Discussion 1: C vs C++, Debugging

Communication

How do we communicate? Language. We have a common protocol to communicate. Computer languages like C serve the same purpose – i.e., to connect humans and computers.

The CPU understands binary (0 and 1), so gcc or g++ converts your C code to a binary executable.

Binary Representation

• Two's complement notation
  • Flip each bit, and add one.
  • Most widely used notation for binary numbers.
• Examples
  • int x = 5;. The real storage in the computer is 00000101, since ints are allocated 8 bits.
  • int x = -5;. The real storage in the computer is 11111011, according to two's complement notation.

C vs C++ differences

C

A racing car that goes incredibly fast, but breaks down every fifty miles.

Features not in C

• Classes and objects
• Namespaces
• new and delete
• cout and cin
• Data Structures
• string datatype
• Pass by reference (&)

C++

An advanced version of the C racing car, with lots of improvements and dozens of extra features that increase the breaking down limit to 250 miles.

Assembly

You are the car. It breaks down every few feet.

C examples

Hello world:

#include <stdio.h>
int main() {
    printf("Hello World");
}

Printing numbers, zero through 9:

#include <stdio.h>
int main() {
    int i;
    for (int i = 0; i < 10; i++) {
        printf("i = %d\n", i);
    }
}

Strings in C

Initializing strings

There is no string type in C. Use char * or char[] instead.

char *text = "Hello";
char text[] = {'H', 'e', 'l', 'l', 'o', '\0'};

Do not forget the null sentinel, if using list initialization!

Copying strings

What's wrong with this?

char *text = "Hello";
char *copy;
copy = text;

Both text and copy are pointing to the same address of the string. If text is changed, so is copy, and you might not want that.

String comparison

What's wrong with this?

char *text = "Hello";
if (text == "Hello") {
    // ...
}

It is comparing a char * of the memory location of the string text, to the memory address of a local string "Hello". This would always return false.

Memory Allocation in C

int *foo;
foo = (int *) malloc(sizeof(int)); // Allocates memory for int
*foo = 5;

// ...

free(foo);

Use this instead of new and delete.

Debugging

Common bugs

• Seg faults
• Memory leaks
• Infinite loops

Common solutions

Bad:

• Shout at computer
• cout statements, if you're a noob
  • If you encounter a segfault, cout isn't going to save you.

Good:

• Breakpoints and gdb.

GDB abilities

• Stack frame (backtrace)
  • All the function calls which were made before the error occurred
• Breakpoint
  • Help monitor a particular statement
• Step execution (step)
  • Go statement by statement to narrow donw the code region for finding bug

Lecture 1 add-on

General purpose processor is a general processor, in contrast to an ASIC processor which is an Application Specific Integrated Circuit.