API request gets error with Postman, different behavior with curl

Question

Windows 11

I'm attempting what should be a pedestrian API request, but I'm getting an error in Postman (which might be a Postman bug).

The API call is possible because it runs on an Android app that logs the JSON request and response (but no other data about the API, sadly enough).

The call is to a small device. The small device has Wi-Fi, so I can connect from my PC to the small device. The API is simple:
- http protocol
- call to a specific Ip address and a specific port
- JSON, with two key:value pairs

And I get this error:

Notice the "Parse Error: Expected HTTP/" there at the top. Here's a Stack Overflow thread that comes to the conclusion that this is a bug in Postman, so... try curl! People do, and it works... for them.

When I try, I get a whole new set of issues. It connects, but gives me an error. Here's my command:

curl.exe -v -d "{"token":0, "msg_id":257}" -H "Content-Type: application/json" http://192.168.42.1:7878

And here's the "response" I get.

*   Trying 192.168.42.1:7878...
* Connected to 192.168.42.1 (192.168.42.1) port 7878 (#0)
> POST / HTTP/1.1
> Host: 192.168.42.1:7878
> User-Agent: curl/8.0.1
> Accept: */*
> Content-Type: application/json
> Content-Length: 21
>
* Received HTTP/0.9 when not allowed
* Closing connection 0
curl: (1) Received HTTP/0.9 when not allowed

"Received HTTP/0.9 when not allowed". OK. Some Googling leads me to this blog, where the blogger confidently says to try the flag --http0.9, so I add it.

curl.exe -v -d "{"token":0, "msg_id":257}" -H "Content-Type: application/json" --http0.9 http://192.168.42.1:7878

*   Trying 10.0.0.254:7878...
* Connected to 10.0.0.254 (10.0.0.254) port 7878 (#0)
> POST / HTTP/1.1
> Host: 10.0.0.254:7878
> User-Agent: curl/8.0.1
> Accept: */*
> Content-Type: application/json
> Content-Length: 21
>
{"rval":-4,"msg_id":0} BLINKING CURSOR, IS HANGING

Not only will the API not finish, that's not the response I should get. I should get this:

{"rval":0, "msg_id":257, "param": <TokenNumber>}

I'm way outside of my knowledge zone here. Does anyone have any ideas on what's happening?

Everything I've tried is described above.

Answer 1

Meta: I'm not sure if testing an API from commandline counts as development; it certainly isn't programming. But as there are no VTC (yet) and this is much too long for a comment:

The commandline syntax -d "{"token":0, "msg_id":257}" does not send valid JSON; the quotes are processed and removed (by the shell on Unix, and by the C runtime and/or PowerShell on Windows which it appears you are using) and what curl actually sends is {token:0, msg_id:257}. I'd bet the "rval":-4 response indicates something like 'invalid data format'.

On Unix you can (easily) use singlequotes to fix this: -d '{"token":0,"msg_id":257}' . (That includes WSL on Windows IF you enter the command directly not as (an) argument(s) to wsl or bash in CMD or PS.)

On (native) Windows syntax varies because different builds use different C implementations (compilers and runtime libraries) AND PS interferes with the argument parsing in a way CMD does not. If you are using the Microsoft-supplied (in Win 10 up) curl.exe in CMD then either backslashing or doubling should send the correct data:

... -d "{\"token\":0,\"msg_id\":257}" ...
... -d "{""token"":0,""msg_id"":257}" ...

(and as you've found --http0.9 should accept the headerless response). If you are using MS curl.exe in PS, it appears only singlequotes AND backslash works:

... -d '{\"token\":0,\"msg_id\":257}' ...

Alternatively in either case you can avoid some/most of this issue by piping the data instead of supplying it as an argument:

CMD> echo {"token:0", "msg_id":257} | curl[.exe] -d@- -H"content-type: application/json" --http0.9 http://whatever 
PS> echo '{"token":0, "msg_id":257}' | curl.exe -d@- -H"content-type: application/json" --http0.9 http://whatever 

(In PS echo is an alias for write-output and you can use that instead to be explicit.)