Lecture: programming interface

preparation

• read the xv6 book: §0, Operating system interfaces

questions from last lecture

• labs 1–5 & X
• review exercise: boot xv6 (see also calling conventions from CSE 351)

overview

• recap: print to screen via MMIO (0xb8000 on x86)
• 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 -f -- 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

if you don’t see clone (possibly with old bash), try:

$ strace -f -- sh -c "uptime && true"

summary

• system calls (and “files”)
  • why do we need them at all - just library functions?
  • what is the “file” abstraction
    • as long as you can open/read/write/close
    • can support different “backends”: terminals, disk files, pipes, system information
• guess how strace is implemented

exercises

draw the system calls for the following (you can use strace but don’t have to):

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