Convert 2 bytes into an integer

Question

I receive a port number as 2 bytes (least significant byte first) and I want to convert it into an integer so that I can work with it. I\'ve made this:

char buf[         


        

Answer 1

I appreciate this has already been answered reasonably. However, another technique is to define a macro in your code eg:

// bytes_to_int_example.cpp
// Output: port = 514

// I am assuming that the bytes the bytes need to be treated as 0-255 and combined MSB -> LSB

// This creates a macro in your code that does the conversion and can be tweaked as necessary
#define bytes_to_u16(MSB,LSB) (((unsigned int) ((unsigned char) MSB)) & 255)<<8 | (((unsigned char) LSB)&255) 
// Note: #define statements do not typically have semi-colons
#include <stdio.h>

int main()
{
  char buf[2];
  // Fill buf with example numbers
  buf[0]=2; // (Least significant byte)
  buf[1]=2; // (Most significant byte)
  // If endian is other way around swap bytes!

  unsigned int port=bytes_to_u16(buf[1],buf[0]);

  printf("port = %u \n",port);

  return 0;
}

Answer 2

char buf[2]; //Where the received bytes are
int number;
number = *((int*)&buf[0]);

&buf[0] takes address of first byte in buf.
(int*) converts it to integer pointer.
Leftmost * reads integer from that memory address.

If you need to swap endianness:

char buf[2]; //Where the received bytes are
int number;  
*((char*)&number) = buf[1];
*((char*)&number+1) = buf[0];

Answer 3

If you make buf into an unsigned char buf[2], you could just simplify it to;

number = (buf[1]<<8)+buf[0];

Answer 4

I receive a port number as 2 bytes (least significant byte first)

You can then do this:

  int number = buf[0] | buf[1] << 8;