Why does the buffering of std::ifstream "break" std::getline when using LLVM?

I have worked around this issue by wrapping POSIX getline() in a simple C API and simply calling that from C++. The code is something like this:

typedef struct pipe_reader {
    FILE* stream;
    char* line_buf;
    size_t buf_size;
} pipe_reader;

pipe_reader new_reader(const char* pipe_path) {
    pipe_reader preader;
    preader.stream = fopen(pipe_path, "r");
    preader.line_buf = NULL;
    preader.buf_size = 0;
    return preader;
}

bool check_reader(const pipe_reader* preader) {
    if (!preader || preader->stream == NULL) {
        return false;
    }
    return true;
}

const char* recv_msg(pipe_reader* preader) {
    if (!check_reader(preader)) {
        return NULL;
    }
    ssize_t read = getline(&preader->line_buf, &preader->buf_size, preader->stream);
    if (read > 0) {
        preader->line_buf[read - 1] = '\0';
        return preader->line_buf;
    }
    return NULL;
}

void close_reader(pipe_reader* preader) {
    if (!check_reader(preader)) {
        return;
    }
    fclose(preader->stream);
    preader->stream = NULL;
    if (preader->line_buf) {
        free(preader->line_buf);
        preader->line_buf = NULL;
    }
}

This works well against libc++ or libstdc++.