Question

I am trying to implemante a client- server program communication using TCP socket programming in C.
It is between between two 64bit machines with linux OS installed.
I want to transfer a c-struct between the two processes.

For this I try used a pack – unpack() functioanlity.

please consider the following code snipt

/*---------------------------------------------------------
on the sending side I have: 
---------------------------------------------------------*/

struct packet {
 int64_t x;
 int64_t y;
 int64_t q[maxSize];
} __attribute__((packed));

int main(void)
{
 // build packet
 struct packet pkt;
 pkt.x = htonl(324);
 pkt.y = htonl(654);
 int i;
 for(i = 0; i< maxSize; i++){
    pkt.q[i] = i; **// I also try pkt.q[i] = htonl(i);**
 }
  // and then do the send
}

/*-----------------------------------------------------------------------------
 in the receiving side: 
 -----------------------------------------------------------------------------*/
struct packet {
 int64_t x;
 int64_t y;
 int64_t q[maxSize];
} __attribute__((packed));

static void decodePacket (uint8_t *recv_data, size_t recv_len)
{
 // checking size
 if (recv_len < sizeof(struct packet)) {
     fprintf(stderr, "received too little!");
     return;
 }

struct packet *recv_packet = (struct packet *)recv_data;

int64_t x = ntohl(recv_packet->x);
int64_t y = ntohl(recv_packet->y);
int i;
printf("Decoded: x=%"PRIu8" y=%"PRIu32"\n", x, y);

 for(i=0;i<maxSize;i++){
  **//int64_t res = ntohl(recv_packet->q[i]);  I also try to print res**          
  printf("%"PRIu32"\n" , recv_packet->q[i]);     
 }
} 

int main(int argc, char *argv[]){
 // receive the data and try to call decodePacket()
 int8_t *recv_data = (int8_t *)&buf; //buf is the data received
 size_t recv_len = sizeof(buf);
 **decode_packet(recv_data, recv_len);**  
}

//-----------------------------------------------------------------------------

Now the problem is that I am receiving the value of x and y in the struct correctly,
but for the array q in the struct I am receiving a strange number, possible a memory grabage value, (I try to use memset() filling the array by zeros before receiving a data from the other side in which case the value of all zeros is received )

I don’t understand why I am not receiving the correct value for the array in struct.

Please Note that I try with and with out htonl() while filling the array before putting in struct,
and on the other side: with and with out ntohl() while decoding the array from struct

Any help will be appreciated,

Answer 1

size_t recv_len = sizeof(buf);
decode_packet(recv_data, recv_len);

This piece of code ensures the wrong size is passed to decode_packet. So when decode_packet goes on to check recv_len < sizeof(struct packet), that test is meaningless – it will always pass, no matter how many bytes were received.

You need to fetch the size from the value returned by the recv call. My best guess is that indeed you’re receiving fewer bytes than you’re expecting.

---

While sending and receiving structs is quite convenient, it’s often an exercise in futility. Manually serializing data or using some explicit mechanisms is probably the way to go.