Input count
Input Count
Version
4 bytes
Marker
1 byte
Flag
1 byte
Input Count
variable
Previous TXID
32 bytes
Output Index
4 bytes
ScriptSig Size
variable
ScriptSig
variable
Sequence
4 bytes
Output Count
variable
Amount
8 bytes
ScriptPubKey Size
variable
ScriptPubKey
variable
Witness
variable
Locktime
4 bytes
Size
variable
Format
Compact Size
Description
Indicates the number of inputs.
Example
01Byte Visualization
01
Byte 1
The number of inputs is stored as a variable-length integer (varint).
Let's examine this real transaction:
Breaking this down:
01000000: Version (4 bytes)01: Input count (1 byte, highlighted in the image below)14e68f...: First input begins
As shown in the highlighted section of the transaction hex above, this transaction has an input count of 1 (0x01).
This single byte tells us that we should expect exactly one input in this transaction.
The encoding format depends on the value range:
| Decimal Range | Hex Range | Bytes used | Format |
|---|---|---|---|
| 0 to 252 | 0x00 to 0xfc | 1 | uint8_t |
| 253 to 2¹⁶-1 | 0xfd to 0xffff | 3 | 0xfd followed by the number as uint16_t |
| 2¹⁶ to 2³²-1 | 0x10000 to 0xffffffff | 5 | 0xfe followed by the number as uint32_t |
| 2³² to 2⁶⁴-1 | 0x100000000 to 0xffffffffffffffff | 9 | 0xff followed by the number as uint64_t |
For example:
- The number 100 is encoded directly as
0x64 - The number 1000 is encoded as
0xfd 0xe8 0x03 - The number 100000 is encoded as
0xfe 0xa0 0x86 0x01 0x00
Note
In our example transaction, the input count is 1 (0x01), so it uses the simplest encoding format of a single byte.
Implementation
Here's a Python implementation for reading and writing varints:
def read_varint(s):
'''Reads a variable integer from a stream
The first byte determines the format:
- If < 0xfd: directly contains the number
- If 0xfd: next 2 bytes contain number
- If 0xfe: next 4 bytes contain number
- If 0xff: next 8 bytes contain number
'''
i = s.read(1)[0]
if i == 0xfd:
return int.from_bytes(s.read(2), 'little')
elif i == 0xfe:
return int.from_bytes(s.read(4), 'little')
elif i == 0xff:
return int.from_bytes(s.read(8), 'little')
else:
return i
def encode_varint(i):
'''Encodes an integer as a varint
- Numbers < 0xfd: 1 byte
- Numbers < 0x10000: 3 bytes (0xfd + 2 bytes)
- Numbers < 0x100000000: 5 bytes (0xfe + 4 bytes)
- Numbers < 0x10000000000000000: 9 bytes (0xff + 8 bytes)
'''
if i < 0xfd:
return bytes([i])
elif i < 0x10000:
return b'\xfd' + i.to_bytes(2, 'little')
elif i < 0x100000000:
return b'\xfe' + i.to_bytes(4, 'little')
elif i < 0x10000000000000000:
return b'\xff' + i.to_bytes(8, 'little')
else:
raise ValueError(f'integer too large: {i}')