Question

I have to dynamically allocate array of words. Words are stored in a file separated by variable count of white-space characters. I don’t know how many words is in the file a they can have variable length.

I have this code:

void readWord(FILE* stream, char *word, char first_c) {

    word[0] = first_c;
    char val;
    int wlen = 1;
    // isWhitespac is my function - tests if char is blank or '\n'
    while ((val = fgetc(stream)) != EOF && isWhitespace(val) == 0) {
        wlen++;
        word = realloc(word, (wlen+1) * sizeof (char));

        word[wlen-1] = val;    

    }
    word[wlen] = '\0';
}

int readList(const char *file) {

    FILE* f;
    char **arr;
    char val;
    int wcount = 0;

    arr = malloc(sizeof (char*));
    f = fopen(file, "r");

    while (fscanf(f, " %c", &val) == 1) {
        wcount++;
        arr = realloc(arr, wcount * sizeof (char *));

        arr[wcount - 1] = malloc(sizeof (char));  

        readWord(f, arr[wcount-1], val); 
        printf("%s\n", arr[wcount-1]);

    }


    for (int i = 0; i < wcount; ++i) {
        free(arr[i]);
    }

    free(arr);

    fclose(f);
    return 0;
}

It appears to work fine, it reads a prints all the words. But when I run the program with Valgrind the are too many errors, which I can’t find. Could anyone help me? (I know I have to test if malloc and others went fine, it is just a test func.)

The Valgrind log is quite long, should I post it too?

Answer 1

One of the issues is that you do realloc inside readWord. If realloc allocates a new buffer and doesn’t just extend the current one then your code will crash (you will double free the pointer) and this is what Valgrind picks up. To fix this I would rewrite the code so it returns a pointer instead of void.

char * readWord(FILE* stream, char *word, char first_c) {
    word[0] = first_c;
    char val;
    int wlen = 1;
    // isWhitespac is my function - tests if char is blank or '\n'
    while ((val = fgetc(stream)) != EOF && isWhitespace(val) == 0) {
      wlen++;
      word = realloc(word, (wlen+1) * sizeof (char));
      word[wlen-1] = val;
    }
    word[wlen] = '\0';
    return word;
}

And then change the loop in readList to this:

while (fscanf(f, " %c", &val) == 1) {
  wcount++;
  arr = realloc(arr, wcount * sizeof (char *));
  arr[wcount-1]=malloc(sizeof(char));
  arr[wcount - 1] = readWord(f, arr[wcount-1],  val);
  printf("%s\n", arr[wcount-1]);
}