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

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Most C programmers, even beginners, would claim that arrays are easy, except those abundant off-by-one errors and they are right. Arrays are easy indeed. However, here are a few points to consider when writing a program that needs a rather large array (By the way, the thought for this article came into mind while helping a colleague to resolve an error a couple of days ago). When creating a large array (e.g. something like int a[1000][1000], which BTW takes up around 3.8 MB on most machines), you might not have any issue at all or might see either a compile time error or worse, runtime errors.
Why do these errors happen? Depending on your machine’s limitations you are most probably consuming the total available stack (if you used a local variable) or data memory. Depending on your compiler, these might be flagged at compile time or surface when you try to run your program. Or it might be that your compiler isn’t able to work with large arrays. What you can do to alleviate these:

• The first and best way is to minimize your array size (This makes for an amazing example here from the bookd “Programming Peals”: http://www.cs.bell-labs.com/cm/cs/pearls/cto.html )
• Use “huge” memory model.
• Use a global variable (or a static variable if you are particular about its visibility to the rest of the program). The stack is generally quite limited as compared to the data memory available to a program. So, a variable with static storage would ensure that you use memory from the data segment.
• Don’t declare it as an array at all and instead use a pointer and dynamically allocate the memory required for it. You might have to use special allocation calls instead of normal malloc/calloc to get this working though (e.g. using farmalloc)
• Create a section in assembly with the “Area” directive (or whatever it is for your particular assembler) and reserve space for your array there and refer to that array as an extern variable.

The first approach is something that you should always look for. But if you can’t mimize your need any further, choose one out of the rest two. But few things to be kept in mind here that these options can still fail, e.g., when you don’t have enough RAM/heap memory remaining at runtime (of course, you would have planned for a graceful exit though in such case instead of the random crash that would have occured otherwise). But still these could be useful to you in case you don’t have any real RAM limitations but just that your compiler isn’t able to work with large objects.

© Safer Code | Large Arrays In C

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What would be the best way to start a blog that talks about building safer code? Yes, it would be to talk about the first thing you think when you start coding. The “main” function (as seen in C/C++).

We’ve been brought up with many different variations of this popular function. Some books/teachers like to write it as “int main”, some write it as “void main” and then there are some who make do with just “main”, forgoing the return type altogether. So, which one out of them is correct? Or does it even make a difference?

The answer to the 2nd question is “YES”. It makes a whole world of difference as in it could:

• do nothing
• or give you a compile time warning
• or crash the program
• or cause problems in your invocation environment

Now, we go back to the first question. Which is the correct form and why?
The answer is “int main” is the correct type for C++.
But for C, it is a bit tricky and I’d say “int main” is the recommended way.
The simple reasoning is “because the C and C++ standards say so”. (See this however, which is what is leads to a bit of confusion though and makes it implementation dependent in c)

But lets take a brief look at the practical reasons for this because you might wonder “My compiler doesn’t give me a warning for void main, so why should I care?” (If your compiler does that, then its time to switch to something else. Did I hear you are using a Microsoft compiler? ).
(more…)

© Safer Code | int main() vs void main()

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