struct-to-json

Latest Version	v1.0.0
Input Content Type	application/octet-stream
Output Content Type	application/json

The struct-to-json transformer takes packed C structures and converts them to JSON according to a user provided schema.

The schema is an ordered list of objects in the Pipeline YAML. Each object describes one member of the struct and must contain a name and type. Additionally, string types must specify a length (more below). The accepted types are:

Schema Value	Description
u8	Unsigned 8-bit integer
i8	Signed 8-bit integer
u16	Unsigned 16-bit integer
i16	Signed 16-bit integer
u32	Unsigned 32-bit integer
i32	Signed 32-bit integer
u64	Unsigned 64-bit integer
i64	Signed 64-bit integer
float	Single precision (32-bit) IEEE 754 floating point number
double	Double precision (64-bit) IEEE 754 floating point number
string	Variable length string. NOT NULL terminated

For string type members, the YAML object must also contain exactly one of length or lengthField.

• length contains a non-negative integer with the constant length of the string.
• lengthField contains the name of a member of the same struct. The member must hold a non-negative integer value, and the length member must come before the string member in the payload.

Example Usage

    transformer:
      type: struct-to-json
      version: v1
      parameters:
        members:
          - name: temperature
            type: float
          - name: string1
            type: string
            length: 5
          - name: string2_len
            type: u8
          - name: string2
            type: string
            lengthField: string2_len

Example Input

struct my_struct {
    float temperature;
    char string1[5];
    uint8_t string2_len;
    char string2[];
} __attribute__((packed));

struct my_struct *s = malloc(sizeof(struct my_struct) + strlen("Golioth!"));

s->temperature = 23.5f;
s->string2_len = strlen("Golioth!");
memcpy(s->string1, "hello", strlen("hello"));
memcpy(s->string2, "Golioth!", strlen("Golioth!"));

This is the hexdump of s as created by the above C snippet:

00000000  00 00 bc 41 68 65 6c 6c  6f 08 47 6f 6c 69 6f 74  |...Ahello.Goliot|
00000010  68 21                                             |h!|
00000012

Example Output

{
  "temperature": 23.5,
  "string1": "Hello",
  "string2_len": 8,
  "string2": "Golioth!"
}

Tip

If you do not require the string length after the struct-to-json transform step, you can remove it using the json-patch transformer before sending the data to your destination.