Error - read or write protected memory in C dll project

[Valli]

Hi,

I have written a C dll project that supports LZO algorithm(lzo.lib). This
dll is called inside a VB.net project.
The dll starts executing. Assignment statements works fine.

#define IN_LEN (1024L)

lzo_byte *in;
lzo_uint in_len;

in_len = IN_LEN;

lzo_memset(in, 0, in_len);

the statement
lzo_memset(in, 0, in_len);

throws error "Attempt to read or write protected memory"

Whats is the problem in my code?
Can anyone help me on this

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Thanks & Regards
Valli