# System Calls
**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.

Fork()
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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](https://www.csl.mtu.edu/cs4411.ck/www/NOTES/process/fork/create.html) -> great explainer on forking with C example



Links
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* https://www.man7.org/linux/man-pages/man2/syscalls.2.html contains a lot of information I should go through some time.