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Encoding Spec


64-bit Words

For the purpose of Cap’n Proto, a “word” is defined as 8 bytes, or 64 bits. Since alignment of data is important, all objects (structs, lists, and blobs) are aligned to word boundaries, and sizes are usually expressed in terms of words. (Primitive values are aligned to a multiple of their size within a struct or list.)


The unit of communication in Cap’n Proto is a “message”. A message is a tree of objects, with the root always being a struct.

Physically, messages may be split into several “segments”, each of which is a flat blob of bytes. Typically, a segment must be loaded into a contiguous block of memory before it can be accessed, so that the relative pointers within the segment can be followed quickly. However, when a message has multiple segments, it does not matter where those segments are located in memory relative to each other; inter-segment pointers are encoded differently, as we’ll see later.

Ideally, every message would have only one segment. However, there are a few reasons why splitting a message into multiple segments may be convenient:

The first word of the first segment of the message is always a pointer pointing to the message’s root struct.


Each segment in a message contains a series of objects. For the purpose of Cap’n Proto, an “object” is any value which may have a pointer pointing to it. Pointers can only point to the beginning of objects, not into the middle, and no more than one pointer can point at each object. Thus, objects and the pointers connecting them form a tree, not a graph. An object is itself composed of primitive data values and pointers, in a layout that depends on the kind of object.

At the moment, there are three kinds of objects: structs, lists, and far-pointer landing pads. Blobs might also be considered to be a kind of object, but are encoded identically to lists of bytes.

Value Encoding

Primitive Values

The built-in primitive types are encoded as follows:

Primitive types must always be aligned to a multiple of their size. Note that since the size of a Bool is one bit, this means eight Bool values can be encoded in a single byte – this differs from C++, where the bool type takes a whole byte.


Enums are encoded the same as UInt16.

Object Encoding


The built-in blob types are encoded as follows:


A list value is encoded as a pointer to a flat array of values.

lsb                       list pointer                        msb
|A|             B               |C |             D              |

A (2 bits) = 1, to indicate that this is a list pointer.
B (30 bits) = Offset, in words, from the end of the pointer to the
    start of the first element of the list.  Signed.
C (3 bits) = Size of each element:
    0 = 0 (e.g. List(Void))
    1 = 1 bit
    2 = 1 byte
    3 = 2 bytes
    4 = 4 bytes
    5 = 8 bytes (non-pointer)
    6 = 8 bytes (pointer)
    7 = composite (see below)
D (29 bits) = Number of elements in the list, except when C is 7
    (see below).

The pointed-to values are tightly-packed. In particular, Bools are packed bit-by-bit in little-endian order (the first bit is the least-significant bit of the first byte).

Lists of structs use the smallest element size in which the struct can fit. So, a list of structs that each contain two UInt8 fields and nothing else could be encoded with C = 3 (2-byte elements). A list of structs that each contain a single Text field would be encoded as C = 6 (pointer elements). A list of structs that each contain a single Bool field would be encoded using C = 1 (1-bit elements). A list of structs which are each more than one word in size must be encoded using C = 7 (composite).

When C = 7, the elements of the list are fixed-width composite values – usually, structs. In this case, the list content is prefixed by a “tag” word that describes each individual element. The tag has the same layout as a struct pointer, except that the pointer offset (B) instead indicates the number of elements in the list. Meanwhile, section (D) of the list pointer – which normally would store this element count – instead stores the total number of words in the list (not counting the tag word). The reason we store a word count in the pointer rather than an element count is to ensure that the extents of the list’s location can always be determined by inspecting the pointer alone, without having to look at the tag; this may allow more-efficient prefetching in some use cases. The reason we don’t store struct lists as a list of pointers is because doing so would take significantly more space (an extra pointer per element) and may be less cache-friendly.

In the future, we could consider implementing matrixes using the “composite” element type, with the elements being fixed-size lists rather than structs. In this case, the tag would look like a list pointer rather than a struct pointer. As of this writing, no such feature has been implemented.

Notice that because a small struct is encoded as if it were a primitive value, this means that if you have a field of type List(T) where T is a primitive or blob type, it is possible to change that field to List(U) where U is a struct whose @0 field has type T, without breaking backwards-compatibility. This comes in handy when you discover too late that you need to associate some extra data with each value in a primitive list – instead of using parallel lists (eww), you can just replace it with a struct list.


A struct value is encoded as a pointer to its content. The content is split into two sections: data and pointers, with the pointer section appearing immediately after the data section. This split allows structs to be traversed (e.g., copied) without knowing their type.

A struct pointer looks like this:

lsb                      struct pointer                       msb
|A|             B               |       C       |       D       |

A (2 bits) = 0, to indicate that this is a struct pointer.
B (30 bits) = Offset, in words, from the end of the pointer to the
    start of the struct's data section.  Signed.
C (16 bits) = Size of the struct's data section, in words.
D (16 bits) = Size of the struct's pointer section, in words.

Fields are positioned within the struct according to an algorithm with the following principles:

Field offsets are computed by the Cap’n Proto compiler. The precise algorithm is too complicated to describe here, but you need not implement it yourself, as the compiler can produce a compiled schema format which includes offset information.

