which public key format should be used for libssh2 library?

I am trying pass publickey (id_rsa.pub) for sftp_write example in libssh2 library.

    const char *pubkey = "/home/username/.ssh/id_rsa.pub";
    const char *privkey = NULL;

    if(libssh2_userauth_publickey_fromfile(session, username,

                                           pubkey, privkey,
                                           password)) {
        fprintf(stderr, "\tAuthentication by public key failed\n");
        goto shutdown;
    }

I have tried openssh format and standard ssh format keys as below.

I got the below errors  for two formats of the 

standard ssh format ,Athentication failed  --> cannot open the file -16 
*---- BEGIN SSH2 PUBLIC KEY ----
AAAAB3NzaC1yc2EAAAADAQABAAABAQCjnPF5vKg7NuUXkGbK0Nh5d3GjLNXHB+VozgVn5Ego
DVSjwezr30YJm8gzGrk5aUAEJKbWSUczVj9vCrn7O54K1/VXOggC1a6pun3+Iqvuv0BINRj3
YuC+c9s803mYPSpzZpNPFf3Uj49lKrroUIdnEz3qo/Z9kFbpWfClJPlb7xjrZaZkSgolbxih
8gZ0w3iAuMR5nwnEGQnHkk751jxOtYopzT4w31gFmFYcq/LRbP4S2xJ0r8wl58J8Mm/U/yZx
EhDARHx/mkqMILofAqFvsFSaaASUuFmB9uUJsgJ69s9FbgXnpGhyGoH0nyVsdM/oqwIXtJji
IvbVBEomnCrX
---- END SSH2 PUBLIC KEY ----*

OPen SSH FOrmat, Athentication failed  --> 
                             LIBSSH2_ERROR_PUBLICKEY_UNRECOGNIZED \ 
                             LIBSSH2_ERROR_AUTHENTICATION_FAILED ,-18

*"ssh-rsa" AAAAB3NzaC1yc2EAAAADAQABAAABAQCjnPF5vKg7NuUXkGbK0Nh5d3GjLNXHB+VozgVn5EgoDVSjwezr30YJm8gzGrk5aUAEJKbWSUczVj9vCrn7O54K1/VXOggC1a6pun3+Iqvuv0BINRj3YuC+c9s803mYPSpzZpNPFf3Uj49lKrroUIdnEz3qo/Z9kFbpWfClJPlb7xjrZaZkSgolbxih8gZ0w3iAuMR5nwnEGQnHkk751jxOtYopzT4w31gFmFYcq/LRbP4S2xJ0r8wl58J8Mm/U/yZxEhDARHx/mkqMILofAqFvsFSaaASUuFmB9uUJsgJ69s9FbgXnpGhyGoH0nyVsdM/oqwIXtJjiIvbVBEomnCrX*

Can some one suggest if its the public key format problem.

I am using Bitvise server on windows  for testing the libssh2 client publickey authentication.

Thnaks & Regards
Vamsi