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

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Many times while going through some perl code, you must have come across snippets like “select((select(fh), $|=1)[0])” and wondered what this means, even though you might know that:

• $|=1 is used for setting autoflush (i.e. unbuffered data output) and that
• select is used to set the default output to a given file handle instead of STDOUT

Whenever I face any issue with a code fragment, I try to break it down into simpler terms to understand it from the beginning (like I did for my random number generation post). So, these are the steps in which I progressed:

• select(fh) replaces the STDOUT with fh and returns the old filehandle (i.e. STDOUT).
• (select(fh),$|=1) does the above and then sets this new output to autoflush
• From perl’s online docs, I found that the output of the above is a list, the first element of which is the old Filehandle
• So, (select(fh),$|=1)[0] gives us STDOUT
• Then select(select(fh, $|=1)[0]) basically just sets the default output back to STDOUT

So, what is the use of all this. Basically, this is nothing but a trick to set autoflush for any filehandle. Now, there is a very simple way to do this. You just need to include the IO::Handle module (by writing “use IO::Handle;” in your script) and then call “fh->autoflush(1)” on your file handle (Use 0 as parameter to disable autoflushing). This is much cleaner although it means longer run times as your script now has to include and compile lot of new lines of code because the module you added.

© Safer Code | Weird Usage Of “select” in perl

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A global or static structure is automatically initialized by all of its elements as 0 or null (In case of pointers). But if you want to initialize a local struct to this, what would you do? Many would say that would use a memset or calloc to set all the elements of that structure instance to 0. But this is incorrect as a null pointer might not be same as 0. So, the correct way to do this would be like:

struct a b = {0};

What this does is that it initializes the first element of the structure b to 0 (or null pointer if it is a pointer) and the rest of the elements are initialized like they would have been done if the structure was global or static.

© Safer Code | Structure Initialization (All Elements to 0) In C

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This is a topic that is quite easy and doesn’t need much explanation but still many people manage to mess it up. Not going into a hyperbole, I’ll get straight to the point. When asked, people can quickly tell you how to get a random number between 0 and b (b being any number below the maximum random number possible, which is defined as RAND_MAX by most C compilers). Using the function “rand” provided by most C libraries, this is as simple as:

result = rand() % b;

Basically, given any random number, if you take its modulus with a, you will obviously get a number between 0 and a -1. This is all fine and dandy, so now someone asks you to generate a random number between  a and b (a < b). This one is also really simple but few people fumble out still. Just think of it this way.

1. If you add 1 to the above equation’s right hand side, your random number will be between 1 and a. So, basically your “lower limit” is raised by one. In the above case, your lower limit is a, so just raise it by a by adding it to right hand side, to arrive at this partial solution.

   result = rand() % b + a;

2. To complete the equation, now think of the gap between the minimum and maximum result obtained from original equation. Minimum is 0 and maximum is (b-1). But your desired gap is (b-a). Since taking modulus with respect to b, gives you a gap of b-1, to get the desired gap, you need to take mod with respect to ((b-a)+1). So, minimum value this will give you is 0 and maximum would be (b-a) +1 -1 = b-a. So, your final equation becomes

   result = rand() % (b - a + 1) + a;

This will give you a minimum value of 0 + a = a

and a maximum of b – a + a = b.

Note that the above solution includes the limits for the result. If you don’t want to include the limits (i.e. minimum result = a + 1 and maximum result = b – 1) and , then just add (a + 1) instead of a in step 1 and use (b – a -1) for your modulus operation instead of (b – a + 1) in step 2 to make the equation:

result = rand() % (b - a - 1) + a + 1;

Note that in the above equation we used b – a -1, though on the surface it looks like we could have gone with just (b-a). After all we just wanted to decrease the upper limit by 1. But the reason we had to decrement by an extra place is because of the 1 we added to raise the lower limit (a +1).

© Safer Code | Random Number Between Two Integers

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