At this point, the sign_server() function needs to sign the message. Signing a message entails returning the message's Message Integrity Code or MIC to the client. Returning the message proves that the message was sent and was unwrapped successfully. To obtain the MIC, sign_server() uses the function gss_get_mic():
maj_stat = gss_get_mic(&min_stat, context, GSS_C_QOP_DEFAULT, &msg_buf, &xmit_buf);
gss_get_mic() looks at the message in msg_buf, produces the MIC, and stores the MIC in xmit_buf. The server then sends the MIC back to the client with send_token(). The client verifies the MIC with gss_verify_mic(). See Reading and Verifying a Signature Block From a GSS-API Client.
Finally, sign_server() performs some cleanup. sign_server() releases the GSS-API buffers msg_buf and xmit_buf with gss_release_buffer(). Then sign_server() destroys the context with gss_delete_sec_context().