在结构中对char *进行排序-获取垃圾

Question

I made an array of Node structs and I am trying to sort nodes in alphabetical order based on their char* variable called "word".

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "memwatch.h"
#include "concord.h"

#define BUFFSIZE 1000

int main(int argc, char** argv)
{
  Node** list;
  printf("%s%s\n","The file name is ",  argv[1]);
  readInputFile(argv[1], list);
  return 0;
}

int compareWords(const void* nodeA, const void* nodeB)
{
  Node* nodeAA = (Node *) nodeA;
  Node* nodeBB = (Node *) nodeB;
  puts("now here\n");
  printf("%s\n", nodeAA->word);
  printf("%s\n", nodeBB->word);
  return strcmp(nodeAA->word, nodeBB->word);
}

void readInputFile(char* filename, Node** wordList)
{
  FILE* file;
  file = fopen(filename, "r");
  wordList = calloc(BUFFSIZE, sizeof(Node*));

  char* currentWord;
  currentWord = (char*) malloc(sizeof(char) *BUFFSIZE);
  int i;
  i = 0;
  while(fscanf(file, "%s", currentWord)  == 1)
  {
    wordList[i] = (Node*) malloc(sizeof(Node));
    wordList[i]->word = strdup(currentWord);
    puts(wordList[i]->word);
  }
  fclose(file);
  qsort(wordList, i, sizeof(Node), compareWords);
}

Before I was printing out garbage when I tried to print out the word in the compare function, now it looks like the function is not even being called.

Answer 1

now it looks like the function is not even being called.

That is because to sort a list of 0 elements you never need to compare two elements:

  // ...
  int i;
  i = 0;    // --- set to 0
  while(fscanf(file, "%s", currentWord)  == 1)
  {
    // i not changed ... causes other problems, too
    // (contents omited)
  }
  fclose(file);
  // i is still 0
  qsort(wordList, i, sizeof(Node), compareWords);
  // ...

Apart from that your usage of an "out parameters" is wrong, as pointed out in the comment by David C. Rankin. In this case, i'd also advise to just use the return value.

Moreover, I'd split that function into multiple functions:

// Does the file opening and closing, calls readInput
Node * readInputFile(char const *);
// The actual reading
Node * readInput(FILE *)
// Probably do the sorting outside of these functions

Answer 2

[First of all you question is incomplete as it misses to show us the definition of Node .]

However, three issues here:

1. I made an array of Node structs

   You don't.

   Here

    wordList = calloc(BUFFSIZE, sizeof(Node*)); 

   you allocate memory for an array of pointers to Node .

   And then here

    wordList[i] = (Node*) malloc(sizeof(Node)); 

   you allocated a separate chunk of memory to each element of the pointer array created previously.

   The latter may be scattered all over the process's memory. They will not be located in a continous block of memory, as expected by qsort() , which might have been the reason for:

   I was printing out garbage [before]

2. On returning from readInputFile() the value of wordList is lost.

3. The reading loop does not increment the index counter i .

---

To fix 1. and 2. create an array and return a reference to it back up to the caller of readInputFile() like so

*wordList = calloc(BUFFSIZE, sizeof **wordList);

and call qsort() like this:

qsort(*wordList, i, sizeof(Node), compareWords);

---

To fix 3. do this:

size_t i = 0; /* No need for negative indexes here .*/
while((i < BUFFSIZE) /* Male sure not to overflow the array. */   
      && (fscanf(file, "%s", currentWord) == 1))
{
  (*wordList)[i].word = strdup(currentWord); /* This is POSIX not Standard C. */
  puts((*wordList)[i].word);
  ++i;
}