Question

Ok I am putting the whole struct here, its specification of a protocol named openflow that is implemented in some of industrial switches , so the struct is like:

struct ofp_packet_in {
    struct ofp_header header;
    uint32_t buffer_id;     /* ID assigned by datapath. */
    uint16_t total_len;     /* Full length of frame. */
    uint16_t in_port;       /* Port on which frame was received. */
    uint8_t reason;         /* Reason packet is being sent (one of OFPR_*) */
    uint8_t pad;
    uint8_t data[0];        /* Ethernet frame, halfway through 32-bit word,
                               so the IP header is 32-bit aligned.  The
                               amount of data is inferred from the length
                               field in the header.  Because of padding,
                               offsetof(struct ofp_packet_in, data) ==
                               sizeof(struct ofp_packet_in) - 2. */
};
OFP_ASSERT(sizeof(struct ofp_packet_in) == 20);

now I have to fill up some data in the last field that is –uint8_t data[0] which can be varied and info is gathered from the length field inside the header. I have to construct a packet in, and for that data has to be put in. Please take a look.

Answer 1

You’ll need to use a dynamic allocation and copy the contents.

Something like:

#include <stdlib.h>
#include <string.h>

void foo(void) {
  struct some_struct *container = malloc(sizeof(struct some_struct) + 100);
  if (!container) {
    // handle out-of-memory situation
  }
  memcpy(container->data, some_data, 100);
}