System Calls

System Calls

AU May-17, 22

• System calls provide the interface between a running program and the operating system. Any single CPU computer can execute only one instruction at a time. If a process is running a user program in user mode and needs a system service, such UTD as reading a data from a file, it has to execute a instruction to transfer control  to the operating system.

• Operating system provides services and system call provides interface to these services. System call is written in language C and C++ as routines. System calls are performed in a series of steps.

• System call is a technique by which a program executing in user mode can request the kernel's service.

•An application programmer interface is a function definition that specifies how to

obtain a given service.

• Fig. 1.12.1 shows the working of system call.

• When application program calls the stub, trap instruction is executed and CPU switches to supervisor mode. Each system call contains its identification number.

• OS maintains the table of system call number. Operating system executes the system call using that number.

• When the function completes, it switches the processor to user mode and then returns control to the user process.

A system call is an explicit request to the kernel mode via a software interrupt. When user mode process invokes a system call, the CPU switches to kernel mode and starts the execution of the kernel function.

• Making a system call is like making a special kind of procedure call. Only system call enters the kernel and procedure call does not enter into the kernel.

• Kernel implements many different types of system calls. The user mode process must pass a parameter called the system call number to identify the required system call. All system calls return an integer value. In the kernel, positive or 0 values denote a successful termination of the system call and negative values denote an error condition.

• An API does not necessarily correspond to a specific system calls.

1. API could offer its services directly in user mode.

2. Single API function could make several system calls.

3. Several API functions could make the same system call.

• API supports a set of functions for application programmer. It includes passing parameter to each function and return values. API used by application programmer are as follows:

1. Windows system: win32 API

2. POSIX system : POSIX API

3. Virtual machine: Java API.

• User function uses trap instruction for using kernel services. Application programmer uses ordinary procedure call. Operating system provides a library of user functions with name corresponding to each actual system call. Each of these stub function contains a trap to the operating system function.

• Trap is also called system call interface.

• Three general methods are used to pass parameters to the operating system.

a. Pass parameters in registers

b. Registers pass starting addresses of blocks of parameters

C. Parameters can be placed, or pushed, onto the stack by the program, and

popped off the stack by the OS

• Fig. 1.12.2 shows passing of parameters as a table

Classification of System Call

• System call is divided into following types:

1. File management                            2. Process management

3. Interprocess communication          4. I/O device management

5. Information processing and maintenance

1. File management

• File management system calls are create file, delete file, open file, close file, read file, write file, get and set file attribute.

• User can create a file using create() system call. File name with attributes are required for creating and deleting a file through system call. After creating  a file, user can performs various operations on the file.

• Read, write, reposition are the operation performed on the file. File is closed after finished using. Same type of operation is performed on directory.

• Every file has file attributes. File attributes includes name of file, type of file, accounting information etc. To perform any operation on the file, set file attribute and get file attribute executed to check the attributes.

2. Process management

• System calls for process management are create, terminate, load, execute, abort, set and get process attributes. Other system call for process management is wait for time, wait event, allocate and free memory.

• In some situation user want to terminate the currently running process abnormally then system call used. Other reasons for abnormal process termination are error message generated, memory dump, error trap.

• Operating system provides debugging facility to determine the problem of dump. Dump is written to secondary storage disk.

• Debugger is a one type of system program. It provides facility for finding and correcting bug.

3. Interprocess communication

• Pipe, socket, message passing and shared memory are used for interprocess communication. Send message, receive message, create and delete connection, attach remote device are the system calls used in interprocess communication. •Shared memory: A process uses memory for communication. One process will create a memory portion which other processes can access. A shared segment can be attached multiple times by the same process. Shared memory is the fastest form of IPC because data does not need to be copied between processes.

• A socket is a bidirectional communication device that can be used to communicate with another process on the same machine or with a process running on other machine.

• Message passing: Two processes communicate with each other by passing messages. Message passing is direct and indirect communication. Indirect communication uses mailbox for sending receiving message from other process.

4. I/O device management

• System calls for device management are request () device, release ( ) device, get of nail and set device attributes, read, write etc.

• Process needs several resources in its life time. When process request for resources, request is granted if it is free otherwise request is rejected. Once request is granted, control is transfer to the process.

5. Information processing and maintenance

• System calls for this category is set time and date, get time and date, get system data, get system data, get and set process, file and device.

• Most of the operating system provides system call for set and get the time and

date.

• This type of system call is used for transferring information from user to operating system and vice versa.

University Questions

1. Discribe system calls and system programs in detail with neat sketch.  AU:May-17,Marks 5

2. List and explain five types of system calls.  AU:May-22, Marks 6