Correct format for JSONRPC for Clangd

Question

I am attempting to send JSON to Clangd on my desktop using Python. I see in the terminal window that Clangd says:

I[15:52:46.580] Warning: Missing Content-Length header, or zero-length message.
"Content-Length": 53

{
    "jsonrpc": "2.0", 
    "id": 1, 
    "method": "exit"
}

But that is not the case. Python code:

import argparse
import sys

if __name__ == "__main__":
    
    parser = argparse.ArgumentParser()
    parser.add_argument("-p", "--process_num", type=int, help="Clangd process number", required=True)
    parser.add_argument("-d", "--dry_run", action="store_true", help="Dry run")
    
    args = parser.parse_args()
    
    filepath = f"/proc/{args.process_num}/fd/0"
    
    my_content = """\
"Content-Length": 53

{
    "jsonrpc": "2.0", 
    "id": 1, 
    "method": "exit"
}"""
    
    if args.dry_run:
        print(my_content.encode("utf-8"))
        sys.exit(0)
    
    with open(filepath, 'wb') as f:
        f.write(my_content.encode("utf-8"))

Is there something in the formatting I am missing?

Update:

Made changes based on insight from HighCommander:

import argparse
import sys
import json

if __name__ == "__main__":
    
    parser = argparse.ArgumentParser()
    parser.add_argument("-p", "--process_num", type=int, help="Clangd process number", required=True)
    parser.add_argument("-d", "--dry_run", action="store_true", help="Dry run")
    
    args = parser.parse_args()
    
    filepath = f"/proc/{args.process_num}/fd/0"
    
    
    my_object = {}
    my_object["jsonrpc"] = "2.0"
    my_object["id"] = 1
    my_object["method"] = "exit"
    
    my_content = json.dumps(my_object).encode()
    
    my_header = "Content-Length: {}\r\n\r\n".format(len(my_content)).encode("ascii")

    
    if args.dry_run:
        print(my_header)
        print(my_content)
        sys.exit(0)
    
    with open(filepath, 'wb') as f:
        f.write(my_header)
        f.write(my_content)

I get the missing Content-Length/zero length message print out from Clangd

You might find it helpful to look at an example of a working JSON-RPC implementation in python: github.com/HighCommander4/ls-interact/blob/master/common.py — HighCommander4
– HighCommander4, Commented Apr 27 at 1:05
@HighCommander4, thanks, unfortunately the issue is still at-hand. — xBACP
– xBACP, Commented Apr 27 at 1:25
Taking another look, I see that you're writing to /proc/<pid>/fd/0 of an existing clangd process. I've never seen it done this way, I've only seen it done in a way where the clangd process is launched by the client as a subprocess. Maybe get it to work that way first, to rule out protocol format issues? — HighCommander4
– HighCommander4, Commented Apr 27 at 19:02

nos · Accepted Answer · 2025-04-26 23:56:26Z

3

You're not supposed to quote the "Content-Length" , just send Content-Length: 53

Make sure you're using \r\n for the newlines too in the header part, as shown in the Language Server Protocol . Make sure your Content-Length is right as well, the json you posted here isn't 53 bytes,

answered Apr 26 at 23:56

nos

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