c - bsearch 未能在结构数组中找到字符串成员

Question

在 C 中使用 bsearch 未能在结构数组中找到字符串“Eva Lam”。此数组按字符串成员的降序排序。检查了很多次，还是不知道bug在哪里？顺便说一句，我正在使用 DEV C++ 5.9.4。请帮助，非常感谢。

#include <stdio.h>
#include <stdlib.h>     // for bsearch
#include <string.h>
#define SIZE 4   
#define NAME_SIZE 20

struct student {
  int id;
  char name[NAME_SIZE];
};

// Function prototypes
int comp_name(const void* a, const void* b);
void print_struct_array(struct student studs[], int size, int serial);

int main(){
  int i, option=0;
  struct student *stud, *target;

  // studs array already sort in descending order of name
  struct student studs[SIZE] = {{14123456, "Mary Chan"}
    , {11001234, "Eva Lam"}
    , {10123456, "David Wong"}
    , {12345678, "Chris So"}
  };

  printf("*** Before Searching ***\n");
  print_struct_array(studs, SIZE, 1);

  target = (struct student*) malloc(sizeof(struct student));
  if (target == NULL) {
    fprintf(stderr, "Out of memory!\n");
    return -1;
  }

  printf("Input student name to search: ");
  scanf("%[^\n]", target->name);
  fflush(stdin);
  printf("name=%s\n", target->name);


  stud = (struct student *)bsearch(target->name, studs, SIZE,  
    sizeof(struct student), comp_name);
  if (!stud)
     printf("name %s not found!\n", target->name);
  else
     printf("[id, name] found is [%d, %s]\n", stud->id, stud->name);


  return 0;
}

int comp_name(const void* a, const void* b) {
   printf("comp_name: a->name=%s, b->name=%s\n", 
     (*(struct student *)a).name, (*(struct student *)b).name);
   return strcmp((*(struct student *)b).name, 
     (*(struct student *)a).name);
}


void print_struct_array(struct student studs[], int size, int serial) {
  int i;

  printf("Student array #%d is {\n", serial);
  for (i=0; i<SIZE; i++) {
    if (i==0) 
      printf("  ");
    else if (i<=SIZE-1)
      printf(", ");
    printf("[%d, %s]\n", studs[i].id, studs[i].name);
  }
  printf("}\n");
}

但是搜索“Eva Lam”时程序的输出是:

*** Before Searching ***
Student array #1 is {
  [14123456, Mary Chan]
, [11001234, Eva Lam]
, [10123456, David Wong]
, [12345678, Chris So]
}
Input student name to search: Eva Lam
name=Eva Lam
comp_name: a->name=Lam, b->name=Eva Lam
comp_name: a->name=Lam, b->name=Mary Chan
name Eva Lam not found!

--------------------------------
Process exited after 8.216 seconds with return value 0

Answer 1

更仔细地阅读bsearch 的文档。

The compar routine is expected to have two arguments which point to the key object and to an array member, in that order.

这意味着您的比较函数的第一个参数将始终与您作为第一个参数提供给 bsearch 的参数完全相同。所以要么将其称为:bsearch(target, studs, ...) 或者更好的是，将您的比较函数重写为:

int
comp_name(const void *av, const void *bv) {
    const char *a = av;
    const struct student *b = bv;
    printf("comp_name: a->name=%s, b->name=%s\n", a, b->name);
    return strcmp(b->name, a);
}

此外，请不要在 C 中强制转换 void * 指针，尤其是 malloc，还有 bsearch 的返回值代码。