Optional Fields

The EO protocol uses two mechanisms for optional data: chunked delimiters and padded fixed-length strings.

Reading optional fields with chunked mode:

In chunked reading mode, attempting to read past the end of the current chunk returns 0 (or an empty string) rather than EO_BUFFER_UNDERRUN. This lets you safely read fields that may or may not be present without extra length checks.

EoReader reader = eo_reader_init(buf, buf_len);
eo_reader_set_chunked_reading_mode(&reader, true);
 
int32_t required_field;
eo_reader_get_short(&reader, &required_field);
 
// This field is only present in some versions of the packet.
int32_t optional_field = 0;
eo_reader_get_char(&reader, &optional_field);
// optional_field remains 0 if the chunk ended before it.
 
eo_reader_next_chunk(&reader);
 
// An optional trailing string.
char *optional_str = NULL;
if (eo_reader_remaining(&reader) > 0) {
    eo_reader_get_string(&reader, &optional_str);
}
free(optional_str);  // safe even if NULL was never reassigned

Writing optional fixed-length strings with padding:

Pass padded = true to eo_writer_add_fixed_string() or eo_writer_add_fixed_encoded_string() to right-pad a shorter string with 0xFF bytes up to the declared length.

// Write a name into a 12-byte padded field.
eo_writer_add_fixed_string(&writer, "Bob", 12, true);
// Produces: 'B','o','b', 0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF

Passing NULL as the string value is treated as an empty string and writes nothing (the field is simply omitted), which is useful when an optional trailing field is not present.