HI, a question on #include

Hi, all,
I have a question on the use of #include:

#include <unistd.h>
#include <asm/unistd.h>
#include "nn.h"

What are the differences between the above three usages?
It seems the third one allows the code to include the nn.h in the same
directory of the code;
the first one allows the code to include the unistd.h from some other
directory without any specification;
the second one asks the code to include the unistd.h under
somedirectory .../asm/unistd.h.

Then another question is:
If there exist 2 files named unistd.h, one is
..../sys/unistd.h, the other is .../asm/unistd.h, which one will be
included by
#include <unistd.h>

Thanks for your help!

Jing

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