Please review the changes and feedback

Yep, I wrote a small client and server just to verify that it is functional.

But the function works and returns the peer's credentials?

Released with libassuan 3.0.2 (T7163)

Here is a patch to support "w32_error" for assuan_sock_get_flag function.

Put it under lower priority, as it's basically programming error.

Here is my proposal to avoid unsynched state for data.

diff --git a/src/client.c b/src/client.c
index 410f940..0989984 100644
--- a/src/client.c
+++ b/src/client.c
@@ -250,6 +250,7 @@ assuan_transact (assuan_context_t ctx,
   int off;
   char *line;
   int linelen;
+  gpg_error_t last_err = 0;

I have confirmed that rA69069bc63e6b fixes the build on macOS.

Fixed in libassuan 3.0.0.

Fixed in: rA69069bc63e6b: Remove an declaration for an unused function

Actually we should get rid of stdio functions and use the es_foo replacements from libgpg-error.