What Is a System Call?

by Carson

Did you know how apps do basic tasks, like reading and writing data, copying files, or creating new processes? It all comes down to hardware, but it’s a system call at the kernel level. Let’s find out how they work and why they’re essential in this article.

The Structure of System Calls

System calls are simple. They are sent by processes, which are then interpreted by the kernel. It then activates the right set of drivers for the hardware action to be taken. Essentially, it’s an API (application programming interface), but dealing with things at a much lower level than the typical APIs that send data over the Internet.

These functions add another layer to the already numerous layers of code that get involved when writing a program. So what’s the point of doing this? Part of the reason has to do with permissions. The apps you use and programs you run operate in user space, which can only access things that the owner of the process can do. That’s a good thing because it keeps the system more secure by not allowing apps to manipulate the kernel maliciously. But many of the actions by these user-space processes need to access the kernel space, which controls everything from system files to process scheduling. Thus, we need a way to reach kernel space from user space safely, and the way to do it is a system call. If the kernel disallows the action, it returns an error on the system call instead of letting it complete.

Also, the other part of the reason deals with development efficiency. Apps run on different hardware, and you need to account for every possible hardware configuration for the app to work for all users. Just imagine how inefficient it would be if the developers had to write and test hundreds of assembly code files for every app they write! Instead, this task is handled by the drivers, which act as the interface between hardware and software. They receive signals according to OS specifications, making OS development easier. This means developers invoke system calls to carry out actions on the hardware instead of writing all hardware interfaces themselves.

What Do System Calls Do?

You might not be familiar with kernel space, but there are many things that depend on it. To access these functionalities, we must issue system calls. They include:

  1. File system and I/O operations
  2. Process management
  3. Communication
  4. Basic information
1. File System and I/O Operations

Many of the basic operations on files require system calls to complete. They include:

  1. Reading or writing to files
  2. Copying files
  3. Renaming or moving files
  4. Deleting or creating files
  5. Reading or writing data to files
  6. Applying and modifying file permissions
2. Process Management

If a program wants to deal with the resources that it has, it involves dealing with the process scheduler or even the hardware. Therefore, a system call is also used to handle these things, which include:

  1. Creating and terminating processes
  2. Allocating and freeing memory
  3. Suspending processes and waiting for an event
3. Communication

Often, programs need to access the Internet. This is another common action that needs access to the hardware and is handled by system calls, too.

  1. Creating and closing connections with a remote server or device
  2. Send and receive data
  3. Connecting to a network
4. Basic Information

Last but not least, there are some basic pieces of information that programs need to access to operate correctly. Examples include:

  1. System time
  2. Basic properties of the operating system (e.g., version)
  3. File system type
  4. Hardware specifications


To conclude, a system call is a way for processes to safely and efficiently perform low-level functions in the operating system. An app requests this function through an API, which then gets sent to the kernel. If it permits, the kernel hands that task out to the drivers and the hardware, returning the output to the program. If you want us to include more details about this, please leave your suggestions in the comments below.

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