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函数结构：
UF_MODL_ask_features_of_exp
(
tag_t exp,
int * number_of_features,
tag_t * * features
)

函数说明：
获取所有使用提供的表达式的功能。

函数参数：
第1个参数为输入：
exp代表参数变量，tag_t 为输入参数类型，查询表达式

第2个参数为输出：
输出int * 整数型的参数，参数的变量格式为number_of_features，返回的特征标签的数

第3个参数为输出：
features代表参数变量，tag_t * * 为输出参数类型，有关定表达式功能。该数组必须通过调用UF_free释放。

UF_MODL_ask_features_of_exp函数实例代码演示：

#include <uf.h>
#include <uf_defs.h>
#include <stdio.h>
#include <uf_modl.h>
#include <uf_part.h>
#include <uf_assem.h>
#define UF_CALL(X) (report( __FILE__, __LINE__, #X, (X)))
static int report( char *file, int line, char *call, int irc)
{
  if (irc)
  {
     char    messg[133];
     printf("%s, line %d:  %s\n", file, line, call);
     (UF_get_fail_message(irc, messg)) ?
       printf("    returned a %d\n", irc) :
       printf("    returned error %d:  %s\n", irc, messg);
  }
  return(irc);
}
static void do_ugopen_api(void)
{
  int i,j, num_exps;
  tag_t  *oexps;
  tag_t part;
  int num_feats;
  tag_t *feats;  char *string;
  char *feat_type;
  char *feat_name;
/* Test UF_MODL_ask_features_of_exp */
  part = UF_ASSEM_ask_work_part();
  UF_CALL(UF_MODL_ask_exps_of_part(part, &num_exps, &oexps));
  printf ("Total number of expressions in part = %d\n", num_exps);
  for (i=0; i<num_exps; i++)
  {
    if (UF_CALL(UF_MODL_ask_exp_tag_string(oexps[i], &string)))
    {
      UF_terminate();
    }
    else
      printf("\nExpression #%d: %s\n",i, string);
    if(UF_CALL(UF_MODL_ask_features_of_exp(oexps[i], &num_feats,
                                           &feats)))
    {
      if (num_feats > 0) UF_free(feats);
      UF_terminate();
    }
    else
    {
      printf ("Number of times expression %s is used = %d\n",
                 string, num_feats);
      printf ("The following features are affected:\n");
      for (j=0; j<num_feats; j++)
      {
        UF_CALL(UF_MODL_ask_feat_name(feats[j], &feat_name));
        UF_CALL(UF_MODL_ask_feat_type(feats[j], &feat_type));
        printf("Feature %u is named %s and is of type %s\n",
               feats[j], feat_name, feat_type);
      }
    }
    if (num_feats > 0) UF_free(feats);
  }
  UF_free(oexps);
}
/*ARGSUSED*/
void ufusr(char *param, int *retcode, int param_len)
{
  if (!UF_CALL(UF_initialize()))
  {
    do_ugopen_api();
    UF_CALL(UF_terminate());
  }
}
int ufusr_ask_unload(void)
{
  return (UF_UNLOAD_IMMEDIATELY);
}