sfixed64

sfixed64 is a 64-bit fixed-width signed integer field type used in Protocol Buffers. It occupies exactly eight bytes on the wire and uses the fixed64 wire encoding, which is the same eight-byte representation as fixed64 but interpreted as a signed value. The difference from other integer types is in interpretation rather than encoding: sfixed64 represents a signed two’s-complement number, with a range from -2^63 to 2^63-1, whereas fixed64 represents an unsigned 64-bit value from 0 to 2^64-1.

In the binary encoding, sfixed64 is stored as eight raw bytes in little-endian order. There is no varint or zigzag encoding for this type. For example, the value -1 is encoded as eight 0xFF bytes; the value 1 is encoded as 01 00 00 00 00 00 00 00. The width is fixed, so the field always consumes eight bytes regardless of the magnitude of the number.

sfixed64 is chosen when a fixed-width signed representation is desirable for performance, alignment, or interoperability reasons. It is useful for fields that are frequently accessed, indexed, or compared, and when a signed 64-bit interpretation is required. It is typically used alongside other fixed-width types like fixed64 and sfixed32, as well as variable-length types such as int64 and sint64, depending on the desired encoding characteristics.

In most languages, sfixed64 maps to a 64-bit signed integer type, and protobuf tooling generates accessors that read and write values using that type, preserving two’s-complement semantics.