Crestron SIMPL+ - Compiler Error 1508: I;O Variable Cannot Be Passed to Read File Functions

Crestron SIMPL+
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Software
Language Reference Guide - DOC. 5797G SIMPL+
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z 325
Compiler Error 1508
function argument error: I/O variable cannot be passed to
read file functions: '<identifier>'
Read file functions (ReadInteger, ReadString, etc.) cannot contain Input or Output
variables for the function’s resulting read buffer.
The following are examples of this error:
DIGITAL_OUTPUT digOut;
STRING_OUTPUT strOut;
FUNCTION MyFunc( SIGNED_INTEGER nHandle )
{
STRING str[100];
INTEGER x;
ReadInteger( nHandle, x ); // ok
ReadString( nHandle, str ); // ok
ReadInteger( nHandle, digOut ); // error
ReadString( nHandle, strOut ); // error
}