Thread: how to cast unsigned char[3] to int?

I have a 3 byte unsigned char array that contains a length value (someone else's control block) and I need to move the value to my int field.

I tried different flavors of casting, but couldn't come up with a method that didn't ring compiler bells or worked. I ended up using a union, but I am wondering if there is a simpler/better/more proper method.

Code:

#include <stdio.h>                                                       
                                                                         
union MyUnion {                                                          
  char ctemp[4] ;                                                        
  int  itemp ;                                                           
} ;                                                                      
                                                                         
int main(void) {                                                         
                                                                         
  union MyUnion myunion ;                                                
  myunion.itemp = 0 ;                                                    
                                                                         
  unsigned char c_len[3] = {0,0,5} ;                                     
                                                                         
  for (int i = 0 ; i < 3 ; i++ ) printf("c_len[%d]=%d\n", i, c_len[i]) ; 
                                                                         
  for (int i = 1 ; i < 4 ; i++ ) myunion.ctemp[i] = c_len[i-1] ;         
                                                                         
  printf("myunion.itemp = %d\n", myunion.itemp) ;                        
                                                                         
  return 0 ;                                                             
}

Output:

Code:

c_len[0]=0        
c_len[1]=0        
c_len[2]=5        
myunion.itemp = 5

Well decide whether it's little endian or big endian.

But something like this is usually the way to go.

Code:

int result = 0;
for ( int i = 0 ; i < 3 ; i++ ) {
    result = ( result << 8 ) + c_len[i];
}

Clever! My platform is big endian. Will never be ported to another platform. I forgot about shifting. Thank you.