sto_subfield

Index into a xrpld serialized object and return the location and length of a subfield

Concepts

• Serialized Objects

Behaviour

• Parse a STObject pointed to by read_ptr
• Find the field specified by field_id
• If the field is found, and:
• 1. It is an array, then return the start and length of the array including the leadin/leadout bytes, or
• 2. It is not an array, then return the start and length of the PAYLOAD of the field (excluding the leadin bytes).

🚧

Field ID encoding

The sto_ apis accept a field_id parameter encoded as follows: (type << 16U) + field
Thus type 1 field 2 would be 0x10002U.

Definition

C

int64_t sto_subfield (
    uint32_t read_ptr,
  	uint32_t read_len,
  	uint32_t field_id
);

Example

C

#define SUB_OFFSET(x) ((int32_t)(x >> 32))
#define SUB_LENGTH(x) ((int32_t)(x & 0xFFFFFFFFULL))
int64_t memo_lookup =
    sto_subfield(memo_ptr, memo_len, sfMemo);
if (memo_lookup < 0)
{
    // sfMemo was not found in the STObject pointed at by memo_ptr
}
else
{
    // sfMemo was found and its location is as follows:
	  uint8_t* memo_ptr = SUB_OFFSET(memo_lookup) + memo_ptr;
	  int64_t  memo_len = SUB_LENGTH(memo_lookup);
}

📘

hookmacro.h already contains the SUB_OFFSET and SUB_LENGTH macros.

Parameters

Name	Type	Description
read_ptr	uint32_t	Pointer to the buffer containing the STObject
read_len	uint32_t	Length of STObject
field_id	uint32_t	The sf code of the field you are searching for. To compute this manually take the serialized type and shift it into the 16 highest bits of uint32_t, then take the field and place it in the 16 lowest bits. For example: sfEmitNonce has type 5 and field 11 thus its value is 0x050BU

Return Code

Type

Description

int64_t

The location of the field within the specified buffer: - The high 32 bits are the offset location. - The low 32 bits are the length. If negative, an error: OUT_OF_BOUNDS - pointers/lengths specified outside of hook memory. TOO_SMALL - Input buffer isn't large enough to possibly contain a valid STObject. DOESNT_EXIST - The searched for field isn't present in the supplied STObject. PARSE_ERROR - The supplied STObject is malformed or not an STObject.