CODEWITHSUNDEEP
lwip
Alex Flint
Alex Flint

Reputation: 6738

When can I re-use memory referenced by a netbuf in lwip?

I am sending UDP packets from an embedded system using lwip, using the netconn API. I create a netbuf like this:

    struct netbuf* buf = netbuf_new();
    netbuf_ref(buf, &signal_packet, sizeof(signal_packet_t));

and then I send the UDP packet like this:

    err_t err = netconn_send(conn, buf);

At what point now can I re-use the memory pointed to by &signal_packet? Does the netconn API make a copy before returning, or does it continue to reference this memory from other threads?

Upvotes: 0

Views: 230

Answers (1)

Itay K
Itay K

Reputation: 1

Your first line,
struct netbuf* buf = netbuf_new();
allocates a new netbuf structure but not a data buffer. (the 'new' keyword being used in similar manner to OOP languages, indicating there should be a netbuf_delete() function coming later for de-allocation.

netbuf_ref(...); will simply update a given netbuf structure's data pointer to whatever buffer you've chosen, but this does not mean it manages it. It is your responsibility to allocate (and de-allocate) your referenced memory.

In conclusion - netconn will not copy your buffer. In fact, as stated in the docs:

Based on Packet buffers (PBUF) internally to avoid copying data around.

If you wish to re-use the buffer across multiple threads after sending, you may also want to know that according to the same docs, this library is not thread-safe (except for netbuf_new() and netbuf_delete()).

Buffers must not be shared accross multiple threads, all functions except netbuf_new() and netbuf_delete() are not thread-safe.

Either implement your own thread safety or use a separate buffer for outgoing data :)

Author of the question did not reference this, but also a worthy mention:

netbuf_alloc(struct netbuf *buf, u16_t size) will allocate a buffer for you, in a given netbuf, and will clear it on deletion.

Upvotes: 0

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