System Calls


In computing, a system call (often abbreviated to syscall) is a mechanism that allows a user-space program to request a service from the operating system's kernel. This provides a controlled interface for programs to access hardware resources (such as disks, network devices, or the display), communicate with other processes, create new processes, and manage system resources.

How System Calls Work

  • Trap or Software Interrupt: The user-space program initiates a system call by executing a special instruction (often called a trap or software interrupt). This instruction causes the processor to switch from user mode to kernel mode, granting the operating system temporary control.
  • System Call Number: Each system call is identified by a unique number. The program places this number, along with any necessary arguments, in specific registers or memory locations as defined by the system's Application Binary Interface (ABI).
  • Kernel Execution: The kernel examines the system call number, retrieves the arguments, and performs the requested operation. This might involve accessing hardware, modifying system data structures, or interacting with other processes.
  • Return to User Mode: Once the operation is complete, the kernel prepares a return value (which could indicate success, failure, or data requested by the program), places it in the appropriate location, and switches the processor back to user mode.
  • Result Handling: The user-space program receives the return value and continues its execution.

Types of System Calls

Some common categories of syscalls include:

  • File I/O: Syscalls for opening, closing, reading, and writing files (open, close, read, write)
  • Process Management: Syscalls for creating, terminating, and controlling processes (fork, execve, waitpid, kill)
  • Memory Management: Syscalls for allocating and deallocating memory (mmap, munmap, brk)
  • Network Communication: Syscalls for creating sockets, sending and receiving data (socket, bind, connect, send, recv)
  • Device Management: Syscalls for interacting with devices like the display, keyboard, or disk drives.

Importance of System Calls

  • Abstraction: Syscalls provide a standardized interface for programs to interact with the operating system, shielding applications from the complexities of low-level hardware management.
  • Security: By controlling access to system resources through syscalls, the kernel enforces security and prevents programs from directly manipulating hardware or interfering with each other.
  • Efficiency: Syscalls are generally optimized by the operating system, making resource access more efficient than if each program implemented its own low-level mechanisms.