to the 3rd argument. It will be less if the total directory information is less than the buffer size.

The data transfered to the buffer is a "list" (i.e. a sequence of bytes containing one structure followed by the next, etc.) of directory entry structures, where each structure is as follows:

dword: inode number (which you don't need in this exercise, but is there)
dword: index of next valid entry in directory, where the first is 0 (you will probably have no need for this one). Note - previous version of this was incorrect, and also the relevant Linux man page is incorrect (which is why I was also confused) - good thing you don't need this item anyway.
word: Size of this entry, including the file name, this item, and the above two. This item is important for the exercise.
variable sized data: null terminated string - the file name. Note that the space USED for this entry is always a multiple of 4, so in some cases there will be "junk" characters after the null character.