Answer / noor alam

Application Programming Interface (API) is a set of
functions, objects, protocols or datastructures for the
support of application development for
developers/programmers. It is actually a kind of function
definition which specifies how to make available of a
specific service of the system/OS. The API's are available
from library or from Operating system itself. Whenever a
programmer need a specific service from OS, he/she can use
appropriate API to do that.

Processes in a system are run in different modes, process
run in user mode have no access to the privileged
instructions. If they want perform any privileged
instructions or need of any services they request kernal for
that service through System Calls. System calls are made by
way of software interrupt. This is actually a request for
the service whereas API is a function description that can
be used by the programmer for his programs, its like a tool
used to obtain a specific task in his programs.

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