This post assumes some basic C skills.
Linux puts you in full control. This is not always seen from everyone’s perspective, but a power user loves to be in control. I’m going to show you a basic trick that lets you heavily influence the behavior of most applications, which is not only fun, but also, at times, useful.
A motivational example
Let us begin with a simple example. Fun first, science later.
int i = 10;
Simple enough, I believe. I compiled it with no special flags, just
gcc random_num.c -o random_num
I hope the resulting output is obvious – ten randomly selected numbers 0-99, hopefully different each time you run this program.
Now let’s pretend we don’t really have the source of this executable. Either delete the source file, or move it somewhere – we won’t need it. We will significantly modify this programs behavior, yet without touching it’s source code nor recompiling it.
For this, lets create another simple C file:
return 42; //the most random number in the universe
We’ll compile it into a shared library.
gcc -shared -fPIC unrandom.c -o unrandom.so
So what we have now is an application that outputs some random data, and a custom library, which implements the rand() function as a constant value of 42. Now… just run random_num this way, and watch the result:
If you are lazy and did not do it yourself (and somehow fail to guess what might have happened), I’ll let you know – the output consists of ten 42’s.
Read the rest of this entry »