C++ struct to Python using UDP socket

Question

I'm trying to send a C++ struct over a UDP socket to a Python app.

This is the C++ code to send the struct:

// my struct
struct S_telemetryPacket  {
    // sensors
    float temperatureSensor1;
    float accelerometer1_x;
    float accelerometer1_y;
    float accelerometer1_z;
    float batteryVoltage1;
    float powerDraw1;

    // motors
    int motor1;
    int motor2;
    int motor3;
    int motor4;

} S_telemetryPacket;

// ... some other code populates the struct

// then the struct is sent over UDP
int res = sendto(relaySocket, (char *)&S_telemetryPacket, sizeof(S_telemetryPacket), NULL, (SOCKADDR *)&addrGroundstation, addrGroundstationSize);

And this is the raw data received in Python:

\x00P\x03E\x00\x00\xfaD\x00\x00\x00\x00\x00@\xfbD\x00`\xfbD\x00@\x03Ed\x00\x00\x00e\x00\x00\x00n\x00\x00\x00o\x00\x00\x00

When I try to unpack it using the struct library, I get an error.

print struct.unpack('eeeeeeiiii', raw_data)

This error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
struct.error: bad char in struct format

Can anyone shed some light? The data I receive looks weird, there are symbols I wouldn't expect like `, or @, or o, etc.

Could it be something wrong with how the struct is sent from the C++ side?

do you extract all the headers ( UDP ,...) ?

ralf htp
– ralf htp

2018-01-14 22:56:34 +00:00
Commented Jan 14, 2018 at 22:56 — ralf htp
– ralf htp, Commented Jan 14, 2018 at 22:56

Stephen Rauch · Accepted Answer · 2018-01-14 23:03:09Z

1

The error message says:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
struct.error: bad char in struct format

This states that format string is wrong.

You should try:

struct.unpack('ffffffiiii', raw_data)

The e format string is for a fairly esoteric 16 bit float, and is unlikely to be what you need. In addition it is not supported on Python 2.

Test Code:

import struct

raw_data = b'\x00P\x03E\x00\x00\xfaD\x00\x00\x00\x00\x00@\xfbD\x00`' \
           b'\xfbD\x00@\x03Ed\x00\x00\x00e\x00\x00\x00n\x00\x00\x00o' \
           b'\x00\x00\x00'
print(struct.unpack('ffffffiiii', raw_data))

Results:

(2101.0, 2000.0, 0.0, 2010.0, 2011.0, 2100.0, 100, 101, 110, 111)

edited Jan 14, 2018 at 23:03

answered Jan 14, 2018 at 22:54

Stephen Rauch♦

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4 Comments

Gordon13 Over a year ago

Ah, great that works. I misread the table and used e instead of f...For some reason, the third data element (0.0) is wrong though. It should be 2001. All the other values are correct. Any ideas why?

Stephen Rauch Over a year ago

It looks like that 0 is in your data... Characters 8-11 are all \x00. At this point I would check the sender...

Stephen Rauch Over a year ago

Every 4 characters is one float. The first 4 characters are \x00P\x03E and are 2101.0

Gordon13 Over a year ago

Perfect. That was it. The sender had a typo and it was using the wrong value... Thanks for the help!

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