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What is the purpose of System Calls?
The interface between a process and an operating system is provided by system calls. In general, system calls are available as assembly language instructions. They are also included in the manuals used by the assembly level programmers. System calls are usually made when a process in user mode requires access to a resource. Then it requests the kernel to provide the resource via a system calls.
In general, system calls are required in the following situations −
- If a file system requires the creation or deletion of files. Reading and writing from files also require a system call.
- Creation and management of new processes.
- Network connections also require system calls. This includes sending and receiving packets.
- Access to a hardware device such as a printer, scanner etc. requires a system call.
Types of System Calls
There are mainly five types of system calls. These are explained in detail as follows -
Process Control
These system calls deal with processes such as process creation, process termination etc.
File Management
These system calls are responsible for file manipulation such as creating a file, reading a file, writing into a file etc.
Device Management
These system calls are responsible for device manipulation such as reading from device buffers, writing into device buffers etc.
Information Maintenance
These system calls handle information and its transfer between the operating system and the user program.
Communication
These system calls are useful for interprocess communication. They also deal with creating and deleting a communication connection.
Some of the examples of all the above types of system calls in Windows and Unix are given as follows -
| Types of System Calls | Windows | Linux |
|---|---|---|
| Process Control | CreateProcess() ExitProcess() WaitForSingleObject() | fork() exit() wait() |
| File Management | CreateFile() ReadFile() WriteFile() CloseHandle() | open() read() write() close() |
| Device Management | SetConsoleMode() ReadConsole() WriteConsole() | ioctl() read() write() |
| Information Maintenance | GetCurrentProcessID() SetTimer() Sleep() | getpid() alarm() sleep() |
| Communication | CreatePipe() CreateFileMapping() MapViewOfFile() | pipe() shmget() mmap() |
Purpose of System Calls
There are many different system calls as shown above. The purpose of some of those system calls is as follows -
open()
The open() system call is used to provide access to a file in a file system. This system call allocates resources to the file and provides a handle that the process uses to refer to the file. A file can be opened by multiple processes at the same time or be restricted to one process. It all depends on the file organization and file system.
read()
The read() system call is used to access data from a file that is stored in the file system. The file to read can be identified by its file descriptor and it should be opened using open() before it can be read. In general, the read() system calls takes three arguments i.e. the file descriptor, the buffer which stores read data and the number of bytes to be read from the file.
write()
The write() system call writes the data from a user buffer into a device such as a file. This system call is one of the ways to output data from a program. In general, the write() system calls takes three arguments i.e. the file descriptor,the pointer to the buffer where data is stored and the number of bytes to write from the buffer.
close()
The close() system call is used to terminate access to a file system. Using this system call means that the file is no longer required by the program and so the buffers are flushed, the file metadata is updated and the file resources are de-allocated.
wait()
In some systems, a process may wait for another process to complete its execution. This happens when a parent process creates a child process and the execution of the parent process is suspended until the child process executes. The suspending of the parent process occurs with a wait() system call. When the child process completes execution, the control is returned back to the parent process.
This system call runs an executable file in the context of an already running process. It replaces the previous executable file. This is known as an overlay. The original process identifier remains since a new process is not created but data, heap, stack etc. of the process are replaced by the new process.
fork()
Processes use the fork() system call to create processes that are a copy of themselves. This is one of the major methods of process creation in operating systems. When a parent process creates a child process and the execution of the parent process is suspended until the child process executes. When the child process completes execution, the control is returned back to the parent process.
exit()
The exit() system call is used by a program to terminate its execution. In a multithreaded environment, this means that the thread execution is complete. The operating system reclaims resources that were used by the process after the exit() system call.
kill()
The kill() system call is used by the operating system to send a termination signal to a process that urges the process to exit. However, kill() system call does not necessarily mean killing the process and can have various meanings.
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