OS Organization

System Call

Below are Xv6 system calls. If not otherwise stated, these calls return 0 for no error, and -1 if there's an error.

System call	Description
int fork()	Create a process, return child’s PID.
int exit(int status)	Terminate the current process; status reported to wait(). No return.
int wait(int *status)	Wait for a child to exit; exit status in *status; returns child PID.
int kill(int pid)	Terminate process PID. Returns 0, or -1 for error.
int getpid()	Return the current process’s PID.
int sleep(int n)	Pause for n clock ticks.
int exec(char *file, char *argv[])	Load a file and execute it with arguments; only returns if error.
char *sbrk(int n)	Grow process’s memory by n bytes. Returns start of new memory.
int open(char *file, int flags)	Open a file; flags indicate read/write; returns an fd (file descriptor).
int write(int fd, char *buf, int n)	Write n bytes from buf to file descriptor fd; returns n.
int read(int fd, char *buf, int n)	Read n bytes into buf; returns number read; or 0 if end of file.
int close(int fd)	Release open file fd.
int dup(int fd)	Return a new file descriptor referring to the same file as fd.
int pipe(int p[])	Create a pipe, put read/write file descriptors in p[0] and p[1].
int chdir(char *dir)	Change the current directory.
int mkdir(char *dir)	Create a new directory.
int mknod(char *file, int, int)	Create a device file.
int fstat(int fd, struct stat *st)	Place info about an open file into *st.
int stat(char *file, struct stat *st)	Place info about a named file into *st.
int link(char *file1, char *file2)	Create another name (file2) for the file file1.
int unlink(char *file)	Remove a file.

• A process may create a new process using the fork syscall. fork creates a new process, called the child process, with exactly the same memory contents as the calling process, called the parent process. fork returns in the both the parent and the child. In the parent, fork returns the child's PID; in the child, fork returns 0.
• The exit system call causes the calling process to stop executing and to release resources such as memory and open files exit takes an integer status argument, conventionally 0 to indicate success and 1 to indicate failure.
• The wait system call returns the PID of an exited (or killed) child of the current process and copies the exit status of the child to the address passed to wait; if none of the caller’s children has exited, wait waits for one to do so. If the caller has no children, wait immediately returns -1. If the parent doesn’t care about the exit status of a child, it can pass a 0 address to wait.

Shell

The shell is an ordinary user program, not a part of kernel, that reads commands from the user and executes them.