Message Format

zcm-msg is a simple typed-value envelope independent of the transport and data format.

Envelope

• magic (u32): 0x5A434D31 (ZCM1)
• version (u16): 1
• type_len (u16): length of type string
• payload_len (u32)
• type (bytes): UTF-8, no NUL terminator
• payload (bytes): sequence of typed items

Typed items

Each item begins with a one-byte item_type followed by item data.

Scalar items

• CHAR (1): 1 byte
• SHORT (2): 2 bytes
• INT (3): 4 bytes
• LONG (4): 8 bytes
• FLOAT (5): 4 bytes
• DOUBLE(6): 8 bytes

Array item

• ARRAY (7):
  • array_type (u8): CHAR/SHORT/INT/FLOAT/DOUBLE
  • elements (u32)
  • data (elements * element_size)

Text item

• TEXT (8):
  • len (u32)
  • data (len bytes)

Bytes item

• BYTES (9):
  • len (u32)
  • data (len bytes)

Value property

For command-line interoperability and generic process handling, zCm defines a standardized scalar value property encoded as three typed items in order:

1. TEXT marker: __zcm_value__
2. TEXT scalar kind: text | double | float | int
3. Value item matching kind:
   • TEXT for text
   • DOUBLE for double
   • FLOAT for float
   • INT for int

Library helpers:

• zcm_msg_put_value_text()
• zcm_msg_put_value_double()
• zcm_msg_put_value_float()
• zcm_msg_put_value_int()
• zcm_msg_get_value()

Endianness

All numeric fields are stored in little-endian encoding, regardless of host endianness.