CSCI 2021 HW13: pmap and Linking

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                            HW 13 QUESTIONS
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- NetID: (THE kauf0095 IN kauf0095@umn.edu)

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PROBLEM 1: Virtual Memory and pmap
==================================

(A) memory_parts memory areas
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  Examine the source code for the provided `memory_parts.c'
  program. Identify what region of program memory you expect the
  following variables to be allocated into:
  - global_arr[]
  - stack_arr[]
  - heap_arr


(B) Running memory_parts and pmap
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  Compile the `memory_parts' using the provided Makefile.
  ,----
  | > make memory_parts
  `----
  Run the program and note that it prints several pieces of information
  - The addresses of several of the variables allocated
  - Its Process ID (PID) which is a unique number used to identify the
    running program. This is an integer.
  For example, the output might be
  ,----
  | > ./memory-parts
  | 0x5605a7c271e9 : main()
  | 0x5605a7c2a0c0 : global_arr
  | 0x7ffe5ff7d600 : stack_arr
  | 0x5605a92442a0 : heap_arr
  | 0x7f1fa7303000 : mmap'd file
  | 0x600000000000 : mmap'd block1
  | 0x600000001000 : mmap'd block2
  | my pid is 8406
  | press any key to continue
  `----
  so the programs PID is 8406

  The program will also stop at this point until a key is pressed. DO
  NOT PRESS A KEY YET.

  Open another terminal and type the following command in that new
  terminal.
  ,----
  | > pmap THE-PID-NUMBER-THAT-WAS-PRINTED-EARLIER
  `----

  Paste the output of pmap below.


(C) Program Addresses vs Mapped Addresses
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  pmap prints out the virtual address space table for the program. The
  leftmost column is a virtual address mapped by the OS for the program
  to some physical location.  The next column is the size of the area of
  memory associated with that starting address. The 3rd column contains
  permissions of the program has for the memory area: r for read, w for
  read, x for execute. The final column is contains any identifying
  information about the memory area that pmap can discern.

  Compare the addresses of variables and functions from the paused
  program to the output. Try to determine the virtual address space in
  which each variable resides and what region of program memory that
  virtual address must belong to (stack, heap, globals, text).  In some
  cases, the identifying information provided by pmap may make this
  obvious.


(D) Min Size of Mapped Areas
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  The minimum size of any virtual area of memory appears to be 4K. Why
  is this the case?


(E) Additional Observations
~~~~~~~~~~~~~~~~~~~~~~~~~~~

  Notice that in addition to the "normal" variables that are mapped,
  there is also an entry for the mmap()'d file 'gettysburg.txt' in the
  virtual address table.  The mmap() function is explored in the next
  problem but note its calling sequence which involves use of a couple
  system calls:
  1. `open()' which is a low level file opening call which returns a
     numeric file descriptor.
  2. `fstat()' which obtains information such as size for an open file
     based on its numeric file descriptor. The `stat() / fstat()' system
     calls are used to ask the Unix Operating System information about
     files such as their size, modification times, and access
     permissions.  This system call is studied more in Operating System
     courses.

  Finally there are additional calls to `mmap()' which allocate memory
  to the program at a specific virtual address. Similar code to this is
  often used to allocate and expand the heap area of memory for programs
  in implementations of `malloc()'.


PROBLEM 2: Linking to System Libraries
======================================

(A)
~~~

  The file `do_math.c' contains some basic usage of the C library math
  functions like `pow()'.  Compile this program using the command line
  ,----
  | > gcc do_math.c
  `----

  and show the results below which should be problematic. Describe why
  the linker complains about functions like `cos' and `pow'.

  *Note*: problems will arise on Linux systems with gcc: other
  OS/compiler combinations may not cause any problems.


(B)
~~~

  In order to fix this problem, one must link the program against the
  math library typically called `libm'. This can be done with the option
  `-l' for "library" and `m' for the math library as shown:
  ,----
  | > gcc do_math.c -lm
  `----


  Show a run of the resulting executable after a successful compile
  below.


(C)
~~~

  After successfully compiling `do_math.c', use the `ldd' command to
  examine which dynamically linked libraries it requires to
  run. Assuming the executable is named `a.out', invoke the command like
  this
  ,----
  | > ldd a.out
  `----

  Show the output for this command and note anything related to the math
  library that is reported.


(D)
~~~

  Run the program which should report its Process ID (pid) before
  pausing.  In a separate terminal, while the program is still running,
  execute the pmap command to see the virtual address space for the
  program (command `pmap <pid>'). Paste the results below and describe
  any relation to the math library that is apparent.


(E)
~~~

  Repeat the general steps above with the C file `do_pthreads.c' which
  will require linking to the PThreads library with `-lpthread'.
  - Compile to show error messages
  - Compile successfully with proper linking and show output
  - Call `ldd' on the executable
  - While the program is paused, run `pmap' to see its virtual address
    space

  Show the output of these commands below.