System calls are how the kernel interacts with the hardware. They are kernel entry points.
Want to open a file? Call open and then call read or write. Want to do some networking? Call connect to open a new connection and send to send something, recv to receive.
So, system calls are the operating system’s universal API.
Some (very little amount of) system calls:
open - opens a file (returns a file descriptor)
read - read bytes from fd (file, socket)
write - write to fd - stat - read file statistics
ioctl - set I/O properties
connect - connect to an IP address
accept - accept a network connection
sendto/recvfrom - using network: send to X, receive from X
socket - open network connections - fork - creates new process
mmap - map a file to the process memory address space (allocate memory for a file)
munmap - unmap the memory
brk - extend the heap pointer (allocate memory)
exec or execve - execute a new program (infect forked process)
System calls let userland programs interact with the OS by switching from user space to kernel space using software interrupts.
This system call is used to create processes identical to their parents. Every time a process requires a child (e.g the shell) it forks itself and, if required, calls execve to replace the child process with a new one.
The fork() System Call → great explainer on forking with C example