You passed no data to the fucntion. Please ask on gnupg-users for help.

It's more complicated, as the same code works when engine is set explicitely to
gpg2 but the gpg1 engine return a no error data.

You can see our git repo :
http://retroshare.git.sourceforge.net/git/gitweb.cgi?p=retroshare/retroshare;a=commit;h=44ff2edce463fd2da0fdadbf54be65617c97a706

We manage to fix it by removing 1 from the signature length :
gpgme_data_new_from_mem(&gpgmeSig, (const char *) sig, siglen - 1, 1) works for
gpg1, and gpgme_data_new_from_mem(&gpgmeSig, (const char *) sig, siglen, 1)
works for gpg2. See git commit :
http://retroshare.git.sourceforge.net/git/gitweb.cgi?p=retroshare/retroshare;a=commit;h=a28d745a4fbde5f4bc5c32f3fd382c86d7195da7

Please give a detailed report, I can't do much with the brief comments in your
repo. Note that gpg1 and gpg2 are slighly different and thus in corener cases
you may get different error codes.

I reassign this to gnupg, because this does not seem to be related to gpgme at
all. Maybe it can be closed due to age, don't know.