在C中的16个char数组中并置32位int块

Question

I have a result buffer of the following data type:

 char result[16];

The problem is, that the result is computed in 4 chunks of 32 bits each, that need to be assigned to the 128-bit result char.

int res_tmp[0] = 0x6A09E667;
int res_tmp[1] = 0x6A09E612;
int res_tmp[2] = 0x6A09E432;
int res_tmp[3] = 0x6A09E123;

Ideally, there should be something like an concatenation operator in C, eg,

result = res_tmp[0] || res_tmp[1] || res_tmp[2] || res_tmp[3];

Finally, the result needs to be send over a socket as follows:

while((connection_fd = accept(socket_fd, 
                          (struct sockaddr *) &address,
                          &address_length)) > -1)
{
  n = write(connection_fd, result, strlen(result));
  if (n < 0) printf("Error writing to socket\n");            
  close(connection_fd);
  break;  
}

Anyone knows the easiest syntax for concatenating the 32-bit words in the 128-bir result char ?

Thanks, Patrick

Answer 1

You must decide if the char array is representing the result in big-endian or little endian order. If the endian-ness of your processor and the array happen to coincide, you can use a union :

union myunion
{
    char result[16];
    int res_tmp[4];
};

Then you don't have to copy at all.

If you need the opposite endian-ness of your processor, you can use htonl

for (i = 0; i < 4; i ++) res_tmp[i] = htonl(res_tmp[i]);

Answer 2

why not just use memcpy ?

memcpy(result, res_tmp, sizeof(res_tmp));

Also note that strlen is for null terminated strings, you should use sizeof for static buffer:

n = write(connection_fd, result, sizeof(result));

And of course you could just send res_tmp

n = write(connection_fd, (char*)res_tmp, sizeof(res_tmp));

Answer 3

Basically the standard technique is to create an int pointer and point it at the char array, then use it to write the data in. Something like this

int temp_res[4]; //the calculated ints
char result[16]; //the target buffer
int *ptr=(int *)result;
for (int i=0;i<4;i+=1) {
  *ptr=temp_res[i];
  ptr++; //move up a int size because ptr is an int type
}