Appendix D P4 core library

The P4 core library contains declarations that are useful to most programs.

For example, the core library includes the declarations of the predefined packet_in and packet_out extern objects, used in parsers and deparsers to access packet data.

\~ Begin P4Example /// Standard error codes. New error codes can be declared by users. error { NoError, /// No error. PacketTooShort, /// Not enough bits in packet for ‘extract’. NoMatch, /// ‘select’ expression has no matches. StackOutOfBounds, /// Reference to invalid element of a header stack. HeaderTooShort, /// Extracting too many bits into a varbit field. ParserTimeout, /// Parser execution time limit exceeded. ParserInvalidArgument /// Parser operation was called with a value /// not supported by the implementation. } extern packet_in { /// Read a header from the packet into a fixed-sized header @hdr /// and advance the cursor. /// May trigger error PacketTooShort or StackOutOfBounds. /// @T must be a fixed-size header type void extract(out T hdr); /// Read bits from the packet into a variable-sized header @variableSizeHeader /// and advance the cursor. /// @T must be a header containing exactly 1 varbit field. /// May trigger errors PacketTooShort, StackOutOfBounds, or HeaderTooShort. void extract(out T variableSizeHeader, in bit\<32> variableFieldSizeInBits); /// Read bits from the packet without advancing the cursor. /// @returns: the bits read from the packet. /// T may be an arbitrary fixed-size type. T lookahead(); /// Advance the packet cursor by the specified number of bits. void advance(in bit\<32> sizeInBits); /// @return packet length in bytes. This method may be unavailable on /// some target architectures. bit\<32> length(); } extern packet_out { /// Write @data into the output packet, skipping invalid headers /// and advancing the cursor /// @T can be a header type, a header stack, a header_union, or a struct /// containing fields with such types. void emit(in T data); } action NoAction() {} /// Standard match kinds for table key fields. /// Some architectures may not support all these match kinds. /// Architectures can declare additional match kinds. match_kind { /// Match bits exactly. exact, /// Ternary match, using a mask. ternary, /// Longest-prefix match. lpm }

/// Static assert evaluates a boolean expression /// at compilation time. If the expression evaluates to /// false, compilation is stopped and the corresponding message is printed. /// The function returns a boolean, so that it can be used /// as a global constant value in a program, e.g.: /// const version = static_assert(V1MODEL_VERSION > 20180000, “Expected a v1 model version >= 20180000”); extern bool static_assert(bool check, string message);

/// Like the above but using a default message. extern bool static_assert(bool check);

\~ End P4Example