Safer Code – Secure Coding In C \ C++ And More..

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Last time we explained the real meaning of const keyword, this time it’s going to be Volatile, the other sibling of this most misunderstood duo in C history. Let’s separate out the myths and the facts first and then we will discuss the how’s and why’s of it.

FACTS:

• A volatile qualifier is important to be used for auto-storage variables within setjmp and longjmp.
• A volatile qualifier must be used when reading the contents of a memory location whose value can change unknown to the current program.
• A volatile qualifier must be used for shared data modified in signal handlers or interrupt service routines.

MYTHS:

• All shared data in multi-threaded programs must be declared volatile.

Now, we’ll see how we made the above classfication.

(more…)

© Safer Code | Volatile: C Keyword Myths Dispelled

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There are a few very basic things in C that are widely misunderstood by programmers of all caders. This is not because these things are very complex, but because books and teachers do not give them their due importance while teaching beginners and the misconceptions stick even years later. A few of these are like the keywords “volatile” (covered in next post) and “const”.

Look at the following piece of code:

#include <stdio .h>
int main()
{
const int a = 1;
a = 2;
printf("%d\n", a);
return 0;
}</stdio>

You’d be quick to point out (correctly) that the above code wouldn’t compile. It would fail with an error on the lines of “assignment of read-only variable a” because a is now a “constant” because of the const type qualifier attached to it. And now comes the issue. Many start assuming (or might even be explicitly taught) that the value of a cannot be changed now. Did you think so too? Well, you thought wrong. Look at the following code:

(more…)

© Safer Code | "const" Keyword Explained

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One of the ways to optimize your code for speed is to unroll the loops you have in your program. If you don’t know what loop unrolling is, then see the following simple example, where we are trying to copy a 5 element long string:
A Simple Loop:

for (i = 0 ; i < 5; ++i)
{
    *j++ = *k++;
}

The same loop unrolled:

*j++ = *k++;
*j++ = *k++;
*j++ = *k++;
*j++ = *k++;
*j++ = *k++;

Now, the unrolled version is definitely going to be faster because of the following reasons:

1. There are no branches any more. In a loop, the compiler has to insert branches to jump back to the for statement.

2. There are no condition checks. In a loop, the value of i is checked every time to see what to do next.

3. A third rather hidden, but really important, advantage is that unrolled code can be pipelined efficiently by the processor hence resulting in faster execution.

The above code in itself might not show you any perceivable difference in execution time, but it does become crucial when the loops are of much higher magnitude and especially in embedded/real time scenarios.

Now, the loop unrolling can be done by the compiler or manually by the coder. In this part, we will see how to facilitate compiler to carry out loop unrolling efficiently. (more...)

© Safer Code | Tweak Your Code For Speed: Unroll Those Loops Part 1

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The biggest bottlenecks in making efficient code today are jumps or branches. You must have always heard of people telling you to use switch-case blocks instead of cascadin if-else’es. They were right, but partially.

This is because a cascading if-else block will check for each condition in each iteration until it reaches the correct one. On the other hand, switch-case blocks are “supposed to” (why supposed to, we’ll see just now) just execute the correct statement and move on.

Optimization Technique 1: Keep the case label values close and small (Answer to “supposed to”)

Actually switch-case statements are also many times changed into if-else cascades by the compilers. This happens when the case label values are too far apart and/or are spread over a fairly large range. If you have a small switch-case statement like below, then the compiler will generate a jump-table instead of the if-else cascade.

switch(a)
{
  case 0: /*do something*/
  case 1: /*do something*/
  case 2: /*do something*/
  case 3: /*do something*/
  default:
}

A jump table is like a “lookup table” or a “dispatch table”. You can assume that compiler creates an internal array of addresses. The array is indexed by the case lable values and the value at that index is the address of instructions corresponding to that case.

Optimization Technique 2: Place those cases first which are more likely to occur

(more…)

© Safer Code | Optimizing Switch-Case Statements In C For Speed

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Thousands of sites around the interwebs are devoted to programming and producing code. But there is something missing. This “something” is actually the most important piece of the puzzle. This piece is about how “safe” and “efficient” your code is. There are programmers all over, but a very small minority is worried about finding all the loopholes in their code. Infact, most don’t even know there could be loopholes even as they start writing their program (more on this very soon ). And many times, you’d see people creating a jet fighter for something that could be solved with a bicycle (although the pace would be vice-versa).

The problem here is that there are many things that are not taught in the schools, the knowledge might be out there on the internet, but either it is fuzzy, not properly explained or is plagued by just too much of information overload. You cannot imagine yourself (and everyone else around you) to sit in a week-long class and emerge a champion programmer. It has to seep in gradually.

So, this is an attempt to solve the problem. We’ll bring you the concepts to make your code a fortress. We’ll bring them at a gradual pace that gives you time to learn, understand, ask questions and imbibe them into your daily routines. The problems and solutions would range from the very basics and trivia to the most advanced. We’d concentrate mostly on examples through C/C++ with a bit of JAVA and others interspersed here and there when needed but most concepts learned could be as well applied to any language. We’ll not tell you how to program, we assume you already know, but we’ll tell you how to program efficiently and securely.

So, if you are a college goer, or a fresher just into the corporate world, or an experienced professional, we have something for you all, to make you so capable that you can take a running program and re-write it so that it runs for years without crashing, being exploited to death, or taking a ton of memory or cycles.

Enough talking now. As Linus once said “Talk is cheap. Show me the code.”. So, lets begin…

© Safer Code | And So It Begins…

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This site is an attempt to solve a very major problem. The problem of people all around us producing code that is inefficient or ridden with security holes. The problem is created and aggravated because everything is not taught in school, and there is too much, too distributed or too vague information available on the internet.

We attempt to solve this problem by brining concepts about the issues of optimization, safety and security to you at a gradual pace that gives you time to learn, understand, ask questions and imbibe them into your daily routines. The problems and solutions would range from the very basics and trivia to the most advanced. We’d concentrate mostly on examples through C/C++ with a bit of JAVA and others interspersed here and there when needed. But we’ll not tell you how to program in C, we assume you already know, but we’ll tell you how to program efficiently and securely in C.

Amit Goel / Shantanu Goel

© Safer Code | About

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