variable-length array in struct with TI compiler in C (socket programming)

Question

I'm writing code that deals with data received from the UDP protocol in the application layer.

This is the function prototype used to receive data from the UDP protocol:

int my_recv_UDP(int s,
                void* mem,
                int len,
                struct sockaddr* from,
                int fromlen);

in which:

s: the socket.

mem: received data is put in it.

len: memory length of mem.

This function returns the data length, which may be less than len.

For example

frameLen = my_recv_UDP(socket, rxBuf, 100, &ra, sizeof(ra));

In which it uses socket as the socket, stores data in rxBuf with a length of 100, and returns the length of received data to frameLen.

The length of the received data may vary from 100. Since I specify a length of 100, I might receive data that is less than 100, or lose data that exceeds 100, depending on the frame.

I intended to create rxBuf_s and let this struct type's pointer point to rxBuf.

i.e:

typedef struct{
    int cmd;
    int data[?];    //you don't know the data length until it's received
    int len;
}rxBuf_s;


rxBuf_s* rxBuf_p = rxBuf;

With this, I can easily "manipulate" data in rxBuf. For example, if I want to read cmd into buffer, I can do this:

int Cmd = rxBuf_p->cmd;

However, I didn't know the data length until the data was received, and I didn't know what offset was for len either; So decided to calculate the length at "run time" and put it in array, like this:

typedef struct{
    int cmd;
    int data[length];    //length is calculated at run time.
    int len;
}rxBuf_s;

But VLA in struct is not allowed in C99.
How can I handle it? Is there a smart way to do this?

Either use the maximum length that ever could be reached, or change data to a pointer int*. — Klaus Gütter
– Klaus Gütter, Commented Dec 22, 2018 at 7:20
You are misunderstand. I've changed the content, plz read it again. — Andy Lin
– Andy Lin, Commented Dec 22, 2018 at 7:27
Ahh, maybe I understand now. So you receive cmd, data[length], len in this order in the frame? Then you are out of luck mapping all three to a common struct. You need to calculate the offset of the len member explicity and extract it from the receive buffer. — Klaus Gütter
– Klaus Gütter, Commented Dec 22, 2018 at 7:53
Yes, in this order. But the problems is, C99 does not allow me to calculate the offset of the len member and put it in array data. English is not my mother tongue, plz bear with me of bad English. Btw, how to put shaded text in comment? — Andy Lin
– Andy Lin, Commented Dec 22, 2018 at 8:14
Read about flexible array member. It has to be the last in its containing struct — Basile Starynkevitch
– Basile Starynkevitch, Commented Dec 22, 2018 at 11:23

Klaus Gütter · Accepted Answer · 2018-12-22 10:59:49Z

2

So you receive cmd, data[length], len in this order in the frame. As you already noted, it is not possible to catch this 1:1 using a C structure. At least the len field has to be extracted by a different means, e.g.

length = ...; // calculate length

int cmd = *(int*)rxBuf;
int *data = (int*)rxBuf + 1;
int len = *(data + length);

answered Dec 22, 2018 at 10:59

Klaus Gütter

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