ntuck@neu
  1. Welcome
  2. Spring 2021
  3. CS3650 - Computer Systems
  4. 2020-01 Notes

./06-syscalls-intro-c/notes.md

Computer Systems

HW Questions?

  • HW03 is due soon.
    • All or nothing tests, so TAs will look for extra partial credit.
  • HW04 is available.

System Calls

  • We've been using the C standard library to access OS functionality, but that's just calling functions made of normal code.
  • In order to do things, these library functions need to call into the OS kernel.
  • This is done with the syscall instruction, which works a lot like the call instruction.
  • The Linux kernel exposes about 300 syscalls. List of Syscalls

Syscall Example

Standard file descriptors:

  • 0: stdin

  • 1: stdout

  • 2: stderr

  • Show lines.S

Programming in C

  • Let's write "prime", which prints the Nth prime number.
  • Top-down design.
    • Allocate an array for N primes.
    • Pre-load first 3 primes.
    • A number is-prime if it's not divisible by any prime up to sqrt(x).
  • Makefile first.
    • gcc command
    • clean rule
    • tabs
  • Specifics:
    • Local variables are "on the stack".
    • Can allocate arrays "on the stack".
    • Standard library functions and headers:
      • stdio.h: printf
      • stdlib.h: atol
      • math.h: ceil, sqrt -- show manpage, talk about libm
    • Casts. For numbers, these convert.
  • prime

Arrays and Pointers

  • Arrays in C are just like arrays in ASM. They're a contigous sequence of memory starting at some address.
  • Array variables store the address of the start of the array.
  • C knows how big the type stored in array is.
  • For arrays in ASM, we had to manually calculate the byte offsets.
  • An array of words in C would be xs[0], xs[1], etc.
  • Pointers are variables that store memory addresses with an associated type so "int*" (int pointer) is the address of an int.
  • C lets you do arthmetic with pointers.
    • If you add one to an int pointer, it steps to the next int-sized chunk of memory. So if p is a pointer to a 4 byte int, then (p + 1) is the address increased by 4 bytes.
    • Show sizeof: char, short, int, long, float, double, char*, long*
  • Array varibles and pointers are the same thing with different notation.
  • If pp is a pointer and aa is an array, both of the same type pointing to the same address, then pp[3] and *(pp + 3) are the same thing.
  • Strangely, pp[ii] and ii[pp] are the same thing too.
    • Why?

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