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

Answer 1

Uninitialized data.

Answer 2

I was dealing with dynamically allocated 2D arrays and I instead of free()'ing N rows, I decided to free M columns. This was fine for smaller inputs where N == M but on large inputs, I only free()'d 50% of what I allocated.

shrug

Live and learn.

Answer 3

while(a)
{ 
   // code - where 'a' never reaches 0 :( 
}

Answer 4

It has been a long time, but some things you never forget ;-).

• forget the \0 at the end of a string.
• allocate n characters for a string with n characters.
• forgetting the break in a switch statement.
• 'creative' macro use.

Answer 5

You should worry more about little mistakes. Big/spectacular mistakes are usually documented in books (with reasons why they are bad, alternative approaches etc.).

It's the little design/coding mistakes that get to you, because they tend to add up.

So my advice is to try and read the books Kernighan wrote or co-authored ("The C Programming Language", "Practice of Programming" etc.) because they are full of common-sense (common for experienced C programmers) advice and list principles that are very useful in avoiding both small and big mistakes.

They also list many potential big mistakes, so they answer your initial question.

Answer 6

Forgetting architecture constraints and happily memcpy()ing into the memory mapped I/O area on a microcontroller. The magic smoke was released from the test rig.