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These lines of code:

long duration; duration = (long)(500*g_sampleTimeInSeconds) / 60; printf("Memory will fill up after %d minutes\n", duration);

are producing a negative number when g_sampleTimeInSeconds is greater than 65. When g_sampleTimeInSeconds = 65 the printed number is 541 but when it is 66 the printed number is -542 rather then the correct number of 550. This is also the case when g_sampleTimeInSeconds is replaced with hardcoded values.

This code is compiled with the XC8 compiler to run on Microchip PICs

UPDATE:

g_sampleTimeInSeconds is an 16 bit int

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  • 2
    Please show the data type for g_sampleTimeInSecond Commented Mar 19, 2014 at 16:23
  • 2
    Change to (500L*g_sampleTimeInSeconds) / 60; Commented Mar 19, 2014 at 16:34
  • @chux your method also works - it seems only one of the multiplication operands needs to be cast to a long for this to work Commented Mar 20, 2014 at 8:39
  • 1
    Note: Should add l: printf("Memory will fill up after %ld minutes\n", duration); Commented Mar 20, 2014 at 15:08

1 Answer 1

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Is this a 16-bit compiler? My guess is that this is a integer conversion issue. where you are exceeding the size of a 16 bit integer, which is being treated as a two's complement number.

So, in your example, you are casting the results of the calculation to a long, after the calculation takes place. Assuming that g_sampleTimeInSeconds is a signed 16-bit int, then

500*66 = 33000, which is larger than the largest positive value of 32767 allowed for a signed 16 bit number. This causes an overflow, and the intermediate value is calculated as 33000 - 65536 = -32536. -32536/60 = -542.xxx.

So, you need to either cast the values within the expression, or change the data type of g_sampleTimeInSeconds to a long.

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2 Comments

Here's all the boring math behind it for those interested.
I'm not in front of the hardware right now so can't test but this makes perfect sense. I didn't realise that the type of g_sampleTimeInSeconds would affect the rest of the calculation like that. I'll accept once fully tested

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