Protocol Buffers Version 2 Language Specification
The syntax is specified using Extended Backus-Naur Form (EBNF):
| alternation
() grouping
[] option (zero or one time)
{} repetition (any number of times)
For more information about using proto2, see the language guide.
Lexical Elements
Letters and Digits
letter = "A" ... "Z" | "a" ... "z"
capitalLetter = "A" ... "Z"
decimalDigit = "0" ... "9"
octalDigit = "0" ... "7"
hexDigit = "0" ... "9" | "A" ... "F" | "a" ... "f"
Identifiers
ident = letter { letter | decimalDigit | "_" }
fullIdent = ident { "." ident }
messageName = ident
enumName = ident
fieldName = ident
oneofName = ident
mapName = ident
serviceName = ident
rpcName = ident
streamName = ident
messageType = [ "." ] { ident "." } messageName
enumType = [ "." ] { ident "." } enumName
groupName = capitalLetter { letter | decimalDigit | "_" }
Integer Literals
intLit = decimalLit | octalLit | hexLit
decimalLit = [-] ( "1" ... "9" ) { decimalDigit }
octalLit = [-] "0" { octalDigit }
hexLit = [-] "0" ( "x" | "X" ) hexDigit { hexDigit }
Floating-point Literals
floatLit = [-] ( decimals "." [ decimals ] [ exponent ] | decimals exponent | "."decimals [ exponent ] ) | "inf" | "nan"
decimals = [-] decimalDigit { decimalDigit }
exponent = ( "e" | "E" ) [ "+" | "-" ] decimals
Boolean
boolLit = "true" | "false"
String Literals
strLit = strLitSingle { strLitSingle }
strLitSingle = ( "'" { charValue } "'" ) | ( '"' { charValue } '"' )
charValue = hexEscape | octEscape | charEscape | unicodeEscape | unicodeLongEscape | /[^\0\n\\]/
hexEscape = '\' ( "x" | "X" ) hexDigit [ hexDigit ]
octEscape = '\' octalDigit [ octalDigit [ octalDigit ] ]
charEscape = '\' ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | '\' | "'" | '"' )
unicodeEscape = '\' "u" hexDigit hexDigit hexDigit hexDigit
unicodeLongEscape = '\' "U" ( "000" hexDigit hexDigit hexDigit hexDigit hexDigit |
"0010" hexDigit hexDigit hexDigit hexDigit
EmptyStatement
emptyStatement = ";"
Constant
constant = fullIdent | ( [ "-" | "+" ] intLit ) | ( [ "-" | "+" ] floatLit ) |
strLit | boolLit | MessageValue
MessageValue is defined in the
Text Format Language Specification.
Syntax
The syntax statement is used to define the protobuf version.
syntax = "syntax" "=" ("'" "proto2" "'" | '"' "proto2" '"') ";"
Import Statement
The import statement is used to import another .proto’s definitions.
import = "import" [ "weak" | "public" ] strLit ";"
Example:
import public "other.proto";
Package
The package specifier can be used to prevent name clashes between protocol message types.
package = "package" fullIdent ";"
Example:
package foo.bar;
Option
Options can be used in proto files, messages, enums and services. An option can be a protobuf defined option or a custom option. For more information, see Options in the language guide.
option = "option" optionName "=" constant ";"
optionName = ( ident | bracedFullIdent ) { "." ( ident | bracedFullIdent ) }
bracedFullIdent = "(" ["."] fullIdent ")"
For examples:
option java_package = "com.example.foo";
Fields
Fields are the basic elements of a protocol buffer message. Fields can be normal fields, group fields, oneof fields, or map fields. A field has a label, type and field number.
label = "required" | "optional" | "repeated"
type = "double" | "float" | "int32" | "int64" | "uint32" | "uint64"
| "sint32" | "sint64" | "fixed32" | "fixed64" | "sfixed32" | "sfixed64"
| "bool" | "string" | "bytes" | messageType | enumType
fieldNumber = intLit;
Normal field
Each field has label, type, name and field number. It may have field options.
field = label type fieldName "=" fieldNumber [ "[" fieldOptions "]" ] ";"
fieldOptions = fieldOption { "," fieldOption }
fieldOption = optionName "=" constant
Examples:
optional foo.bar nested_message = 2;
repeated int32 samples = 4 [packed=true];
Group field
Note that this feature is deprecated and should not be used when creating new message types – use nested message types instead.
