CodeVetter · Repo Unpack · Extension points
curl/curl
curl is extended by adding a record to a table, not by subclassing. New URI schemes are `struct Curl_scheme` records pointing at a `struct Curl_protocol` vtable; new transport behaviour is a `struct Curl_cftype` inserted into the filter chain; new response processing is a `struct Curl_cwtype` registered at a phase; new TLS/SSH/DNS backends are entries in a backend array guarded by a `USE_*` define. On the tool side, a new command-line option means one row in the alphabetical `aliases[]` table, one `C_*` enum value, one case in `getparameter()`, and one curldown page. Several of these tables are additionally mirrored by generated files or documentation that CI verifies.
- Adding a protocol means adding a `Curl_scheme_*` record in lib/protocol.c, externing it in lib/protocol.h, teaching the length-bucketed lookup functions about the new name, and supplying a `struct Curl_protocol` vtable; docs/internals/NEW-PROTOCOL.md documents the social and technical bar for that.
- The `struct Curl_protocol` vtable is the per-scheme hook set: setup_connection, connect, connecting, do_it, doing, done, proto_getsock/doing_pollset/domore_pollset/perform_pollset, disconnect, write_resp, write_resp_hd, connection_is_dead, attach, and follow.
- PROTOPT_* flags are the declarative knobs a new scheme uses to opt into behaviour — SSL, DUAL (two connections), NEEDSPWD, NOURLQUERY, WILDCARD, CREDSPERREQUEST, PROXY_AS_HTTP, CONN_REUSE, SSL_REUSE, NO_TRANSFER, HTTP_PROXY_TUNNEL — rather than special-casing the scheme in shared code.
- New transport layers plug in as `struct Curl_cftype` instances; `Curl_cf_setup_add()` / `cf_setup_create()` build the base filter and the `cf_setup_add_*` helpers decide insertion order, so a new filter is added by writing one more `cf_setup_add_*` step.
- Response-processing extensions register as `struct Curl_cwtype` with a `CURL_CW_*` phase; `cwriter_add()` inserts each writer first within its phase, so ordering is declared rather than coded.
- Content and transfer decodings are two explicit registries — `general_unencoders[]` and `transfer_unencoders[]` — resolved by name in `find_unencode_writer()`, which is where a new `Content-Encoding` would be added alongside deflate/gzip/brotli/zstd/identity.
- Upload-side extensions use the mirrored client-reader registry (`struct Curl_crtype` with `CURL_CR_*` phases); HTTP's Expect: 100-continue handling is implemented this way via `http_exp100_add_reader()`.
- A new TLS backend is a `struct Curl_ssl` added to `available_backends[]` behind a `USE_*` define; `curl_global_sslset()` and the `CURL_SSL_BACKEND` lookup both iterate that same array, so no other file needs to change to make it selectable.
- Resolver strategies are swappable at build time behind a common `lib/vdns/asyn.h` interface with three implementations — c-ares (`asyn-ares.c`), threaded (`asyn-thrdd.c`) and the shared base (`asyn-base.c`) — plus `cf-dns.c` exposing resolution as a connection filter.
- SSH has two interchangeable backends selected at build time (libssh2 and libssh) behind lib/vssh/vssh.h, following the same one-directory-per-backend-family convention as vtls and vquic.
- Adding a CLI option touches the alphabetically-sorted `aliases[]` table (the comment above it is explicit: "this array MUST be alphasorted based on the 'lname'"), the `C_*` id enum, a case in `getparameter()`, and the generated help in src/tool_listhelp.c.
- New `--write-out` variables are one row in `variables[]` mapping a name to a `VAR_*` id, a `CURLINFO_*` id, and a writer function (`writeString`/`writeLong`/`writeOffset`/`writeTime`), with NULL signalling a special form handled inline.
- New libcurl options plug into three parallel registries that must stay in sync: the `CURLOPT_*` enum in include/curl/curl.h, the typed setters in lib/setopt.c, and one curldown page under docs/libcurl/opts/ (422 of them) plus an entry in docs/libcurl/symbols-in-versions.
- Multi-state entry hooks are themselves a small registry: the `state_enter[MSTATE_LAST]` function table in `mstate()` lets a state gain on-entry behaviour without touching `multi_runsingle()`'s switch.
- Cross-handle caches are extended through the share interface: `share_setup()` in the tool shows the enumerable set (`CURL_LOCK_DATA_COOKIE`, `DNS`, `SSL_SESSION`, `PSL`, `HSTS`, `CONNECT`), and `CURLSHE_NOT_BUILT_IN` is tolerated so a disabled feature does not break setup.