in src/signing.c [150:161]
void aws_crt_signing_config_aws_set_should_sign_header_fn(
aws_crt_signing_config_aws *signing_config,
aws_crt_should_sign_header_fn *should_sign_header_fn,
void *user_data) {
/* store the target function and user data */
signing_config->should_sign_header = should_sign_header_fn;
signing_config->should_sign_header_user_data = user_data;
/* auth will call the FFI version with the FFI user data, which will dispatch to the target function */
signing_config->config.should_sign_header = should_sign_header_thunk;
signing_config->config.should_sign_header_ud = signing_config;
}