Sprint-Challenge: Intro to C and Operating Systems

Complete both challenges, below.

The Challenge 2 code needs a Unix-like environment to work! That includes Linux, macos, Cygwin, WSL, BSD, etc.

If you want to test if your environment is set up for it, compile and run the testdir.c program in the examples/ directory. (You can type make in the examples/ directory.) It should print Testing: PASS.

Challenge 1

Name at least three things that an operating system is responsible for handling?

Challenge 2

Write a program in C, lsls.c, that prints out a directory listing for the directory the user specifies on the command line. If the user does not specify a directory, print out the contents of the current directory, which is called ..

Example runs:

$ ./lsls
.
..
lsls.c
lsls

$ ./lsls /home/exampleuser
.
..
.config
.vim
.yarnrc
.bashrc
foo.c
.vscode
Downloads
.gitconfig
.bash_history
.viminfo
src

Hint: Start by just printing out the contents of the current directory ., and then add the command line parsing later after you have it working.

General approach

You are expected to use Google to find examples of how to use these functions. Also see Details, below.

1. Call opendir().
2. Then repeatedly call readdir()--printing the filenames as you go--until it lets you know there are no more directory entries by returning NULL.
3. Then call closedir().

You don't have to write the three functions, above. They're system calls built into the OS.

Details

You will be using functionality included in <dirent.h>. This header file holds the declarations for DIR, struct dirent, opendir(), readdir(), and closedir(), below.

• DIR *opendir(char *path): This function opens the directory named in path (e.g. .) and returns a pointer to a variable of type DIR that will be used later. If there is an error, opendir() returns NULL.

  You should check for errors. If there is one, print an error message and exit (using the exit() function).

• struct dirent *readdir(DIR *d): Reads the next directory entry from the DIR* returned by opendir(). Returns the result as a pointer to a struct dirent (see below). Returns NULL if there are no more directory entires.

• closedir(DIR *d): Close a directory (opened previously with opendir()) when you're done with it.

The struct dirent * returned by readdir() has the following fields in it:

struct dirent {
  ino_t  d_ino       // file serial number
  char   d_name[]    // file name, a string
};

(You don't need to declare this struct dirent type. It's already included in <dirent.h>.)

For output, you should print the field d_name from your struct dirent * variable, e.g.

struct dirent *ent;

// ... some of your code ...

ent = readdir(d);

printf("%s\n", ent->d_name);

To parse the command line, you'll have to look at argc and argv specified in your int main(int argc, char **argv) function. Example code to print all command line arguments can be found in commandline.c. Modify that example to look at the command line parameters, if any, and pass those to opendir().

Stretch Goal: Print file size in bytes

Modify the program to print out the file size in bytes as well as the name.

Example output (suggestion: use %10lld to print the size in a field of width 10):

$ ./lsls
    224  .
    992  ..
   1722  lsls.c
   8952  lsls

You'll need to use the stat() call in <sys/stat.h>.

• int stat(char *fullpath, struct stat *buf): For a given full path to a file (i.e. the path passed to opendir() following by a / followed by the name of the file in d_name), fill the fields of a struct stat that you've pointed to. Returns -1 on error.

  // Example stat() usage
  
  struct stat buf;
  
  stat("./lsls.c", &buf);
  
  printf("file size is %lld\n", buf.st_size);

Stretch Goal: Mark Directories

Instead of a size in bytes for a directory (which is marginally useful), replace the number with the string <DIR>.

Example output:

$ ./lsls
     <DIR>  .
     <DIR>  ..
      1717  lsls.c
      8952  lsls

The st_mode field in the struct stat buffer holds information about the file permissions and type of file.

If you bitwise-AND the value with S_IFDIR and get a non-zero result, the file is a directory.

(If you bitwise-AND the value with S_IFREG and get a non-zero result, the file is a regular file, as opposed to a device node, symbolic link, hard link, directory, named pipe, etc.)