How to plot a pixel with a negative stride/pitch?

  allegro, c++

Well i don’t know, the locker() in Allegro 5 lib is returning me a negative stride.
I don’t know what it means.
The thing is that i don’t know how to plot pixels/write 32 bit (RGB) pixels when this happens.
In windows all is OK but in linux it throws a Segmentation Fault.

Source: Windows Questions C++