Default Values

A default struct is always all-zeros. To achieve this, fields in the data section are stored xor’d with their defined default values. An all-zero pointer is considered “null” (such a pointer would otherwise point to a zero-size struct, which might as well be considered null); accessor methods for pointer fields check for null and return a pointer to their default value in this case.

There are several reasons why this is desirable:

Inter-Segment Pointers

When a pointer needs to point to a different segment, offsets no longer work. We instead encode the pointer as a “far pointer”, which looks like this:

lsb                        far pointer                        msb
|A|B|            C              |               D               |

A (2 bits) = 2, to indicate that this is a far pointer.
B (1 bit) = 0 if the landing pad is one word, 1 if it is two words.
    See explanation below.
C (29 bits) = Offset, in words, from the start of the target segment
    to the location of the far-pointer landing-pad within that
    segment.  Unsigned.
D (32 bits) = ID of the target segment.  (Segments are numbered
    sequentially starting from zero.)

If B == 0, then the “landing pad” of a far pointer is normally just another pointer, which in turn points to the actual object.

If B == 1, then the “landing pad” is itself another far pointer that is interpreted differently: This far pointer (which always has B = 0) points to the start of the object’s content, located in some other segment. The landing pad is itself immediately followed by a tag word. The tag word looks exactly like an intra-segment pointer to the target object would look, except that the offset is always zero.

The reason for the convoluted double-far convention is to make it possible to form a new pointer to an object in a segment that is full. If you can’t allocate even one word in the segment where the target resides, then you will need to allocate a landing pad in some other segment, and use this double-far approach. This should be exceedingly rare in practice since pointers are normally set to point to new objects, not existing ones.

Capabilities (Interfaces)

When using Cap’n Proto for RPC, every message has an associated “capability table” which is a flat list of all capabilities present in the message body. The details of what this table contains and where it is stored are the responsibility of the RPC system; in some cases, the table may not even be part of the message content.

A capability pointer, then, simply contains an index into the separate capability table.

lsb                    capability pointer                     msb
|A|              B              |               C               |

A (2 bits) = 3, to indicate that this is an "other" pointer.
B (30 bits) = 0, to indicate that this is a capability pointer.
    (All other values are reserved for future use.)
C (32 bits) = Index of the capability in the message's capability

In rpc.capnp, the capability table is encoded as a list of CapDescriptors, appearing along-side the message content in the Payload struct. However, some use cases may call for different approaches. A message that is built and consumed within the same process need not encode the capability table at all (it can just keep the table as a separate array). A message that is going to be stored to disk would need to store a table of SturdyRefs instead of CapDescriptors.

Serialization Over a Stream

When transmitting a message, the segments must be framed in some way, i.e. to communicate the number of segments and their sizes before communicating the actual data. The best framing approach may differ depending on the medium – for example, messages read via mmap or shared memory may call for a different approach than messages sent over a socket or a pipe. Cap’n Proto does not attempt to specify a framing format for every situation. However, since byte streams are by far the most common transmission medium, Cap’n Proto does define and implement a recommended framing format for them.

When transmitting over a stream, the following should be sent. All integers are unsigned and little-endian.


For cases where bandwidth usage matters, Cap’n Proto defines a simple compression scheme called “packing”. This scheme is based on the observation that Cap’n Proto messages contain lots of zero bytes: padding bytes, unset fields, and high-order bytes of small-valued integers.

In packed format, each word of the message is reduced to a tag byte followed by zero to eight content bytes. The bits of the tag byte correspond to the bytes of the unpacked word, with the least-significant bit corresponding to the first byte. Each zero bit indicates that the corresponding byte is zero. The non-zero bytes are packed following the tag.

For example, here is some typical Cap’n Proto data (a struct pointer (offset = 2, data size = 3, pointer count = 2) followed by a text pointer (offset = 6, length = 53)) and its packed form:

unpacked (hex):  08 00 00 00 03 00 02 00   19 00 00 00 aa 01 00 00
packed (hex):  51 08 03 02   31 19 aa 01

In addition to the above, there are two tag values which are treated specially: 0x00 and 0xff.


unpacked (hex):  00 (x 32 bytes)
packed (hex):  00 03

unpacked (hex):  8a (x 32 bytes)
packed (hex):  ff 8a (x 8 bytes) 03 8a (x 24 bytes)

Notice that both of the special cases begin by treating the tag as if it weren’t special. This is intentionally designed to make encoding faster: you can compute the tag value and encode the bytes in a single pass through the input word. Only after you’ve finished with that word do you need to check whether the tag ended up being 0x00 or 0xff.

It is possible to write both an encoder and a decoder which only branch at the end of each word, and only to handle the two special tags. It is not necessary to branch on every byte. See the C++ reference implementation for an example.

Packing is normally applied on top of the standard stream framing described in the previous section.


When Cap’n Proto messages may contain repetitive data (especially, large text blobs), it makes sense to apply a standard compression algorithm in addition to packing. When CPU time is scarce, we recommend Google’s Snappy. Otherwise, zlib is slower but will compress more.

Security Notes

A naive implementation of a Cap’n Proto reader may be vulnerable to DoS attacks based on two types of malicious input:

To defend against these attacks, every Cap’n Proto implementation should implement the following restrictions by default: