What about

gpgme_get_dirinfo ("agent-socket")

? For testing you can use

GNUPGHOME=/foo/bar gpgme/tests/t-engine-info 2>&1 | grep agent-info

What about gpgme_get_dirinfo ("agent-socket")?

I did not know about that, and that helps a bit, but has the downside that it
uses the GNUPGHOME from the process' environment.

I'm thinking about the following use case. I have created an ephemeral home
directory to contain the results or side-effects of some operation, and now I
want to talk to the agent that serves that particularly home directory. I
cannot use gpgme_get_dirinfo because that uses GNUPGHOME, and I don't want to
change the environment variable because that is a process-global thing and I
don't want to interfere with other threads.

I see. Let's get back to this after the release of 1.9

Well, at least this works without changing the environment:

$ gpgconf --homedir /tmp/uiux/ --list-dirs | grep agent-socket
agent-socket:/run/user/1000/gnupg/d.sazbxpsor1wn5xsqw4b4jf34/S.gpg-agent

We should add a new function gpgme_op_dirinfo(gpgme_ctx_t ctx, const char *what) that's implemented by GPGME_PROTOCOL_GPGCONF. That would mimic the way gpgconf works and allows setting the home_dir via the context.

Now you can do this:

char *result = NULL;
gpgme_error_t err = gpgme_ctx_set_engine_info (ctx, GPGME_PROTOCOL_GPGCONF, NULL, "/tmp/foo");
fail_if_err (err);
err = gpgme_op_conf_dir (ctx, "agent-socket", &result);
fail_if_err (err);

Please see my comments on rM9f24e6c9010e171fd11c5cdac797cb8ce2e501dd