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Operating system services

Operating system services. Program execution I/O operations File-system manipulation Communications Error detection Resource allocation Accounting Protection. System Calls.

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Operating system services

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  1. Operating system services • Program execution • I/O operations • File-system manipulation • Communications • Error detection • Resource allocation • Accounting • Protection

  2. System Calls • Provide an interface between a process and the operating system. Generally available as assembly-language instructions • UNIX system calls maybe invoked directly from a C or C++ program • System calls for modern Microsoft Windows platforms are part of the Win32 application programmer interface (API), which is available for use by all the compilers written for Microsoft Windows

  3. Example of how system calls are used… • Consider writing a simple program to read data from one file and to copy them to another file • What is the first thing the program needs? • Which would be the process? Details in the lecture…

  4. Passing parameters to the operating system • The simplest approach is to pass parameters in registers. In some cases, there may be more parameters than registers. In these cases, the parameters are generally stored in a block or table in memory, and the address of the block is passed as a parameter ina register. This is the approach taken by LINUX. Parameters can also be placed, or pushed onto a stack by the program, and popped off the stack by the operating system. Some operating systems prefer the stack or block method, because they do not limit the number or length of parameters being passed.

  5. Process control End, abort Load, execute Create process, terminate Get, set attributes Wait for time Wait event, signal event Allocate free memory File management Create, delete file Open, close Read, write, reposition Get, set file attributes Device management Request, release device Read, write Get, set device attributes Logically attch, detach devices Information maintenance Get, set time/date Get, set system data Get, set process, file or device attribute Communications Create, delete communciation connection Send, receive messages Transfer status information Attach or detach remote devices Groups of system calls

  6. SYSTEM CALLS FOR THE FILE SYSTEM

  7. System Calls • System calls that return file descriptors for use in other system calls • System calls that use the namei algorithm to parse a path name • System calls that assign and free inodes, using algorithms ialloc and ifree • System calls that set or change the attributes of a file • System calls that do I/O to and from a process, using algorithms alloc, free and the buffer application algorithms • System calls that change the structure of the file system • System calls that allow a process to change its view of the fiel system tree

  8. OPEN • The open system call is the first step a process must take to access the file • open(pathname,flags,modes) Algorithm open Inputs: file name, type of open, file permissions (for creation type of open) Output: file descriptor { convert file name to inode (algorithm namei); if (file does not exist or not permitted access) return (error); allocate the file table entry for inode, initialise count, offset; allocate user file descriptor entry, set pointer to file table entry; if (type of open specifies truncate file) free all file blocks (algorithm free); unlock (inode); /*locked above in namei */ return (user file descriptor); }

  9. Data structures after OPEN User file descriptor table file table inode table 0123. . ……………. .. . . Count 2 (/etc/passwd) Count 1 Read Count 1 Rd-Wrt Count 1 (local) Count 1 Write

  10. Data structures after two processes OPEN User file descriptor table (proc A) 0123. . ……………. file table inode table .. . . Count 2 (/etc/passwd) Count 1 Read User file descriptor table (proc B) Count 1 Rd-Wrt Count 1 (local) 0123. . ……………. Count 1 Read Count 1 (private) Count 1 Write .. . . Count 1 Read

  11. READ algorithm read inputs: user file descriptor, address of buffer in user process, number of bytes to read output: count of bytes copied into user space { get file table entry from user file descriptor; check file accessibility; set parameters in u area for user address, byte count, I/O to user; get inode from file table; lock inode; set byte offset in u area from file table offset; while (count not satisfied) { convert file offset to disk block (algorithm bmap); calculate offset into block, number of bytes to read; if (number of bytes to read is 0) break; read block (algorithm breada if with read ahead, algorithm bread otherwise); copy data from system buffer to user address; update u area fields for file byte offset, read count, address to write into user space; release buffer; } unlock inode; update file table offset for next read; return (total number of bytes read); } read (fd,buffer,count)

  12. WRITE Reading a file via two descriptors: #include <fcntl.h> main() { int fd1, fd2; char buf1[512], buf2[512]; fd1 = open(“/etc/passwrd”, O_RD); fd2 = open(“/etc/passwrd”, O_RD); read(fd1,buf1,sizeof(buf1)); read(fd2,buf2,sizeof(buf2)); } write (fd,buffer,count)

  13. Tables after CLOSING a file User file descriptors 0123. . ……………. file table inode table .. . . Count 2 (/etc/passwd) Count 1 Count 1 Count 1 (local) 0123. . ……………. Count 1 Count 1 (private) Count 1 .. . . Count 1

  14. FILE CREATION algorithm creat input: file name, permission settings output: file descriptor { get inode for file name (algorithm namei); if (file already exists) { if (not permitted access) { release inode (algorithm iput); return (error); } } else /*file does not exist yet */ { assign free inode from file system (algorithm ialloc); create new directory entry in parent directory; include new file name and newly assigned inode number; } allocate file table entry for inode, initialise count; if (file did exist at time of create) free all file blocks (algorithm free); unlock (inode); return (user file descriptor); }

  15. chdir, chroot • chdir(pathname) • chroot(pathname) • chown(pathname, owner, group) • chmod(pathname, mode)

  16. PIPES • Pipes allow transfer of data between processes in a FIFO manner. • Named & unnamed pipes • Pipe System call: pipe(fdptr) • Opening a named pipe • Reading and writing pipes • Closing pipes

  17. MOUNT - UNMOUNT • The mount system call connects the file system in a specified section of a disk to the existing file system hierarchy • The unmount system disconnects a file system from the hierarchy • mount(special pathname, directory pathname, options) • unmount(special pathname)

  18. File System tree before and after Mount / Root file system bin etc usr . . . cc date sh getty passwd /dev/dsk1 file system / bin include src uts awk banner yacc stdio.h

  19. LINK - UNLINK • The link system call links a file to a new name in the file system directory structure, creating a new directory entry for an exitsing node • link(source file name, target file name) • The unlink system call removes a directory entry for a file • unlink(pathname)

  20. FILE SYSTEM MAINTENANCE • The kernel maintains consistency of the file system during normal operation. • The command fsck checks for consistency and repairs the file system in case of extraordinary circumstances (power failure)

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