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, MinGW, 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. It should print Testing: PASS.

Challenge 1

What are some of the tasks that a general operating system is responsible for handling?

Describe in as much detail as you can how these tasks are handled by the operating system.

The general tasklist that the OS is responsible for includes:

Processor Management. In order to manage multiple programs, the OS has to switch between processes thousands of time per second. Each process occupies an amount of RAM, as well as registers, stacks and queues in the OS memory space. When two processes multitask, the OS gives a certain number of CPU cycles to each process. After those cycles, the OS copies the state of the process (called the process control block) before switching to the next process. It then loads the state of the next process. Managing the hardware's resources(processors, memory, storage, I/O).
Memory Management. On boot, the OS goes to the top of memory and backs up far enough to meet the OS needs. It then goes to the bottom of memory and starts building device driver memory. The rest of the remaining memory can be used for application processes. As applications start, the OS assigns block sizes and boundaries for each, making sure apps can't overwrite each others space.
Device Management. The OS translates the high-level programming of the apps to the electrical signals of the various devices used by the computer through translators called drivers. Since the OS has no way to know where to place the bits and bytes needed by the different pieces of hardware, the driver provides that bridge.
APIs. APIs are what allow the applications installed to access the functions of the hardware and OS without direct manipulation. The system is essentially split between a user mode (the user and applications) and kernel mode (the OS down to hardware). The basic flow is that the user makes a request to an application, which uses a set of library routines to make system calls to the OS, which then processes that request to the hardware.
User Interface. One of the most important features of any OS is providing a user friendly means for the user to interact with the system. This also forces structure upon the user's interaction. With the modern OS, this is typically a GUI (Graphical User Interface);

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

To parse the command line, you'll have to look at argc and argv specified in your int main(int argc, char **argv). Example code to print all command line arguments can be found in commandline.c.

General approach: Call opendir(), then repeatedly call readdir()--printing the filenames as you go--until it says there are no more entries, then call closedir()

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 and returns a pointer to a variable of type DIR that will be used later. If there is an error, opendir() returns NULL.
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
};

For output, you should print the field d_name.

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, 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.)

justinborek/Sprint-Challenge--Intro-C-OS

Sprint-Challenge: Intro to C and Operating Systems

Challenge 1

Challenge 2

Stretch Goal: Print file size in bytes

Stretch Goal: Mark Directories