使用左移和按位或将两个无符号字节组合为单个整数值

Question

I'm reading 2 bytes which together build up an unsigned short value, from 0 to 65536. I want to combine them to a single value, so here what I've done:

int32_t temp; 
uint8_t buffer[2]; 

.............
temp = (buffer[1] << 8) /* [MSByte]*/| (buffer[0]/* [LSByte]*/);

printf (" %d" ,temp) ;

I still get an overflow at 32767. Any idea why?

Answer 1

Cast byte to int before shifting, ie:

((int32_t)buffer[1] << 8) | buffer[0] 

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PS 2 bytes can store an unsigned integer value in range of [0, 65535]; the value of 65536 you've mentioned is out of range.

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Complete test program — try different byte values in buffer :

#include <stdio.h>
#include <stdint.h>

int main()
{
//uint8_t buffer[2] = {255, 0  }; // prints   255
//uint8_t buffer[2] = {255, 127}; // prints 32767
  uint8_t buffer[2] = {255, 255}; // prints 65535

  int32_t temp = ((int32_t)buffer[1] << 8) | buffer[0];

  printf("%d", temp);
}