SPI_execp is the same as SPI_execute_plan, with the latter's read_only parameter always taken as false.
prepared statement (returned by SPI_prepare)
An array of actual parameter values. Must have same length as the statement's number of arguments.
An array describing which parameters are null. Must have same length as the statement's number of arguments. n indicates a null value (entry in values will be ignored); a space indicates a nonnull value (entry in values is valid).
If nulls is NULL then SPI_execp assumes that no parameters are null.
maximum number of rows to return, or 0 for no limit
Read the upstream documentation. For corrections, first check the current version.