CODEWITHSUNDEEP
swift
CpeGeo
CpeGeo

Reputation: 78

Swift Data.subdata fails with EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)

I am attempting to retrieve a subset of data from a Data object. When I attempt to get the data using subdata(in:) I get the mentioned error. I cannot figure out what I am doing wrong as all values appear correct. The code in question is:

let tempData = incomingDataBuffer.subdata(in: 0..<headerSizeInBytes)

using the lldb i've investigated and found that everything looks correct.

(lldb) po incomingDataBuffer.count
8

(lldb) po headerSizeInBytes
6

(lldb) po incomingDataBuffer
▿ 8 bytes
  - count : 8
  ▿ pointer : 0x0000600000002a42
    - pointerValue : 105553116277314
  ▿ bytes : 8 elements
    - 0 : 17
    - 1 : 6
    - 2 : 29
    - 3 : 49
    - 4 : 2
    - 5 : 0
    - 6 : 1
    - 7 : 6

(lldb) po incomingDataBuffer.subdata(in: 0..<headerSizeInBytes)
error: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0).
The process has been returned to the state before expression evaluation.

This Doesn't make any sense to me. All the values appear correct. Nothing is nil. Why would I get this failure? Thanks for help. :)

Upvotes: 3

Views: 654

Answers (1)

Martin R
Martin R

Reputation: 539915

The indices of a Data value (or of collections in general) are not necessarily zero-based. A slice shares the indices with the originating data. Example:

let buffer = Data(bytes: [1, 2, 3, 4, 5, 6])[2..<4]

print(buffer.count) // 2
print(buffer.indices) // 2..<4

let tmpData = buffer.subdata(in: 0..<2)
// 💣 Thread 1: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)

Therefore you have to take the start index into account:

let tmpData = buffer[buffer.startIndex ..< buffer.startIndex + 2]
print(tmpData as NSData) // <0304>

or simply use the prefix:

let tmpData = buffer.prefix(2)
print(tmpData as NSData) // <0304>

Applied to your case:

let tempData = incomingDataBuffer.prefix(headerSizeInBytes)

Upvotes: 9

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