how to write a structure in a shared mem segment.

[wysota]

You should operate on it as on any other structure - fill it field by field (byte by byte) using regular memory-manipulating operations, like:

struct myStruct *ptr = (myStruct*)file_memory;
ptr->field1 = 1;
ptr->field2 = 2;

struct myStruct *ptr = (myStruct*)file_memory;
ptr->field1 = 1;
ptr->field2 = 2;

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For simple structures (meaning ones that consist only of primitive types) this should be enough.