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Iterating over an array in c

In the first semester of grad school, I wanted to revisit my basics. Therefore, I took the Operating Systems class. We take an existing teaching operating system (MIT’s xv6) and implement features like memory mapping and threads on top. Having only sparingly worked with C and never with assembly, I knew I was in for quite a ride.

As a part of the course, we were required to write a simple graphics driver. The driver itself was pretty simple, where we wrote data to a memory region and sent control commads on a pre-defined port. To test the driver we had to display an image on the screen. So as things go with writing programs it was 2 in the morning and me questioning my computer “Why aren’t you printing the god damned image!”. But of course, the computer was doing exactly what I asked it to.

To write the file to the memory buffer, I had a loop which was iterating over the contents of the file and writing it to the memory location. Sounds simple enough right, this is what it looked like:

int* i;

for(i = 0; i < n; i++) // n is the number of bytes to write
    display[(i + f->off)] = buf[i]; // display is a pointer to the region in memory

And this worked for the most part, but the image displayed was zoomed in and alternate pixles were not coloured in. After a lot of time spent in gdb unable to understand what was going on, I asked what was going on the class’ discussion forum. The professor pointed out that when we have an index of a given type, when incrementing the index, we increment by the sizeof the index variable. So in my loop, instead of iterating over each byte in the file, I was iterating by sizeof(int).

Once I used the correct type, it just worked.

So to conclude. When using an index to iterate over an array, everytime you increment the index, you are actually incrementing by the sizeof(type).