Function pomp_buffer_set_capacity 

pub unsafe extern "C" fn pomp_buffer_set_capacity(
    buf: *mut pomp_buffer,
    capacity: usize,
) -> c_int

Set the capacity of the buffer. @param buf : buffer. @param capacity : new capacity of buffer (shall be greater than used length). @return 0 in case of success, negative errno value in case of error. -EPERM is returned if the buffer is shared (ref count is greater than 1).