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.

/// 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 an array or 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<T>(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<T>(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<T>();
    /// 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<T>(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);