c - strcmp not returning 0 for equal strings

问题:

So I've tried searching for a solution to this extensively but can only really find posts where the new line or null byte is missing from one of the strings. I'm fairly sure that's not the case here.

I am using the following function to compare a word to a file containing a list of words with one word on each line (dictionary in the function). Here is the code:

int isWord(char * word,char * dictionary){
  FILE *fp;
  fp = fopen(dictionary,"r");
  if(fp == NULL){
    printf("error: dictionary cannot be opened\n");
    return 0;
  }
  if(strlen(word)>17){
    printf("error: word cannot be >16 characters\n");
    return 0;
  }
  char longWord[18];
  strcpy(longWord,word);
  strcat(longWord,"\n");
  char readValue[50] = "a\n";
  while (fgets(readValue,50,fp) != NULL && strcmp(readValue,longWord) != 0){
    printf("r:%sw:%s%d\n",readValue,longWord,strcmp(longWord,readValue));//this line is in for debugging
  }
  if(strcmp(readValue,longWord) == 0){
    return 1;
  }
  else{
    return 0;
  }
}

The code compiles with no errors and the function reads the dictionary file fine and will print the list of words as they appear in there. The issue I am having is that even when the two strings are identical, strcmp is not returning 0 and so the function will return false for any input.

eg I get:

r:zymoscope
w:zymoscope
-3

Any ideas? I feel like I must be missing something obvious but have been unable to find anything in my searches.

回答1:

I see you are appending a newline to your test strings to try to deal with the problem of fgets() retaining the line endings. Much better to fix this at source. You can strip all trailing stuff like this, immediately after reading from file.

readValue [ strcspn(readValue, "\r\n") ] = '\0';   // remove trailing newline etc

回答2:

The string you are reading contains trailing character(s), and hence is not the same as the string you are comparing it against.

Remove the trailing newline (and CR if that is there); then you do not need to add any newline or carriage return to the string being compared:

int isWord(char *word, char *dictionary){
  FILE *fp;
  fp = fopen(dictionary, "r");
  if (fp == NULL){
    fprintf(stderr, "error: dictionary cannot be opened\n");
    return 0;
  }
  if (strlen(word) > 16){
    fprintf(stderr, "error: word cannot be >16 characters\n");
    return 0;
  }
  char readValue[50];
  while (fgets(readValue, 50, fp) != NULL){
    char *ep = &readValue[strlen(readValue)-1];

    while (*ep == '\n' || *ep == '\r'){
      *ep-- = '\0';
    }
    if (strcmp(readValue, word) == 0){
      return 1;
    }
  }
  return 0;
}