binary.Read无法按预期处理结构填充

Question

In a recent Go project I need to read a binary data file generated by Python, but due to padding, binary.Read in Go doesn't read it properly. Below is a minimal example of my problem.

The struct I deal with if of the following format

type Index struct{
    A int32
    B int32
    C int32
    D int64
}

As you can see the size of the struct is 4+4+4+8=20, but Python added an extra 4 bytes for alignment. So the size is actually 24.

Below is the runnable Python code I use to write this struct:

#!/usr/bin/env python
# encoding=utf8

import struct

if __name__ == '__main__':
    data = range(1, 13)
    format = 'iiiq' * 3
    content = struct.pack(format, *data)
    with open('index.bin', 'wb') as f:
        f.write(content)

the iiiq format means there are three 32 bit integers and one 64 bit integer in the struct, which is the same with the Index struct I defined earlier. And running this code will generate a file named index.bin of size 72, which equals to 24 * 3.

And below is the Go code I use to read index.bin:

package main

import (
        "encoding/binary"
        "fmt"
        "os"
        "io"
        "unsafe"
)

type Index struct {
        A int32
        B int32
        C int32
        D int64
}

func main() {
        indexSize := unsafe.Sizeof(Index{})
        fp, _ := os.Open("index.bin")
        defer fp.Close()
        info, _ := fp.Stat()
        fileSize := info.Size()
        entryCnt := fileSize / int64(indexSize)
        fmt.Printf("entry cnt: %d
", entryCnt)

        readSlice := make([]Index, entryCnt)
        reader := io.Reader(fp)
        _ = binary.Read(reader, binary.LittleEndian, &readSlice)
        fmt.Printf("After read:
%#v
", readSlice)
}

And this is the output:

entry cnt: 3
After read:
[]main.Index{main.Index{A:1, B:2, C:3, D:17179869184}, main.Index{A:0, B:5, C:6, D:7}, main.Index{A:8, B:0, C:9, D:47244640266}}

Obviously the output is messed up when reading from the Python generated file.

So my question is, how can I read the python generated file(with padding) in Go properly?

Answer 1

You can just pad your Go struct to match:

type Index struct {
    A int32
    B int32
    C int32
    _ int32
    D int64
}

Which produces:

[]main.Index{main.Index{A:1, B:2, C:3, _:0, D:4}, main.Index{A:5, B:6, C:7, _:0, D:8}, main.Index{A:9, B:10, C:11, _:0, D:12}}

binary.Read knows to skip the _ field:

When reading into structs, the field data for fields with blank (_) field names is skipped; i.e., blank field names may be used for padding.

(So the 0 values for _ are not because the padding in the file was set to zero, but because the struct field was initialized to 0 and never changed, and the padding in the file was skipped rather than read.)