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In C you should use the malloc() function. It will allocate for you a memory region that isn't used for anything else.
Alternatively if you know at compile time how much space you need, you can also declare a static variable (a char array for example).
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Thank you for your response Daixiwen
But if I want to write data periodically (T= T1 write data + T2 do nothing ) using vhdl logic and then I want to access it through Nios (when vhdl logic does not write, that is T2) , will that be also work with malloc () ?