3.6 Protocols and handles

The extensible nature of UEFI is built, to a large degree, around protocols. Protocols serve to enable communication between separately built modules, including drivers.

Drivers create protocols consisting of two parts. The body of a protocol is a C-style data structure known as a protocol interface structure, or just "interface". The interface typically contains an associated set of function pointers and data structures.

Every protocol has a GUID associated with it. The GUID serves as the name for the protocol. The GUID also indicates the organization of the data structure associated with the GUID. Note that the GUID is not part of the data structure itself.

The example below shows a portion of the Component Named 2 Protocol definition from the UEFI Driver Model chapter of the UEFI Specification. Notice that the protocol data structure contains two functions and one data field.

Example 2-Protocol structure in EDK II

///
/// Global ID for the Component Name Protocol
///
#define EFI_COMPONENT_NAME2_PROTOCOL_GUID \
  {0x6a7a5cff, 0xe8d9, 0x4f70, { 0xba, 0xda, 0x75, 0xab, 0x30, 0x25, 0xce, 0x14 } }

typedef struct _EFI_COMPONENT_NAME2_PROTOCOL EFI_COMPONENT_NAME2_PROTOCOL;

///
/// This protocol is used to retrieve user readable names of drivers
/// and controllers managed by UEFI Drivers.
///
struct _EFI_COMPONENT_NAME2_PROTOCOL {
  EFI_COMPONENT_NAME2_GET_DRIVER_NAME      GetDriverName;
  EFI_COMPONENT_NAME2_GET_CONTROLLER_NAME  GetControllerName;

  ///
  /// A Null-terminated ASCII string array that contains one or more
  /// supported language codes. This is the list of language codes that
  /// this protocol supports. The number of languages supported by a
  /// driver is up to the driver writer. SupportedLanguages is
  /// specified in RFC 4646 format.
  ///
  CHAR8 *SupportedLanguages;
};

Protocols are gathered into a single database. The database is not "flat."Instead, it allows protocols to be grouped together. Each group is known as a handle, and the handle is also the data type that refers to the group. The database is thus known as the handle database. Handles are allocated dynamically. Protocols are not required to be unique in the system, but they must be unique on a handle. In other words, a handle may not be associated with two protocols that have the same GUID.