将字符缓冲区类型转换为来自套接字的结构

Question

I am using the following code to read data from a socket:

while (true) 
{

        int len = 0;
        ioctl(sd, FIONREAD, &len);
        if (len > 0)
        {
                len = read(sd, databuf, len);                   
        }

        else
                std::cout << "error" << std::endl;
}

There is a stream of structures that is coming from the socket. the sizes of the structs is defined an I have them.

So how can I read a struct like :

 typedef struct
 {
      short msglen;
      short id;
      int seqno;
 }HEADER;

I thought of typecasting it, but I cannot figure out how to do it. The character buffer will contain multiple structs one after the other. I need to read just the first 8 bytes to get the first.

Also, there is a while loop because I am receiving a continuous stream of multicast messages from the network.

Answer 1

std::memcpy(&header, buffer, sizeof(header));

Answer 2

The standard compliant and safe way is to use memcpy :

HEADER header;
char buffer[/*size*/]; 

/* read data into buffer */

std::memcpy(&header, buffer, sizeof(header));

Modern compilers are fully capable of optimizing away the function call.

Do not try casting buffer with something like reinterpret_cast . That's undefined behavior because it violates the strict aliasing rules (you can access the stored value of an object of any type through a glvalue of char or unsigned char type, but the reverse doesn't hold).

As a side note, typedef struct { /* ... */ } HEADER; is a C-ism. Just do struct HEADER { /* ... */ }; .