How to convert struct to char array in C? - c

Question

I am trying to send an ethernet packet using RAW socket in C Linux. I have following struct definition in my code:
typedef struct vlink_header_s
{
uint8_t verCmd;
uint8_t reverseVerCmd;
}vlink_header_t;
typedef struct vlink_reg_rd_s
{
vlink_header_t header;
uint32_t address;
uint16_t length;
}vlink_reg_rd_t;
In main i created a struct:
vlink_reg_rd_t g_pkt;
g_pkt.header.verCmd = 0x10|VLINK_CMD_REG_RD;
g_pkt.header.reverseVerCmd = ~(g_pkt.header.verCmd);
g_pkt.address = 0x0007 .....
and message:
char sendbuf[1024];
struct ether_header *eh = (struct ether_header *) sendbuf;
how do I add all the info from the struct g_pkt to this sendbuf after ether_header so I can send a complete packet using:
sendto(sockfd, sendbuf, txLen, 0, (struct sockaddr*)&socket_address, sizeof(struct sockaddr_ll));
Everything else in my code is working, I tried other ways by adding info to sendbuf one by one and it works fine and my machine receive the packets on the other side too. I just want to make it more versatile because there are a bunch of commands and structs for each commands will work best. Thanks.

Answer 1

Try to use the memcpy function:
#include<iostream>
typedef struct vlink_header_s
{
uint8_t verCmd;
uint8_t reverseVerCmd;
}vlink_header_t;
typedef struct vlink_reg_rd_s
{
vlink_header_s header;
uint32_t address;
uint16_t length;
}vlink_reg_rd_t;
using namespace std;
int main()
{
vlink_reg_rd_t data;
//TODO: Set values into typedef data.
int size = sizeof(vlink_reg_rd_t); //get
char* buffer = new char[size];
memset(buffer, 0x00, size);
memcpy(buffer, &data, size); //Copy data from vlink_reg_rd_t to char*
//TODO: Send the buffer.
delete[] buffer; //free memory
return 0;
}
IMPORTANT: be aware of the order in which data types number are written into the buffer. Also it is necessary check the align the data into the structure to avoid extra bytes at the moment of use memcpy. Here you can check this topic:
for Microsoft:
https://msdn.microsoft.com/en-us/library/xh3e3fd0.aspx
https://msdn.microsoft.com/en-us/library/83ythb65.aspx
For Gcc:
https://gcc.gnu.org/onlinedocs/gcc-3.3/gcc/Type-Attributes.html

Answer 2

I have done this way with protocol buffer , you can take a look at : https://www.google.com.vn/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=protocol+buffer&* .
Edit : this way called serialize data , as you serialize your data into a proto file then compile it to the packet file that you use on both server and client .