Week 3 - May 20-24

Week 3 lecture recording here.

Introduction

We will look at programming concepts and file I/O. In Linux, everything is a file, so it is important to know how to use them. We will also look at IOCTLs which are user-space calls into the Linux kernel. We also will look at the gdb debugger.

Videos

Quiz

• Quiz 2 will be on File I/O. Study Chapter 4 (System Programming Concepts) of The Linux Programming Interface.

Lecture Material

• File I/O and IOCTLs - Chapter 4 of The Linux Programming Interface
• Controlling Hardware with ioctls.

Labs

• Lab 2 - using a static library.
• Lab 3 - I/O control.

Assignment(s)

None.

Sample Code

File I/O

• For an example of a program that simply copies a file from one location to another, see SimpleFile.cpp.
• For an example of a program that does exactly the same thing but uses lseek to start 100 bytes into the input file, see SeekFile.cpp.
• For a simple makefile to create these, see Makefile.
• The C library function fopen is compared with the Linux system function open via two test files - openTest.cpp and fopenTest.cpp. They both copy large amounts of data (LargeOpen.txt and LargeFOpen.txt) and the time to copy is measured as a function of open's buffer size. For the makefile see Makefile.

IOCTLs

• For an introduction to IOCTL's see Controlling Hardware with ioctls.
• For sample code using IOCTL's see diskDrive.cpp. This performs IOCTL's on the device driver for two file systems to get sector size, number of head, cylinders etc... Documentation on how to use this IOCTL can be found at Summary of HDIO_ IOCTL calls and sd - driver for SCSI disk drives.

GDB - a Linux debugger

• We looked briefly at the Linux debugger GDB. A tutorial on GDB can be found at Debugging Under Unix: gdb Tutorial or
  Red Hat Customer Portal: Chapter 20. Debugging a Running Application.
  
  Source code that was used to demo GDB can be found at Makefile, Math.cpp, Math.h, Conversions.cpp, Conversions.h, General.cpp, General.h, Geometry.cpp, Geometry.h.
  In this source code the function CelsiusToFahrenheit was made to crash when the flag CRASH was enabled in the Makefile.
• To run a program with the gdb debugger, for example Math, execute the following from the command line:
  $ gdb ./Math
  Then run the program from the gdb console:
  (gdb) run
  If the program crashes, you can execute the backtrace command from within the gdb console to see the flow of function calls that led to the crash:
  (gdb) bt
  
  To attach gdb to an already existing process, first get the process id then execute gdb attach. For instance, say Math is already running and has pid 5467:
  $ gdb attach 5467
• Valgrind is also available on Linux for testing whether your program has a memory leak. You can install valgrind as follows:
  $ sudo apt install valgrind