What was the most dangerous programming mistake you have made in C?

Question

I am an intermediate C programmer. If you have made any coding mistake that you came to know later that it was the most hazardous / harmful to the total application please share that code or description. I want to know this because in future I may come across such situations and I want to have your advice to avoid such mistakes.

Answer 1

if (c = 1) // insert code here

Answer 2

if(a == true);
{
  //Do sth when it is true. But it is allways executed.
}

Edit: Another variant of the same mistake.

for(i=0; i<max_iterations;i++);
{
  //Do sth but unexpectedly only once
}

Answer 3

Few years ago I've got a call from my ex-colleague telling me about the problem he had to fix with my code, which was a router for credit card transactions.

Card number prefix consists of 6-digit BIN (Bank Identification Number) and extra few digits that banks use at own discretion, e.g. bank has BIN for Visa Classic card 456789, and reserve 2 extra digits to indicate sub-product, like 01 for student's card, 02 for co-branded card with local department store and so on. In this case card prefix, which is basically product identifier, becomes 8 digits long. When I coded this part, I decided that 9 digits "ought to be enough for everyone". It was running ok for 2 years until one day bank make a new card products with 10-digit-long prefix (have no idea why they needed it). Not too hard to imagine what has happened - router segfaulted, the whole system halted because it cannot function without transaction router, all ATMs of that bank (one of the biggest in the country) became non-operational for few hours, until problem was found and fixed.

I cannot post the code here firstly because I don't have it and secondly it is copyrighted by the company, but it is not hard to imagine the strcpy() without checking size of target buffer.

Just like man strcpy says:

If the destination string of a strcpy() is not large enough (that is, if the programmer was stupid or lazy, and failed to check the size before copying) then anything might happen. Overflowing fixed length strings is a favorite cracker technique.

I was very embarrassed. But I've learned the lesson and do not forget (usually :) ) to check size of target buffer. I wouldn't recommend you to learn it the hard way - just develop a habit to check target buffer before strcpy() and strcat().

Edit: good suggestion from Healthcarel - use strncpy() rather than strcpy(). It doesn't add trailing 0 but I usually use following macro to get around it:

#define STRNCPY(A,B,C) do {strncpy(A,B,C); A[C] = 0; } while (0)