System calls are interfaces provisioned by the operating system to allow user-level applications to interact with low-level hardware components & make use of all the services provided by the kernel, which is a core component and the heart of an operating system that manages all the hardware and the services provided by the OS. These system calls are essential for every process to interact with the kernel and leverage the services provided by it.
System calls are basically an interface between a process and the operating system. And they’re the only way to switch from user mode to kernel mode.
Pre-requisite: Introduction to System Calls
Types of System Calls
Services provided by an OS are typically related to any kind of operation that a user program can perform like creation, termination, forking, moving, communication etc. Similar types of operations are grouped into one single system call category. System calls are classified into the following categories :
Types of System Calls
1. File System Operations
These system calls are made while working with files in OS, File manipulation operations such as creation, deletion, termination etc.
- open(): Opens a file for reading or writing. A file could be of any type like text file, audio file etc.
- read(): Reads data from a file. Just after the file is opened through open() system call, then if some process want to read the data from a file, then it will make a read() system call.
- write(): Writes data to a file. Wheneve the user makes any kind of modification in a file and saves it, that’s when this is called.
- close(): Closes a previously opened file.
- seek(): Moves the file pointer within a file. This call is typically made when we the user tries to read the data from a specific position in a file. For example, read from line – 47. Than the file pointer will move from line 1 or wherever it was previously to line-47.
Algorithm for Reading a File:
algorithm read
input: 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);
}
2. Process Control
These types of system calls deal with process creation, process termination, process allocation, deallocation etc. Basically manages all the process that are a part of OS.
- fork(): Creates a new process (child) by duplicating the current process (parent). This call is made when a process makes a copy of itself and the parent process is halted temporarily until the child process finishes its execution.
- exec(): Loads and runs a new program in the current process and replaces the current process with a new process. All the data such as stack, register, heap memory everything is replaced by a new process and this is known as overlay. For example, when you execute a java byte code using command – java “filename”. Then in the background, exec() call will be made to execute the java file and JVM will also be executed.
- wait(): The primary purpose of this call is to ensure that the parent process doesn’t proceed further with its execution until all its child processes have finished their execution. This call is made when one or more child processes are forked.
- exit(): It simply terminates the current process.
- kill(): This call sends a signal to a specific process and has various purpose including – requesting it to quit voluntarily, or force quit, or reload configuration.
3. Memory Management
These types of system calls deals with memory allocation, deallocation & dynamically changing the size of a memory allocated to a process. In short, the overall management of memory is done by making these system calls.
- brk(): Changes the data segment size for a process in HEAP Memory. It takes an address as argument to define the end of the heap and explicitly sets the size of HEAP.
- sbrk(): This call is also for memory management in heap, it also takes an argument as an integer (+ve or -ve) specifying whether to increase or decrease the size respectively.
- mmap(): Memory Map – it basically maps a file or device into main memory and further into a process’s address space for performing operations. And any changes made in the content of a file will be reflected in the actual file.
- munmap(): Unmaps a memory-mapped file from a process’s address space and out of main memory
- mlock() and unlock(): memory lock defines a mechanism through which certain pages stay in memory and are not swapped out to the swap space in the disk. This could be done to avoid page faults. Memory unlock is the opposite of lock, it releases the lock previously acquired on pages.
4. Interprocess Communication (IPC)
When two or more process are required to communicate, then various IPC mechanism are used by the OS which involves making numerous system calls. Some of them are :
- pipe(): Creates a unidirectional communication channel between processes. For example, a parent process may communicate to its child process through a pipe making a parent process as input source of its child process.
- socket(): Creates a network socket for communication. Processes in same or other networks can communicate through this socket, provided that they have necessary network permissions granted.
- shmget(): It is short for – ‘shared-memory-get’. It allows one or more processes to share a portion of memory and achieve interprocess communication.
- semget(): It is short for – ‘semaphore-get’. This call typically manages the coordination of multiple processes while accessing a shared resource that is, the critical section.
- msgget(): It is short for – ‘message-get’. IPC mechanism has one of the fundamental concept called – ‘message queue’ which is a queue data structure inside memory through which various processes communicate with each other. This message queue is allocated through this call allowing other processes a structured way of communication for data exchange purpose.
5. Device Management
The device management system calls are used to interact with various peripherial devices attached to the PC or even the management of the current device.
- SetConsoleMode(): This call is made to set the mode of console (input or output). It allows a process to control various console modes. In windows, it is used to control the behaviour of command line.
- WriteConsole(): It allows us to write data on console screen.
- ReadConsole(): It allows us to read data from console screen (if any arguments are provided).
- open(): This call is made whenever a device or a file is opened. A unique file descriptor is created to maintain the control access to the opened file or device.
- close(): This call is made when the system or the user closes the file or device.
Conclusion
System calls are a great way to take full advantage of the services provided by the kernel. Every system call is made in background completely hidden from the user. And every action you perform in a computer like creating, deleting, copy and other operations have many number of system calls done in background which are hidden from the user for simplicity.
- System calls define an interface between the user-level applications and the kernel.
- These are example of some of the most common system calls, however there are many system calls, some of them even specific to an Operating System.
- The system calls name may vary depending upon the OS but the overall functionality will be the same.
- System calls provide a high-level abstraction and simplifies the process of interacting with low level hardware components which the computer users don’t typically have to worry about.
- System calls are implemented in C-Language.
FAQs on System Calls
Q.1: What is the purpose of System Calls?
Answer:
The purpose of the system calls is to allow user-level applications access of the services provided by the kernel. The user apps do not have the privelege to perform operations, so they make system calls which further requests a kernel to provide a specific service.
Q.2: How do device management System Calls work?
Answer:
Device management system calls work by allowing a certain process or a device interact and get access to other hardware resources and perform various operations.
Q.3: What is user mode and kernel mode?
Answer:
User mode and kernel modes are two different privilege modes of a Computer System, that separates the execution of operations by the user applications and the kernel on hardware. This separation provides security, stability, and a level of control over system resources.
Q.4: What happens when a System Call is executed?
Answer:
When a system call is executed, a context switch occurs and the computer system switches from user mode to kernel mode and now the kernel performs the desired operation.
Q.5: What is a software interrupt?
Answer:
A software interrupt is a mechanism through which the OS performs a context switch and transition from user mode to kernel mode. A software interrupt use is not limited only to the system calls but it can also be made when a high priority task is required to be executed by the CPU.