Groups are one way to nest information in message definitions. The group name must begin with capital letter.
group = label "group" groupName "=" fieldNumber messageBody
Example:
repeated group Result = 1 {
required string url = 1;
optional string title = 2;
repeated string snippets = 3;
}
Oneof and oneof field
A oneof consists of oneof fields and a oneof name. Oneof fields do not have labels.
oneof = "oneof" oneofName "{" { option | oneofField } "}"
oneofField = type fieldName "=" fieldNumber [ "[" fieldOptions "]" ] ";"
Example:
oneof foo {
string name = 4;
SubMessage sub_message = 9;
}
Map field
A map field has a key type, value type, name, and field number. The key type can be any integral or string type. Note, the key type may not be an enum.
mapField = "map" "<" keyType "," type ">" mapName "=" fieldNumber [ "[" fieldOptions "]" ] ";"
keyType = "int32" | "int64" | "uint32" | "uint64" | "sint32" | "sint64" |
"fixed32" | "fixed64" | "sfixed32" | "sfixed64" | "bool" | "string"
Example:
map<string, Project> projects = 3;
Extensions and Reserved
Extensions and reserved are message elements that declare a range of field numbers or field names.
Extensions
Extensions declare that a range of field numbers in a message are available for third-party extensions. Other people can declare new fields for your message type with those numeric tags in their own .proto files without having to edit the original file.
extensions = "extensions" ranges ";"
ranges = range { "," range }
range = intLit [ "to" ( intLit | "max" ) ]
Examples:
extensions 100 to 199;
extensions 4, 20 to max;
Reserved
Reserved declares a range of field numbers or field names in a message that can not be used.
reserved = "reserved" ( ranges | strFieldNames ) ";"
strFieldNames = strFieldName { "," strFieldName }
strFieldName = "'" fieldName "'" | '"' fieldName '"'
Examples:
reserved 2, 15, 9 to 11;
reserved "foo", "bar";
Top Level definitions
Enum definition
The enum definition consists of a name and an enum body. The enum body can have options, enum fields, and reserved statements.
enum = "enum" enumName enumBody
enumBody = "{" { option | enumField | emptyStatement | reserved } "}"
enumField = ident "=" [ "-" ] intLit [ "[" enumValueOption { "," enumValueOption } "]" ]";"
enumValueOption = optionName "=" constant
Example:
enum EnumAllowingAlias {
option allow_alias = true;
EAA_UNSPECIFIED = 0;
EAA_STARTED = 1;
EAA_RUNNING = 2 [(custom_option) = "hello world"];
}
Message definition
A message consists of a message name and a message body. The message body can have fields, nested enum definitions, nested message definitions, extend statements, extensions, groups, options, oneofs, map fields, and reserved statements. A message cannot contain two fields with the same name in the same message schema.
message = "message" messageName messageBody
messageBody = "{" { field | enum | message | extend | extensions | group |
option | oneof | mapField | reserved | emptyStatement } "}"
Example:
message Outer {
option (my_option).a = true;
message Inner { // Level 2
required int64 ival = 1;
}
map<int32, string> my_map = 2;
extensions 20 to 30;
}
None of the entities declared inside a message may have conflicting names. All of the following are prohibited:
message MyMessage {
optional string foo = 1;
message foo {}
}
message MyMessage {
optional string foo = 1;
oneof foo {
string bar = 2;
}
}
message MyMessage {
optional string foo = 1;
extend Extendable {
optional string foo = 2;
}
}
message MyMessage {
optional string foo = 1;
enum E {
foo = 0;
}
}
Extend
If a message in the same or imported .proto file has reserved a range for extensions, the message can be extended.
extend = "extend" messageType "{" {field | group} "}"
Example:
extend Foo {
optional int32 bar = 126;
}
Service definition
service = "service" serviceName "{" { option | rpc | emptyStatement } "}"
rpc = "rpc" rpcName "(" [ "stream" ] messageType ")" "returns" "(" [ "stream" ]
messageType ")" (( "{" { option | emptyStatement } "}" ) | ";" )
Example:
service SearchService {
rpc Search (SearchRequest) returns (SearchResponse);
}
Proto file
proto = [syntax] { import | package | option | topLevelDef | emptyStatement }
topLevelDef = message | enum | extend | service
An example .proto file:
syntax = "proto2";
import public "other.proto";
option java_package = "com.example.foo";
enum EnumAllowingAlias {
option allow_alias = true;
EAA_UNSPECIFIED = 0;
EAA_STARTED = 1;
EAA_RUNNING = 1;
EAA_FINISHED = 2 [(custom_option) = "hello world"];
}
message Outer {
option (my_option).a = true;
message Inner { // Level 2
required int64 ival = 1;
}
repeated Inner inner_message = 2;
optional EnumAllowingAlias enum_field = 3;
map<int32, string> my_map = 4;
extensions 20 to 30;
}
message Foo {
optional group GroupMessage = 1 {
optional bool a = 1;
}
}