Team Members:
	Sumit Bindal		(109913379)
	Moiz Ali			(110100333)
	Pratik Bagaria		(110013587)

We have implemented preemptive os (70 marks project)

Readme:

Features:
- COW Fork
- Preemptive Scheduler
- On demand heap

List of Binaries created to test the system calls:
1)fork: to test fork and execve
2)listdir: to test opendir, readdir and closedir
3)open: to test open, read and lseek 
4)ps, pwd: To display process and directory info.
5)ls: for listing the files of a directory
6)echo: to echo on a terminal
7)pipe: to test pipe use the normal pipe symbol to give commands in the shell: Below is an example to test0 3 level pipe in our shell:
 ls | scan_pipe | scan_pipe2
  ls|scan_pipe

Working flow of fork():
	 In fork after the child is created, we first resume the parent process and then the child process is scheduled.

Limitations:
- We disable scheduler while performing scan
- argv, envp handling for user processes

List of SYSTEM CALLS	    
    SYS_exit()
    Syntax       :  exit(int status)
    Description  :  The exit() function causes normal process termination        


	
    SYS_brk()
    Syntax       : int brk(void *addr)
    Description  : brk() sets the end of the data segment to the value specified by addr
    return       : On success, brk() the base pointer from where the memory was allocated. On error 0 is returned.


    SYS_fork()
    Syntax       :  pid_t fork (void); 
    Description  :  Whenever called a child process is created. It returns pid = 0 for child process and non zero for parent process.
    return       :  This call returns the pid for parent process or 0 for the child  process.
    


    SYS_getpid()
    Syntax       :  pid_t getpid(void)
    Description  :  getpid() returns the process ID of the calling process.
    
    SYS_getppid()
    Syntax       :  pid_t getppid(void)
    Description  :  getppid() returns the process ID of the parent of the calling process.


    SYS_execve()
    Syntax       :  int execve(const char *filename, char *const argv[], char *const envp[])
    Description  :  execve() executes the program pointed to by filename along with any necessary arguments argv[] in the environment envp[].
    return       :  This call returns -1 on failure, and 0 on success
    
    SYS_waitpid()
    Syntax       :  pid_t waitpid(pid_t pid, int *status, int options)
    Description  :  It suspends execution of the calling process until a child specified by pid argument has changed state.
    return       :  On success, returns the process ID of the child whose state has changed; on error -1 is returned.

    
    SYS_nanosleep()
    Syntax       :  int sleep(unsigned int seconds)
    Description  :  sleep() makes the calling thread sleep until seconds seconds have elapsed
    return       :  Zero if the requested time has elapsed.



   SYS_open()
    Syntax       :  int open(const char *pathname, int flags)
    Description  :  Whenever called, a file pointed by the pathname is opened and a file descriptor is returned. It opens files from 'tarfs'.
    Dependencies :  The flags can be one of the following: O_RDONLY = 0, O_WRONLY = 1, O_RDWR = 2, O_CREAT = 0x40, O_DIRECTORY = 0x10000
    return       :  This call returns file descriptor number on success and  returns 0 on failure.


    SYS_read()
    Syntax       :  uint64_t read(int fd, void *buf, uint64_t count)
    Description  :  read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf.
    Dependencies :  Before read, always open the file first and after read is finished close the file.
    return       :  Returns 0 on failure and on success returns number of bytes read into buf.


    SYS_write() 
    Syntax       :  uint64_t write(int fd, const void *buf, uint64_t count)
    Description  :  write() writes up to count bytes from the buffer pointed buf to the file referred to by the file descriptor fd.
    return       :  On success, the number of bytes written is returned or 0 on error
       


    SYS_lseek()
    Syntax       :  off_t lseek(int fildes, off_t offset, int whence)
    Description  :  The lseek() function repositions the offset of the open file associated with the file descriptor fd to the argument offset
                    according to the directive whence
    Dependencies :  Values of whence could be any of the following:
       SEEK_SET
              The offset is set to offset bytes.
       SEEK_CUR
              The offset is set to its current location plus offset bytes.
       SEEK_END
              The offset is set to the size of the file plus offset bytes.
    return      : Upon successful completion, lseek() returns the resulting offset location from the beginning of the file else -1 on error.


    SYS_close()
    Syntax       : int close(int fd)
    Description  : It closes a file descriptor, so that it no longer refers to any file.
    return       : returns 1 on success. On error, 0 is returned

    SYS_pipe()
    Syntax       :int pipe(int filedes[])
    Description  : pipe() creates a pipe, a unidirectional data channel that can be used for interprocess communication. The array pipefd is used to return two file descriptors referring to the ends of the pipe. pipefd[0] refers to the read end of the pipe. pipefd[1] refers to the write end of the pipe. Data written to the write end of the pipe is buffered by the kernel until it is read from the read end of the pipe.
    return       :On success, zero is returned. On error, 1 is returned

    SYS_dup2()
    Syntax       :int dup2(int oldfd, int newfd)
    Description  :makes newfd be the copy of oldfd, closing newfd first if necessary
    return       :On success, it returns the new descriptor. On error, -1 is returned,


    SYS_getdents() 

    Syntax       : int getdents(unsigned int fd, struct linux_dirent *dirp,unsigned int count)
    Description  : It reads several linux_dirent structures from the directory referred to by the open file descriptor fd into the
       buffer pointed to by dirp.The argument count specifies the size of that buffer.
    Dependency   :getdents is called by opendir
    return       : On success, the number of bytes read is returned.