From QB64 Wiki
The _MEMEXISTS function returns true (-1) if the memory block variable name specified exists in memory and false (0) if it does not.
- ret = _MEMEXISTS (memblock)
- The memblock variable name should have been created using DIM blockname AS _MEM type (DIM.
- The function verifies that the memory variable exists in memory before using a passed block to avoid generating QB64 errors.
- Typically, this function is used by a LIBRARY SUB or FUNCTION which accepts a _MEM structure as input to avoid an error.