STRUCTure EXtensions

structex provides annotation rules that extend Go structures for implementation of encoding and decoding of byte backed data frames.

structex aims to make it easier to represent industry standard data formats that can be directly embedded in code.

Example

Given the first three bytes T10 SCSI Primary Commands definition of a SCSI INQUIRY data

A structex annotated struct type declaration might be

package main

import (
    "fmt"

    "github.com/HewlettPackard/structex"
)

type SCSI_Standard_Inquiry struct {
    PeripheralDeviceType uint8 `bitfield:"5"`          // Byte 0
    PeripheralQualifier  uint8 `bitfield:"3"`
    reserved0            uint8 `bitfield:"6,reserved"` // Byte 1
    LU_Cong              uint8 `bitfield:"1"`
    RMB                  uint8 `bitfield:"1"`
    Version              uint8                         // Byte 2
}

func main() {
    var inquiry = new(SCSI_Standard_Inquiry)

    // Perform IOCTL on device, returns byte reader to byte response

    if err := structex.Decode(byteReader, inquiry); err != nil {
        panic("a problem")
    }

    fmt.Printf("SCSI Peripheral Device Type: %d\n", inquiry.PeripheralDeviceType)
    fmt.Printf("SCSI Peripheral Qualifier: %d\n", inquiry.PeripheralQualifier)
    fmt.Printf("Inquiry Version: %#02x\n", inquiry.Version)
    // etc
}

Constants

const (
	// EnvVarDefaultEndianness is the name of the environment variable used to
	// define the default endian format for all structure elements unless otherwise
	// sepcified. Defaults to little-endian
	EnvVarDefaultEndianness = "X_STRUCTEX_DEFAULT_ENDIANNESS"
)

Annotation Format

Endianness

Little-endian (default) or big-endian tags support for integer and unsigned integer types little:""

big:""

Bitfield

Bitfields define a structure field with an explicit size in bits. They are analogous to bit fields in the C specification.

bitfield:"[size][,reserved]"

`size`:     Specifies the size, in bits, of the field.

`reserved`: Optional modifier that specifies the field contains reserved
            bits and should be encoded as zeros.

Self-Described Layout

Many industry standards support dynamically sized return fields where the data layout is self described by other fields. To support such formats two annotations are provided.

sizeof:"[name][,relative]"

name        Specifies that the field describes the size of `name` within the
            structure. Should be used with slices.

            During decoding, if field is non-zero, the field's value is
            used to limit the number elements in the array or slice of
            name `name`.

relative    Optional modifier that specifies the value describing the size of
            `name` is relative to the field offset within the structure.
            This is often used in T10.org documentation

countof:"[name]"

name        Specifies that the value describes the count of elements in
            the `name` field.

            During decoding, if field is non-zero, the field's value is
            used to limit the number elements in the array or slice of
            name `name`.

Truncation

Structex expects sufficient data for decoding the desired structure. When data structures are used to define a maximum size of the response buffer, you can use the truncate tag on an arry or slice to permit structex to truncate the returned data with what is provided by the source buffer. If truncate is not specified and the provided buffer is of smaller size than the data structure, an io.EOF error is expected.

truncate:""

Alignment

Annotations can specified the byte-alignment requirement for structure fields. Analogous to the alignas specifier in C. Can only be applied to non-bitfield structure fields.

align:"[value]"

value       An integer value specifying the byte alignment of the field.
            Invalid non-zero alignments panic.

Full Tags

The tags documented above are abbreviated for ease of use; if desired, the full tag format is supported. This provides clarity that tags are part of the structex package, but it means typing more and using every sort of quote and backtic at your disposal.

Examples

`structex:"little"`
`structex:"big"`
`structex:"bitfield='3'"`
`structex:"bitfield='3,reserved'"`
`structex:"countOf='D'"`
`structex:"sizeOf='E'"`
`structex:"sizeOf='F,relative'"`
`structex:"align='8'"`
`structex:"truncate"`

Performance

structex places code readability ahead of any pack/unpack performance. Reflection is used to analyze the structure definitions, which itself carries heavy overhead. Parsing the tags also takes time.

For those looking for more performant code, consider code generation or a cache based solutions