Author: Dennis Kubes

Compiling x86 Assembly on x64 Linux

I am working my way through Professional Assembly Language by Richard Blum. Why? I like lower level programming and I find assembly language interesting.

My setup is a Mint Linux x64 box. Professional Assembly Language, and many of the other books on assembly language, tend to use i386 32-bit assembly. In fact there are more books on 32-bit assembly language on the market than there are on 64-bit assembly language. It is nice to learn by going through book examples, but I don’t want to have to change them too much. I don’t mind changing command line switches, but I don’t want to have to convert all code to 64-bit assembly while learning.

Trying to compile and link 32-bit assembly on an 64-bit machine, you can run into some issues. This post goes over how to get setup so you can assemble and link both 32-bit and 64-bit assembly on an x64 Linux machine.
Read more →

The 5-Minute Guide to C Pointers

If you are reading this you want to know more about c pointers. That’s a good thing. Even if you don’t program in C very often, understanding pointers gives you a deeper understanding how programming and memory works “under the hood”. Learning pointers will make you a better programmer. Read more →

An Interesting Pointer Puzzle

A reader of my blog sent me a question the other day asking to explain a piece of code with pointers. I found it to be a very interesting puzzle, not just because I had to drop into an object dump with a friend to work through it. The error is consistent, even across platforms. Here is a slightly modified version of the original code. We will call this file bad.c. See if you can notice the error. Read more →

How to Think About Variables in C

C is memory with syntactic sugar and as such it is helpful to think of things in C as starting from memory. One of the pieces that I think is often overlooked is variables and data types. If you have the right mental model for variables and data types it makes other concepts in C, and other langauages, easier. Let’s start with three definitions. Read more →

Basics of Function Pointers in C

This post is very detailed because I am attempting to create a mental model to help beginners understand the syntax and basics of function pointers. If you are ok with detail happy reading.

Function pointers are an interesting and powerful tool but their syntax can be a little confusing. This post will going into C function pointers from the basics to simple usage to some quirks about function names and addresses. In the end it will give you an easy way to think about function pointers so their usage is more clear. Read more →

Read Android Data Folder Without Rooting

I got my new galaxy s3 and I am all excited doing new android development. I fire up the adb shell to look around and I see this:

[bash]
dennis@foofive ~ $ adb shell
shell@android:/ $ ls -al

drwxrwx–x system system 2012-09-24 23:38 data

shell@android:/ $ cd data/
shell@android:/data $ ls
opendir failed, Permission denied
[/bash]

Grrrr. Ok so I can’t access the data folder because it is read protected. What do I do now? I can root the phone, but I don’t really want to do that for various reasons. All I really want is to see into my data folder application directories, be able to remove test files such as databases and preferences. Read more →

Is C Pass by Value or Reference?

Pass by Value

In the strictest sense of the word, everything in C is pass-by-value. This often confuses beginning C programmers, especially when it comes to pointers, arrays, and structs. So what do we mean when we say pass-by-value and pass-by-reference.

When we pass-by-value we are passing a copy of the variable to a function. When we pass-by-reference we are passing an alias of the variable to a function. C can pass a pointer into a function but that is still pass-by-value. It is copying the value of the pointer, the address, into the function. In C++ a reference is an alias for another variable. C doesn’t have this concept. Read more →

Basics of Pointers and Arrays in C

Discussions of pointers and arrays in C seem to be a holy war. On one side you have the people who say pointers are not arrays and that everybody must know that. On the other you have the people who say arrays are treated as pointers and so there shouldn’t be a distinction, it just confuses people. Turns out both sides are right. Read more →

Basics of Memory Addresses in C

When C was created, in 1972, computers were much slower. Most programs were written in assembly. C came along as a better assembly allowing programmers to manipulate memory directly with pointers. Programmers worked much closer to the machine and had to understand how memory worked to make their programs efficient. Read more →

Do you know what *p++ does in C?

When first learning C pointers there is one thing I wish had been better explained; operator precedence vs order of operations.
[c]
int myarray[4]= {1,2,3,0};
int *p = myarray;
int out = 0;
while (out = *p++) {
printf("%d ", out);
}
[/c]

The above example prints out 1 2 3. Code like *p++ is a common sight in C so it is important to know what it does. The int pointer p starts out pointing to the first address of myarray, &myarray[0]. On each pass through the loop the p pointer address is incremented, moves up one index in the array, but the previous p unincremented address (index) is dereferenced and assigned to the out variable. This happens until it hits the fourth element in the array, 0 at which point the while loop stops. But what does *p++ do? And how does it move from one element in the array to the next. Read more →