Lab6: UNIX System Calls

Due Date: Sun 29 March 2009 by 23:59

Background:

One of the key services provided by any operating system is the management of files. The C language meshes with UNIX nicely in this regard. In fact all UNIX system calls and most libraries are implemented in C. In this programming assignment we will implement a new version of the UNIX command ls. In the UNIX file system, a directory contains names and attributes of files stored in it. A number of UNIX library functions are available for accessing directories. To use any of them you need to include the header dirent.h

The function opendir with prototpe DIR *opendir(const char *name); opens the named directory and associates a directory stream with it. If the named directory cannot be accessed or there is not enough memory to hold the contents of the directory, opendir returns NULL. Once a directory stream is opened, the function readdir sequentially access its contents. The prototype of the readdir is struct dirent *readdir(DIR *dir);. This routine returns a pointer to the next directory entry of type struct dirent. The pointer value becomes NULL when it reaches the end of the directory.

Look at /usr/include/dirent.h for details on the struct dirent. Also, take a look at /usr/include/sys/stat.h for details on the struct stat. There are other UNIX system library routines such as getcwd which gets the current working directory. RTFM for more details (i.e., man 3 getcwd). I have provided two demo programs which make use of some of these calls, in addition to the code we started in lecture. A demo program showing how to use getcwd and search a directory is given in search.c. I've also provided the demo program filetimes.c that allows us to get the last modified time of a file. This can be modified to find the size of the file as well. For more info on what stat returns, check out man 2 stat.

Assignment:

The purpose of this program is to implement the UNIX command, ls. You must write a program named lab6.c (executable: myls) that does the following:

./myls dir_name -- lists all files in the named directory (default is alphabetical order by name)
./myls -s dir_name -- lists all files in the named directory sorted by size (largest first) with their sizes, one per line
./myls -t dir_name -- lists all files in the named directory sorted by time of last modification (most recent first) with their modification times, one per line
./myls foo+ dir_name -- lists all files in the named directory with the prefix foo in their filename in alphabetical order, one per line

Note . and .. should NOT be printed! Also, all commands must work if dir_name is not specified, in which case the current directory should be used, i.e., ./myls -s should list all the files in current directory sorted by size.

Implementation:

I would suggest using a dynamic array of structs to read and store the information in the directory and then manipulate this array to do the sorting and searching. One way to do this would be to read the directory to determine the number of files contained within, malloc the array of structs, and then use the rewinddir command to reposition to the front of the directory and read the information in. Of course, you may choose another implementation if you desire.

You may find the UNIX qsort utility handy for this assignment. We will discuss qsort and function pointers in lecture on Tuesday. For more information, RTFM. As above, you are not required to use qsort to do the sorting...

Handing in your Solution

Your solution should be in the form of one .c file. Just hand in the .c file and your design document for your program.

Don't hand in any input or .o or exe files.