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EVERYTHING CHANGES

Using the Const Modifier
by George Martin

Start ı Define and Qualify ı In Other Words ı Sources and PDF

Although itıs true that change is inevitable, quite often in embedded software design you want to specify some things as constant. Certainly you donıt want any unexpected changes. If you are writing code in C, you are no doubt using char, int, long, and float to tell the compiler about your data size and representation. There is another type modifier (or qualifier) that Iıll bet you know about and donıt use. My New Yearıs resolution is to use the const modifier, and then Iıll let the compiler watch out on my behalf.

The const modifier declares that the data will not be modified during execution. For example:

int a = 5; // a is an integer type set to 5

const int b = 10; // b is also an integer type set to 10 and wonıt change

int const b = 10; // identical in meaning to line above

Both a and b are variables and have initial values. But, a can be modified by the program, whereas b cannot. If you go on to write:

a = a+7; // increment this by 7

b++; // increment this by 1

the compiler will flag the second statement, which alters the variable b. This example is obvious and probably too simple to be much help unless there are several designers working on different modules for the same project. But, I bet you have a listing that looks something like Listing 1.

I hate questions like, "What do you think this code does?" So I wonıt ask. FLINFO is a structure describing and characterizing the flash memory devices, DevTable[] is an array of all the devices you might find soldered on the board, and FLinfo[MAX_FLASH_DEV] is a table of all the devices actually found on the board.

As I search and find flash memory devices, I copy the data from DevTable into FLINFO and update the BaseAddr. I donıt want DevTable to change as I run the program, so itıs qualified and const. And, any compiled code that would modify any of the DevTable values would be flagged at compile time. In fact, if your code has RAM and ROM, the compiler probably will place DevTable in ROM.

If you work on larger projects, no doubt youıve come across the situation where youıve designed a data structure and another programmer is needed to view your data, perhaps to write a report or to log the data. That other programmer or code module should not be modifying your data. What to do?

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