f77: Undeclared Variable Types (-u)

The -u option checks for any undeclared variables. (Not available with f90.)

The -u option causes all variables to be initially identified as undeclared, so that all variables that are not explicitly declared by type statements, or by an IMPLICIT statement, are flagged with an error. The -u flag is useful for discovering mistyped variables. If -u is set, all variables are treated as undeclared until explicitly declared. Use of an undeclared variable is accompanied by an error message.