使用fgets和strstr在C中计算字符串

Question

This is part of an assignment, so the instructions are clear and I'm not allowed to use anything other than what is specified.

The idea is simple:

1) Create an array of structs which hold a string and a count

2) Count the occurrence of the string in each struct and store the count in that struct

3) Print the strings and their number of occurrences

I have been explicitly told to use the fgets and strstr functions

Here is what I've got so far,

#define MAX_STRINGS 50
#define LINE_MAX_CHARS 1000
int main(){
  int n = argc - 1;
  if (n > MAX_STRINGS) {
    n = MAX_STRINGS;
  }
  Entry entries[MAX_STRINGS];
  char **strings = argv+1;
  prepare_table(n, strings, entries);
  count_occurrences(n, stdin, entries);
  print_occurrences(n, entries);
}

void prepare_table (int n, char **strings, Entry *entries) {
  // n = number of words to find
  // entries = array of Entry structs
  for (int i = 0; i < n; i++){
      Entry newEntry;
      newEntry.string = *(strings + 1);
      newEntry.count = 0;
      *(entries + i) = newEntry;
  }
}

void print_occurrences (int n, Entry *entries) {
  for (int i = 0; i < n; i++){
      printf("%s: %d\n", (*(entries + i)).string, (*(entries + i)).count); 
  }
}

void count_occurrences (int n, FILE *file, Entry *entries) {
  char *str;
  while (fgets(str, LINE_MAX_CHARS, file) !=  NULL){
      for (int i = 0; i < n; i++){ // for each word
          char *found;
          found = (strstr(str, (*(entries + i)).string)); // search line
          if (found != NULL){ // if word found in line
            str = found + 1; // move string pointer forward for next iteration
            i--; // to look for same word in the rest of the line
            (*(entries + i)).count = (*(entries + i)).count + 1; // increment occurrences of word
          }
      }
  }
}

I know for a fact that my prepare_table and print_occurrences functions are working perfectly. However, the problem is with the count_occurrences function.

I've been given a test file to run which just tells me that I'm not producing the correct output. I can't actually see the output to figure out whats wrong

I'm new to pointers, so I'm expecting this to be a simple error on my part. Where is my program going wrong?

Answer 1

fgets(char * restrict str, int size, FILE * restrict stream) writes into the buffer at str ... but you don't have a buffer at str . What is str ? It's just a pointer. What's it pointing at? Garbage, because you haven't initialized it to something. So it might work or it might not ( edit: by which I mean you should expect it not to work, and be surprised if it did, thank you commenters! ).

You could fix that by allocating some memory first:

char *str = malloc(LINE_MAX_CHARS);
// do your stuff
free(str);
str = NULL;

Or even statically allocating:

char str[LINE_MAX_CHARS];

That's one problem I can see anyway. You say you don't have output, but surely you can add some debug statements using fprintf(stderr, "") at the very least..?