Enables or disables handling of undefined variables as errors. With UNDEF, GT.M handles all references to undefined local or global variables as errors. With NOUNDEF, GT.M handles all references to undefined local or global variables as if the variable had a value of the null string. In other words, GT.M treats all variables appearing in expressions as if they were the argument of an implicit $GET(). UNDEF is the default.

Linking a GT.M image with a modified GTM$DEFAULTS alters the default for that image. For more information on GTM$DEFAULTS see the "Program Development Cycle" chapter in this manual.