A concept related to parameter passing modes is that of read-only parameters. The term “read-only” refers, not to a passing mode, but to a restriction on the ways a parameter can be used.

Formal parameter declarations in a Pascal program can include a CONST clause, which causes a parameter to be treated as a read-only parameter. The CONST clause prevents the receiving Pascal program or procedure from making any changes to the value of the formal parameter. However, the CONST clause does not guarantee that the formal parameter has a constant value. The formal parameter value can change because the CONST clause does not affect the passing mode. If the actual parameter is passed by name or by reference, then any changes made by the initiator to the value of the actual parameter are immediately reflected in the value of the formal parameter.