• System calls are functions implemented by the Kernel, but are meant to be called in user space.
  • Kernel 5.3 for example has 340 system calls.
  • When have the Linux Kernel source code, can look at the include/uapi/asm-generic/unistd.h file to see all of the system calls.
  • asm is Assembly Langauge.
  • System calls vary between architecture, x86-64 and ARM changes.
  • Functions users can call are in the man pages.
  • For example, if you look up read in the manual, it is called through the standard library (libc)
  • Standard library uses architecture-dependent means to invoke the system call mechanism.
  • Protocol and Library which tells the Kernel which system call to make.
  • Suitable sised parameters are placed into registers.
  • Kernel is invoked via the library, determines the system call and then calls it.
  • If there is an error, the system call returns a negative value (function in the Kernel) to the library.
  • If that value is negative, the library is going to set a global variable in the process address space. This error is called errorno to abs (absolute value or return value) and returns -1 back to the program. The error number provided has information on what the error is.
  • The library is what sets errorno.
    • When there is no error, the library does not set errorno and returns the value it obtained from the Kernel.
  • Useful shell shortcut –>
    • grep -i read !$
      • The !$ means it will read the last thing on the previous line.
  • If you download the Kernel source code, go to /usr/src/line-(number)/include/uapi/asm-generic. An example link is: 
    howard@skwigelf:/usr/src/linux-headers-5.15.0-100-generic/include/uapi/asm-generic
  • To get an estimate of the number of system calls, we can do the following in the above directory:
    • grep "define __NR" unistd.d
  • Don’t confuse user-space header file adnd Kernel space header files.
  • Can check the man page for read for further information.
  • System Call man pages have sections in the top-right hand corner, for example KILL(1) says we are in the first section of the kill command’s man page.
  • kill -9 will kill a process.
  • To go to another section in a man page, we perform man 2 kill for example.
  • RETURN VALUE is always listed in each library.

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