Why is my variable not being updated?

Question

I have defined a variable with an initial value.
When stepping through the code:

• I can see the initial value
• My function changes the value
• When I use the variable later it has the wrong value

What is happening?

_{Note: this is intended as a reference question for common problems. If the generic answers here don't help you, post a question containing your complete, actual code.}

Answer 1

There are several reasons why a variable might not keep a value. While some are arcane and difficult to debug, some of the most common reasons include:

The variable is modified by an interrupt

//---in main()---
unint8_t rxByte = 0;
printf("%d", rxByte);   //prints "0"

//---later in Uart0_Rx_Handler()---
rxByte = U0RXREG;       //rxByte set to (for example) 55

//---later in main()---
printf("%d", rxByte);   //still prints "0"!!!

If a variable is modified by an interrupt handler, it needs to be declared volatile. Volatile lets the compiler know that the variable could be modified asynchronously and that it shouldn't used a cached copy in a register.

//---in main()---
volatile unint8_t rxByte = 0;
printf("%d", rxByte);   //prints "0"

//---later in Uart0_Rx_Handler()---
rxByte = U0RXREG;       //rxByte set to 55

//---later in main()---
printf("%d", rxByte);   //corectly prints 55

Overrunning an array's bounds

There are no checks in C to prevent you from going beyond the bounds of an array.

int array[10];
int my_var = 55;

printf("%d", my_var);   //prints "55"
for(i=0; i<11; i++)   // eleven is one too many indexes for this array
{
  array[i] = i;
}
printf("%d", my_var);   // prints "11"!!!

In this case, we go through the loop 11 times, which is one index bigger than the array. In most compilers, this will result in overwriting variables declared after the array (anywhere on the page, they don't even have to be declared on the next line). This scenario can occur in many different circumstances, including multi-dimensional arrays and stack corruption.

Forgetting to dereference a pointer

While trivial, forgetting the asterisk on a pointer when making assignments will not set the variable correctly

int* pCount;
pCount = 10;   //forgot the asterisk!!!
printf("%d", *pCount);   //prints ??

Masking a variable with the same name

Reusing a variable name in an inner scope (like inside an if/for/while block or inside a function) hides a variable with the same name elsewhere.

int count = 10;   //count is 10
if(byteRecevied)
{
  int count = U0RXREG;   //count redeclared!!!
  DoSomething(count); 
  printf("%d", count);   //prints "55"
}
printf("%d", count);   //prints "10"