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how i can implement variable length data base ( or data storage structure)in keil C programe?
Think about how you implement them "normally", then check if C51 poses any restrictions on that - Please read the manual: http://www.keil.com/support/man/docs/c51/
You also need to be aware of the limitations of the underlying architecture - why do you actually want to do this on an 8051??
the 8052 Max RAM is 64K. If you need more than that you need external storage. You can Use Banking to increase it, but it adds a lot of complexity. Serial memory or SD Cards are better since you will not be time critical (see how a little info from you changes the answers you get) If you are looking for malloc(). It is there but you need to configure it. Also it is a very poor choice for a small embedded system.
how i can implement variable length data base
you will need memory space for the max size, why not just allocate that or "all available memory"?
Erik