Lecture 1: Programming interface

Preparation

• Read OSPP: §3, The programming interface.
• Skim xv6: §0, Operating system interfaces.

Questions from last lecture

• how to do the labs on my Linux
  • install the stock gcc/gdb/binutils: apt-get or yum or pacman
  • compile our patched qemu - read the tools guide
  • or, simply install the stock qemu - easy but not recommended
• how to do the labs on my Mac
  • follow the tools guide
  • keep homebrew up to date
  • reinstall packages if some are incomplete
• why the kernel image is at 0x00100000 but the entry point is at 0x0010000c
  • check kern/entry.S
  • required by Multiboot
• is the BIOS-bootloader handover protocol the same for cdrom as for harddisk
  • no - see lab 1

Overview

• Recall JOS: print to screen via MMIO (0xb8000 on x86)
  • arguably the lowest-level print: compare to print in C, Java, Python, etc.
  • is it a good programming interface
• History: punch cards - see Holzmann’s To Code Is Human
• Unix system calls & files - see the readings page
• This lecture will use Linux as an example, via strace

Example: hello

$ strace -- echo hello

output:

execve("./a.out", ["./a.out"], [/* 23 vars */]) = 0
...
write(1, "hello\n", 6)                          = 6
...
exit_group(0)                                   = ?

• system calls: execve (first), write, exit_group (last)
• what’s the first argument of write
• what’s the relationship between printf and write
• what’s the first argument of exit_group

Example: uptime

$ strace -- uptime

output:

execve("/usr/bin/uptime", ["uptime"], [/* 23 vars */]) = 0
...
open("/proc/uptime", O_RDONLY)                         = 3
lseek(3, 0, SEEK_SET)                                  = 0
read(3, "1244101.16 14920448.20\n", 2047)              = 23
...
exit_group(0)                                          = ?

• system calls: open, lseek, read, …
• file: /proc/uptime - is this a real file on disk? (source code)
• other pseudo files: e.g., procfs
  • cat /proc/cpuinfo
  • cat /proc/iomem
  • compare to JOS’s output

Example: sh and uptime

$ strace -f -- sh -c uptime

output:

execve("/bin/sh", ["sh", "-c", "uptime"], [/* 23 vars */]) = 0
...
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f831c8ac9d0) = 24844
wait4(-1,  <unfinished ...>
[pid 24844] execve("/usr/bin/uptime", ["uptime"], [/* 23 vars */]) = 0
[pid 24844] exit_group(0)               = ?
[pid 24844] +++ exited with 0 +++
<... wait4 resumed> [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 24844
...

• system calls: clone (old days: fork), wait4
  • what do they do
  • how do they communicate
• child vs parent processes: what are copied/shared

Exercises

draw the system calls for the following

$ uptime | tr '[a-z]' '[A-Z]'
$ uptime > file

Summary & questions

• system calls (and “files”)
  • JOS exposes a lower-level set of system calls
  • JOS implements POSIX system calls (e.g., like in Linux) as user-space libraries
• why do we need system calls at all - just library functions?
• guess how strace is implemented