threads.adoc

Threads

The main() function starts the main bitcoind process thread, usefully named bitcoind. All subsequent threads are currently started as children of the bitcoind thread, although this is not an explicit design requirement.

The Bitcoin Core Developer docs contains a section on threads, which is summarised below in two tables, one for net threads, and one for other threads.

Table 1. Non-net threads

Name	Function	Description
bitcoind	main()	Responsible for starting up and shutting down the application, and spawning all sub-threads
b-loadblk	ThreadImport	Loads blocks from blk*.dat files or -loadblock=<file> on startup
b-scriptch.x	ThreadScriptCheck	Parallel script validation threads for transactions in blocks
b-http	ThreadHTTP	Libevent thread to listen for RPC and REST connections
b-httpworker.x	StartHTTPServer	HTTP worker threads. Threads to service RPC and REST requests
b-txindex.x	ThreadSync	Indexer threads. One thread per indexer
b-scheduler	SchedulerThread	Does asynchronous background tasks like dumping wallet contents, dumping addrman and running asynchronous validationinterface callbacks
b-torcontrol	TorControlThread	Libevent thread for tor connections

Net threads

Table 2. Net threads

Name	Function	Description
b-msghand	ThreadMessageHandler	Application level message handling (sending and receiving). Almost all net_processing and validation logic runs on this thread
b-dnsseed	ThreadDNSAddressSeed	Loads addresses of peers from the ThreadDNS
b-upnp	ThreadMapPort	Universal plug-and-play startup/shutdown
b-net	ThreadSocketHandler	Sends/Receives data from peers on port 8333
b-addcon	ThreadOpenAddedConnections	Opens network connections to added nodes
b-opencon	ThreadOpenConnections	Initiates new connections to peers
b-i2paccept	ThreadI2PAcceptIncoming	Listens for and accepts incoming I2P connections through the I2P SAM proxy

Thread debugging

In order to debug a multi-threaded application like bitcoind using gdb you will need to enable following child processes. Below is shown the contents of a file threads.brk which can be sourced into gdb using source threads.brk, before you start debugging bitcoind. The file also loads break points where new threads are spawned.

threads.brk

set follow-fork-mode child
break node::ThreadImport
break StartScriptCheckWorkerThreads
break ThreadHTTP
break StartHTTPServer
break ThreadSync
break SingleThreadedSchedulerClient
break TorControlThread
break ThreadMessageHandler
break ThreadDNSAddressSeed
break ThreadMapPort
break ThreadSocketHandler
break ThreadOpenAddedConnections
break ThreadOpenConnections
break ThreadI2PAcceptIncoming