Here’s another quick C programming thing.

Consider the following code, which nest two for/next loops together. Notice the variable it uses for each one.

for (int i=0 ; i < 10; i++) // using i
{
    printf("%d\n", i);

    for (int i=0 ; i < 3 ; i++) // using i
    {
        printf ("   %d\n", i);
    }
}

What do you think it will print? Will this even work? Running it produces…

0
   0
   1
   2
1
   0
   1
   2
2
   0
   1
   2
3
   0
   1
   2
4
   0
   1
   2
5
   0
   1
   2
6
   0
   1
   2
7
   0
   1
   2
8
   0
   1
   2
9
   0
   1
   2

In C, you create a new instance (scope) of a variable “i” within the braces. The second for/next declares its own local i, and any references to i within those braces will be that second instance of i.

This is confusing, of course, but it works.

A “bug” could exist if the second scope forgot the “int”:

for (int i=0 ; i < 10; i++) // using i
{
    printf ("%d\n", i);

    for (i=0 ; i < 3 ; i++) // where's the int?
    {
        printf ("   %d\n", i);
    }
}

By forgetting that “int”, it re-uses the first instance of i, and does not do what was expected.

0
   0
   1
   2
4
   0
   1
   2
4
   0
   1
   2
4
   0
   1
   2
4
   0
   1
   2
4
...repeat...

This was based on something recently fixed in some production code at my day job.

So, yeah. Use different variable names and avoid this from ever happening.

Next, revisiting a recent post, also dealing with what braces do, I stumbled upon a much better reason to always put braces around things, even if it’s just one statement.

Suppose you had a simple DEBUG macro (this is not a good one, but pretend it is):

#define DEBUG(x) printf(x);

if (x==1) DEBUG("hello");

Looks fine! But if you decide to make the macro contain multiple statements, using the backslash character like this:

#define DEBUG(x) printf("ERROR!\n"); \ printf(X);

It now breaks. That macro changes the resulting code into something like this:

if (x==1) printf("ERROR!\n");
printf("hello");

Not what was expected. To solve this, the macro creator should have put the statements in braces like:

#define DEBUG(x) { printf("ERROR!\n"); \ printf(X); }

Problem solved. Assuming the macro creator did it that way.

So, to prevent things like this that look fine and work fine from doing neither, just get in the habbit of using braces even for one statement:

if (x==1) { DEBUG("hello"); }

Or just always do them in multiple lines:

if (x==1)
{
   DEBUG("hello");
}

…and you don’t ever have to worry about that again.

Until next time…

Updates:

• 2019-08-12 – Added note about using unions.

In the C programming language, memcpy (memory copy) is a function used to copy a range of bytes from one location in memory to another. I have used it often. Today, my boss mentioned something about not liking memcpy() because of all the dangers of using it. I understand that incorrect addresses, bad calculations, and simple omissions from a copy/paste can cause a memcpy() to have bad results. But, what’s the alternative?

He mentioned that he prefers to use C structures to copy data around, as opposed to using memcpy() to copy blocks of data.

Instead of doing this…

char Buffer1[64];
char Buffer2[64];

// Load Buffer1 with something.
strncpy (Buffer1, "Copy this string over", 64);

// Copy Buffer1 into Buffer2.
memcpy (Buffer2, Buffer1, 64);

That’s a simple, straight-forward example that seems hard to mess up. But doing something like this would be impossible to mess up:

typedef struct
{
    char Buffer[64];
} CopyStruct64;

CopyStruct64 Buffer1;
CopyStruct64 Buffer2;

// Load Buffer1 structure with something.
strncpy (Buffer1.Buffer, "Copy this string over", 64);

// Copy Buffer1 structure into Buffer2 structure.
Buffer2 = Buffer1;

Interesting. With properly defined structures, copying a block of data from one place to the other is done using code generated by the compiler.

(Update) And it seems I misspoke. He was actually talking about using unions, not structures. I’ve used unions as unions, but never for something like this. I did a similar test. (I realize there are several ways to declare a union, but I am using typedef so it looks similar to the above structure example.)

typedef union
{
    char Buffer[64];
} CopyUnion64;

CopyUnion64 Buffer1;
CopyUnion64 Buffer2;

// Load Buffer1 structure with something.
strncpy (Buffer1.Buffer, "Copy this string over", 64);

// Copy Buffer1 structure into Buffer2 structure.
Buffer2 = Buffer1;

Using union appears to work the same way, and since my example was not using structure elements, it seems to make more sense to do it that way. (End of Update)

I’m just now wrapping my brain around this, trying to figure out how this approach could be used to avoid using memcpy() and pointers.

What do you think? Am I on the right track to what he was talking about?

Until next time…

Just a quick rant, based on how example code from a new compiler I am using is presented.

In C (and similar languages), it is very common to see simple logic presented in one line, such as:

if (AlertLevel == RED) TurnOnSiren();

That is simple to understand, and only takes up one line on the screen or a printout. However, at a previous job, our coding standard forbid that, and always required the use of braces around any actions of an “if”.

if (AlertLevel == RED)
{
   TurnOnSiren();
}

Now, ignoring style preferences of where the braces should go, the use of braces for something like this has some advantages when it comes to preventing potential bugs if this code is changed in the future.

In the case of the compiler examples I have been seeing, they show them without braces, such as:

if (AlertLevel == RED)
   TurnOnSiren();

For simple and short logic, this is no different than my first example — it just places the “what to do” stuff on an indented line after the “if”.

However, a common bug I see occurs when someone expands upon that like this:

if (AlertLevel == RED)
   TurnOnSiren();
   FlashRedLight();

In some languages, where program flow is determined by indentation, this would work. But in C, indentation does not matter. The above code would actually be processed as:

if (AlertLevel == RED) TurnOnSiren();

FlashRedLight();

The red lights would flash every time through, regardless of the status of AlertLevel.

Unintended consequences of not using braces, and using indents to format each statement on its own line.

By always using braces (even on simple one-statement things like this example), you avoid that:

if (AlertLevel == RED)
{
   TurnOnSiren();
   FlashRedLight();
}

Now we have the desired result, as adding new code or function calls inside the braces will work as we intended.

It’s 2019. The C Programming Language came out 47 years ago, and I still occasionally run into bugs caused by the lack of braces.

Thanks to a former job, I no longer make this mistake. I always use braces.

This has been a public service announcement of Sub-Etha Software.