bit_counter.c

# include<stdio.h>
# include<stdlib.h>
# include<string.h>

///////////////////////////////////////
// TYPE DEFINITION
/////////////////////
typedef struct ST_32_BITFIELD
{
    unsigned char   lsb  : 1;
    long int        rest : 30;
    unsigned char   msb  : 1;
}ST_32_BITFIELD;

typedef union UN_UL_VAL
{
    ST_32_BITFIELD st_32bitfield;
    long int value;
} UN_UL_VAL;

typedef enum EM_COUNT_METHOD
{
  EM_COUNT_METHOD_BUNKATSU_TOUCHI,
  EM_COUNT_METHOD_BITFIELD,
  EM_COUNT_METHOD_MAX
} EM_COUNT_METHOD;

///////////////////////////////////////
// MACRO & DEFINITION
/////////////////////
# define STR_BUNKATSU_TOUCHI "BUNKATSU_TOUCHI"
# define STR_BITFIELD "BITFIELD"
# define STR_UNKNOWN  "Unknown"

///////////////////////////////////////
// Internal functions
/////////////////////
static void showUsage(void);
static unsigned int count_by_bunkatsu_touchi_method(unsigned int);
static unsigned int count_by_bitfield(unsigned int);
static char* methodToString(EM_COUNT_METHOD);

static void showUsage(void)
{
  printf("USAGE: ./a.out\n");
  printf("\t -c : input unsigned int value in decimal format. (mandatory argument).\n");
  printf("\t -m : select count method.\n");
  printf("\t         0:bunkatsu(default)    1:bitf\n");
}

//単純にカウントする場合は、分割統治法だとか、popcountだとかを強く推奨。
//これは分割統治法での例です。原典はPage下記に記載させて頂いております。
static unsigned int count_by_bunkatsu_touchi_method(unsigned int x)
{
    unsigned int _x = x;
    _x = _x - ((_x >> 1) & 0x55555555);
    _x = (_x & 0x33333333) + ((_x >> 2) & 0x33333333);
    _x = (_x + (_x >> 4)) & 0x0f0f0f0f;
    _x = _x + (_x >> 8);
    _x = _x + (_x >> 16);
    _x = _x & 0x0000003F;

     return _x;
}

//アプリの特性上、カウントついでに
//ごちょごちょする必要があるなら、こういうやり方もある。
//ただし、このコードはカウントすることしか書いてないですヨ。
static unsigned int count_by_bitfield(unsigned int x)
{
    UN_UL_VAL un_val = {0};
    un_val.value = x;
    long int count = 0;

    for(int i = 0; i < 32; i++, un_val.value >>= 1)
    {
      //        printf("lsb:%d value=%ld \n",un_val.st_32bitfield.lsb, un_val.value);

        if(1 == un_val.st_32bitfield.lsb)
        {
	    count++;
        }
   }

    return count;
}

static char* methodToString(EM_COUNT_METHOD _em_method)
{
  char* result = NULL;
  
    switch(_em_method)
    {
        case(EM_COUNT_METHOD_BUNKATSU_TOUCHI):
	{
	  result = STR_BUNKATSU_TOUCHI;
	  break;
	}
	case(EM_COUNT_METHOD_BITFIELD):
	{
	  result = STR_BITFIELD;
	  break;
	}
        default:
	{
	    result = STR_UNKNOWN;
	}
    }
    return result;
}

int main(int argc,char *argv[]) {

    long int target = 0;
    EM_COUNT_METHOD em_method = EM_COUNT_METHOD_BUNKATSU_TOUCHI;//default
    int initCompFlag = 0;
    unsigned int countResult = 0;

    if((argc < 2) || (0 == argc%2))
    {
      showUsage();
      exit(0);
    }
    else
    {
        for (int i=1; i < argc; i++)
  	{
  	    if(0 == strncmp(argv[i], "-c", 4))
	    {
	        target = strtol(argv[++i], NULL, 10);
		initCompFlag = 1;
	    }
	    else if(0 == strncmp(argv[i], "-m", 4))
	    {
	        em_method = strtol(argv[++i], NULL, 10);
	    }
	    else
	    {
	        showUsage();
	        exit(0);
	    }
	}

	//confirm initialization status.
	if(1 == initCompFlag)
	{
	    printf("evaluation target value is = %ld\n", target);
            printf("count method is = %s.\n", methodToString(em_method));
	    if((EM_COUNT_METHOD_BUNKATSU_TOUCHI != em_method) &&
	       (EM_COUNT_METHOD_BITFIELD != em_method))
	    {
        	printf("method:%d is invalid.\n", em_method);      
	        showUsage();
	        exit(0);	        	
	    }
	}
	else //including : 0 == init_comp_flag
	{
	    showUsage();
	    exit(0);
	}
	    
    }

    switch(em_method)
    {
        case(EM_COUNT_METHOD_BUNKATSU_TOUCHI):
	{
	    countResult = count_by_bunkatsu_touchi_method(target);
	    break;
	}
	case(EM_COUNT_METHOD_BITFIELD):
	{
      	    countResult = count_by_bitfield(target);
	     break;
	}
        default:
	{
	    printf("method:%d is invalid.\n", em_method);      
	    showUsage();
	    exit(0);
	}
    }

    printf("# of true bit is %u\n",countResult);
    
    return EXIT_SUCCESS;
}

こんな感じでどうでしょう。。。
注意として、bitfield構造体を定義するときに、1bitの部分(本例だと、msb, lsb)を符号有り型で定義すると、
評価するときにマイナスの値になってしまうので、countが増えないプログラムになります。

Oct/18/2016:
※ご指摘がありましたので、分割統治法でのカウント方法サンプルコードと、ごちゃごちゃ言い訳コメントもつけてみました。
分割統治法のコードをご紹介してくださっているURL：https://www.kaeruspoon.net/articles/720