System calls operating system ppt by rohit malav
- 1. 1 System Calls and I/O SUBMITTED BY:- ROHIT MALAV SID :- 38725
- 2. BASIC TOPICS Goals Get you familiar with necessary basic system & I/O calls to do programming Things covered Basic file system calls I/O calls Signals
- 3. System Calls 3 System Calls A request to the operating system to perform some activity System calls are expensive The system needs to perform many things before executing a system call The computer (hardware) saves its state The OS code takes control of the CPU, privileges are updated. The OS examines the call parameters The OS performs the requested function The OS saves its state (and call results) The OS returns control of the CPU to the caller
- 4. Examples of System Calls 4 Examples getuid() fork() exec() //get the user ID //create a child process //executing a program
- 5. Major System Calls 5 Process Management pid = fork( ) Create a child process identical to the parent pid = waitpid(pid, &statloc, options) Wait for a child to terminate s = execve(name, argv, environp) Replace a process’ core image exit(status) Terminate process execution and return status File Management Today fd = open(file, how, ...) Open a file for reading, writing or both s = close(fd) Close an open file n = read(fd, buffer, nbytes) Read data from a file into a buffer n = write(fd, buffer, nbytes) Write data from a buffer into a file position = lseek(fd, offset, whence) Move the file pointer s = stat(name, &buf) Get a file’s status information
- 6. Directory and File System Management s = mkdir(name, mode) Create a new directory s = rmdir(name) Remove an empty directory s = link(name, name) Create a new entry, name, pointing to name s = unlink(name) Remove a directory entry s = mount(special, name, flag) Mount a file system s = umount(special) Unmount a file system Miscellaneous s = chdir(dirname) Change the working directory s = chmod(name, mode) Change a file’s protection bits s = kill(pid, signal) Send a signal to a process seconds = time(&seconds) Get the elapsed time since January 1, 1970 Major System Calls
- 7. File System and I/O Related System Calls Copyright ©: University of Illinois CS 241 Staff 7 A file system A means to organize, retrieve, and updated data in persistent storage A hierarchical arrangement of directories Bookkeeping information (file metadata) File length, # bytes, modified timestamp, etc Unix file system Root file system starts with “/”
- 8. Why does the OS control I/O? 8 Safety The computer must ensure that if a program has a bug in it, then it doesn't crash or mess up The system Other programs that may be running at the same time or later Fairness Make sure other programs have a fair use of device
- 9. Basic Unix Concepts Input/Output – I/O Per-process table of I/O channels Table entries describe files, sockets, devices, pipes, etc. Table entry/index into table called “file descriptor” Unifies I/O interface user space pipe file socket Per-process file descriptor table kernel system open file table 9
- 10. System Calls for I/O Copyright ©: University of Illinois CS 241 Staff 10 Get information about a file int stat(const char* name, struct stat* buf); Open (and/or create) a file for reading, writing or both int open (const char* name, in flags); Read data from one buffer to file descriptor size_t read (int fd, void* buf, size_t cnt); Write data from file descriptor into buffer size_t write (int fd, void* buf, size_t cnt); Close a file int close(int fd);
- 11. System Calls for I/O Copyright ©: University of Illinois CS 241 Staff 11 They look like regular procedure calls but are different A system call makes a request to the operating system by trapping into kernel mode A procedure call just jumps to a procedure defined elsewhere in your program Some library procedure calls may themselves make a system call e.g., fopen() calls open()