11 02 variables

Local variables are declared with a type, a name, and an optional initializer (as well as an optional annotation):

variableDeclaration
    : variableDeclarationWithoutSemicolon ";"
    ;

variableDeclarationWithoutSemicolon
    : annotations typeRef name optInitializer
    | typeRef name optInitializer
    ;

optInitializer
    : /* empty */
    | "=" initializer
    ;

Variable declarations without an initializer are uninitialized (except for headers and other header-related types, which are initialized to invalid in the same way as described for direction out parameters in Section Calling convention: call by copy in/copy out). The language places few restrictions on the types of the variables: most P4 types that can be written explicitly can be used (e.g., base types, struct, header, header stack, tuple). However, it is impossible to declare variables with type int, or with types that are only synthesized by the compiler (e.g., set) In addition, variables of type parser, control, package, or extern types must be declared using instantiations (see Section Instantiations).

Reading the value of a variable that has not been initialized yields an undefined result. The compiler should attempt to detect and emit a warning in such situations.

Variables declarations can appear in the following locations within a P4 program:

In a block statement,
In a parser state,
In an action body,
In a control block’s apply sub-block,
In the list of local declarations in a parser, and
In the list of local declarations in a control.

Variables have local scope, and behave like stack-allocated variables in languages such as C. The value of a variable is never preserved from one invocation of its enclosing block to the next. In particular, variables cannot be used to maintain state between different network packets.