将bsearch与字符串数组一起使用时出现问题

问题描述

尝试在C中的字符串数组上使用c内置bsearch时，我遇到一些令人困惑的行为.这是代码.我知道您可以使用内置的strcmp搜索字符串数组，但是我将myStrCmp用于调试目的，因为我不知道为什么它不起作用.

I am getting some confusing behaviour trying to use the c builtin bsearch on an array of strings in C. Here is the code. I know you can use the builtin strcmp for searching arrays of strings, but I included myStrCmp for debugging purposes because I didn't know why it wasn't working.

const char *stateNames[] = {"Alabama", "Alaska", "Arizona", "Arkansas", "California", "Colorado", "Connecticut", "Delaware", "Florida", "Georgia", "Hawaii", "Idaho", "Illinois", "Indiana", "Iowa", "Kansas", "Kentucky", "Louisiana", "Maine", "Maryland", "Massachusetts", "Michigan", "Minnesota", "Mississippi", "Missouri", "Montana", "Nebraska", "Nevada", "New Hampshire", "New Jersey", "New Mexico", "New York", "North Carolina", "North Dakota", "Ohio", "Oklahoma", "Oregon", "Pennsylvania", "Rhode Island", "South Carolina", "South Dakota", "Tennessee", "Texas", "Utah", "Vermont", "Virginia", "Washington", "Washington DC", "West Virginia", "Wisconsin", "Wyoming"};

int myStrCmp(const void *s1, const void *s2) {
  printf("myStrCmp: s1(%p): %s, s2(%p): %s\n", s1, (char *)s1, s2, (char *)s2);
  return strcmp(s1, s2);
}

int determineState(char *state) {
  printf("state: %s\n", state);
  for(int i = 0; i < 51; i++)
    printf("stateNames[%i](%p): %s\n", i, &(stateNames[i]), stateNames[i]);

  char *found = (char *) bsearch(state, stateNames, 51, sizeof(char *), myStrCmp );

  if(found == NULL)
    return -1;

  return 0;
}

这是调用此函数以查找阿拉巴马州时的一些输出.

and here is some of the output when this function is called to look for Alabama.

stateNames[0](0x618440): Alabama
stateNames[1](0x618448): Alaska
stateNames[2](0x618450): Arizona
...
stateNames[24](0x618500): Missouri
stateNames[25](0x618508): Montana
stateNames[26](0x618510): Nebraska
stateNames[27](0x618518): Nevada
stateNames[28](0x618520): New Hampshire
stateNames[29](0x618528): New Jersey
stateNames[30](0x618530): New Mexico
stateNames[31](0x618538): New York
stateNames[32](0x618540): North Carolina
stateNames[33](0x618548): North Dakota
stateNames[34](0x618550): Ohio
stateNames[35](0x618558): Oklahoma
stateNames[36](0x618560): Oregon
stateNames[37](0x618568): Pennsylvania
stateNames[38](0x618570): Rhode Island
stateNames[39](0x618578): South Carolina
stateNames[40](0x618580): South Dakota
stateNames[41](0x618588): Tennessee
stateNames[42](0x618590): Texas
stateNames[43](0x618598): Utah
stateNames[44](0x6185a0): Vermont
stateNames[45](0x6185a8): Virginia
stateNames[46](0x6185b0): Washington
stateNames[47](0x6185b8): Washington DC
stateNames[48](0x6185c0): West Virginia
stateNames[49](0x6185c8): Wisconsin
stateNames[50](0x6185d0): Wyoming
myStrCmp: s1(0x415430): Alabama, s2(0x618508):
                                               UA
myStrCmp: s1(0x415430): Alabama, s2(0x618570): A
myStrCmp: s1(0x415430): Alabama, s2(0x618540): PUA
myStrCmp: s1(0x415430): Alabama, s2(0x618528): 1UA
myStrCmp: s1(0x415430): Alabama, s2(0x618538): GUA
myStrCmp: s1(0x415430): Alabama, s2(0x618530): <UA

如您所见，bsearch在搜索过程中访问的位置应该具有有效的字符串(就像在调用bsearch之前检查过的一样)，但是如果您尝试在该位置打印char *，则输出将是垃圾.谁能看到我的错误?顺便说一句，当我调用bsearch并将最终参数设置为:

As you can see, the locations visited by bsearch in the course of its search should have valid strings (as was just checked before calling bsearch), but the output if you try to print the char * at that location is garbage. Can anyone see my mistake? Incidentally I get the same bad behaviour (but don't get to follow it as closely obviously) when I call bsearch with the final parameter set to:

(int(*)(const void*, const void*))strcmp

谢谢！

推荐答案

由于您使用的是const char *数组，因此bsearch()会将指向这些元素的指针传递给比较函数.换句话说，它将在第二个参数中接收const char * const *.

Since you are using an array of const char *, bsearch() will pass to the comparison function a pointer to those elements. In other words, it will receive const char * const * in its second argument.

int myStrCmp(const void *s1, const void *s2) {
  const char *key = s1;
  const char * const *arg = s2;
  printf("myStrCmp: s1(%p): %s, s2(%p): %s\n", s1, key, s2, *arg);
  return strcmp(key, *arg);
}

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