CSCI 235 — Scope and Object files

This lab is going to cover Chapter 7.

Sort of.

Externals

• Download some software.
• Let’s take a look.
• Why is there a fetchandadd.h?
• Look at the Makefile .
• Make it.
• Run it.
• That’s not quite right. Why?
• Would a FORTRAN COMMON be useful here?
• What about a C extern?
• Let’s try out the nm program.
  • nm fauser.o
  • nm fetchandadd.o
• Where does accumulator appear in these nm listings?
• Put the keyword extern at the beginning of the accumulator declarator in fauser.c.
• Make it.
• Run it.
• !
• Retry nm on those two object files.
• Now also add the extern to the accumulator declaration in fetchandadd.c.
• make it.
• !!!!
• Re-retry nm on those two object files. In both accumulator is listed as U, for undefined.
• Remove the extern in fetchandadd.c , the one with FetchAndAdd.
• make and nm. Now the accumulator in fetchandadd.c is listed as C, for common. Sounds like FORTRAN.
• Initialize the accumulator in fetchandadd.c to 0.
• make and nm. Now the accumulator in fetchandadd.c is listed as B for uninitialized which is odd. However, all unitiailized data in the BSS is really initialized to zero.
• Initialize the accumulator in fetchandadd.c to 235.
• make and nm. Now the accumulator in fetchandadd.c is listed as D for initialized which is expected.
• Three next to-last changes:
  • Move both accumulator declarations back inside the functions, main and FetchAndAdd.
  • Remove the extern inside main.
  • make and try it out.
• Ugh. It always adds to 235.
• Two last changes:
  • Put the keyword static in front of the declaration of accumulator inside of FetchAndAdd.
  • make and try it out.
• Now accumulator retains its value between calls.
• Finally, try to determine the connection between the static of Java and C.

Memory

• Take a look at some examples of variable scope
• Address segments

Libraries and object forms

• Unix tools from IBM Developer Works
• Download some more software. It’s a CSCI343 homework from ancient times.
• Look at the btree.h include file. Give some attention to the struct definitions.
• Look at the C code in btmisc.c. Notice how it uses structures in expressions like i < p->head.numUsed.
  • What is the type of p->head.numUsed?
  • What is the address of p->head.numUsed?
• Type gcc -S btmisc.c
• Look at btmisc.s
• For N from 0 to 3, do the following to get four files: (Use the up-arrow)
  • gcc -ON -S btmisc.c; mv btmisc.s btmisc.s-ON
• Also, to get the debugging optimization, try one more:
  • gcc -Og -S btmisc.c; mv btmisc.s btmisc.s-Og
• Let’s compare the assembly files.
  • Notice the sizes.
  • Try to find p->head.numUsed . The offset is 9.
• Let’s examine the Makefile.
  • It uses an static library!
  • make
• Look at the created files.
  • Run nm on some of object (.o) files.
  • Try out some of the many options of objdump on some object files.
    • --info
    • --file-headers
    • --section-headers
    • --disassemble
    • --debugging
  • Use ar to list and extract members of the archive. Look at the ar manual page for ideas.
  • readelf -all bupdate | more
• Resolve to master the GNU Binutils.

Dynamic library

Have you heard of DLL Hell?

• Where are the libraries — /usr/lib/x86_64-linux-gnu
• Shared library tutorial
• My experience
• A longer tutorial

A very busy program

BusyBox: Bet it’s in your router and your workstation.

• busybox
• busybox ls -l /etc
• which busybox
• ln -s /bin/busybox ls
• ls -l ls
• which ls
• ./ls -l /etc

Process creation in Unix

• fork(), wait() and execve()