Passing a structure through Sockets in C

This is a very bad idea. Binary data should always be sent in a way that:

• Handles different endianness
• Handles different padding
• Handles differences in the byte-sizes of intrinsic types

Don't ever write a whole struct in a binary way, not to a file, not to a socket.

Always write each field separately, and read them the same way.

You need to have functions like

unsigned char * serialize_int(unsigned char *buffer, int value){  /* Write big-endian int value into buffer; assumes 32-bit int and 8-bit char. */  buffer[0] = value >> 24;  buffer[1] = value >> 16;  buffer[2] = value >> 8;  buffer[3] = value;  return buffer + 4;}unsigned char * serialize_char(unsigned char *buffer, char value){  buffer[0] = value;  return buffer + 1;}unsigned char * serialize_temp(unsigned char *buffer, struct temp *value){  buffer = serialize_int(buffer, value->a);  buffer = serialize_char(buffer, value->b);  return buffer;}unsigned char * deserialize_int(unsigned char *buffer, int *value);

Or the equivalent, there are of course several ways to set this up with regards to buffer management and so on. Then you need to do the higher-level functions that serialize/deserialize entire structs.

This assumes serializing is done to/from buffers, which means the serialization doesn't need to know if the final destination is a file or a socket. It also means you pay some memory overhead, but it's generally a good design for performance reasons (you don't want to do a write() of each value to the socket).

Once you have the above, here's how you could serialize and transmit a structure instance:

int send_temp(int socket, const struct sockaddr *dest, socklen_t dlen,              const struct temp *temp){  unsigned char buffer[32], *ptr;  ptr = serialize_temp(buffer, temp);  return sendto(socket, buffer, ptr - buffer, 0, dest, dlen) == ptr - buffer;}

A few points to note about the above:

• The struct to send is first serialized, field by field, into buffer.
• The serialization routine returns a pointer to the next free byte in the buffer, which we use to compute how many bytes it serialized to
• Obviously my example serialization routines don't protect against buffer overflow.
• Return value is 1 if the sendto() call succeeded, else it will be 0.

Using the 'pragma' pack option did solved my problem but I am not sure if it has any dependencies ??

#pragma pack(1)   // this helps to pack the struct to 5-bytesstruct packet {int i;char j;};#pragma pack(0)   // turn packing off

Then the following lines of code worked out fine without any problem

n = sendto(sock,&pkt,sizeof(struct packet),0,&server,length);n = recvfrom(sock, &pkt, sizeof(struct packet), 0, (struct sockaddr *)&from, &fromlen);

There is no need to write own serialisation routines for short and long integer types - use htons()/htonl() POSIX functions.