GNU Compiler Collection (GCC) Internals: GGC Roots

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### 23.4 Marking Roots for the Garbage Collector

In addition to keeping track of types, the type machinery also locates the global variables (roots) that the garbage collector starts at. Roots must be declared using one of the following syntaxes:

- `extern GTY(([options])) type name;`
- `static GTY(([options])) type name;`

The syntax

- `GTY(([options])) type name;`

is not accepted. There should be an `extern` declaration of such a variable in a header somewhere—mark that, not the definition. Or, if the variable is only used in one file, make it `static`.