将文件中的单词读入C中的节点struct

Question

I'm having trouble with reading word from a file into a node struct. This is the code:

//open file
FILE* fp = fopen("fp", "r");

//create node structure
typedef struct node  
{
      char* word;
      struct node* next;
} node;

node* newNode = malloc(sizeof(node));

fscanf(fp, "%s", newNode->word);

printf("w: %s\n", newNode->word);

the fp file looks like this

word1
word2
word3
word4

The newNode->word is NULL , why is this?

Answer 1

This is because you have not allocated space for the word . When you allocate node , only a pointer to the word gets allocated. However, the pointer remains uninitialized until you assign it. Reading into it using fscanf is undefined behavior, because there is no memory behind it.

One way to deal with it is as follows: allocate a temporary buffer of fixed size large enough to hold the largest word, read the word into it, then allocate the space for the string, copy the content, and set the pointer into the node:

char buf[100];
...
fscanf(fp, "%99s", buf); // 99, not 100, because '\0' must fit in the buf as well
int len = strlen(buf);
newNode->word = malloc(len+1); // +1 for null terminator
strcpy(newNode->word, buf);

Answer 2

newNode->word is just a pointer. You need to allocate memory to newNode->word before actully storing the data in it!

node* newNode = malloc(sizeof(node));
newNode->word = (char*) malloc(sizeof(char)*10);
fscanf(fp, "%s", newNode->word);

This shud do the trick.

Answer 3

The newNode->word is NUL because you didn't allocate memory for it. You need either

typedef struct node  
{
      char word[100];  // maximum word length 99
      struct node* next;
} node;

or

node* newNode = malloc(sizeof(node));
newNode->word = malloc(100);    // allocate space for word

Answer 4

You either need to allocate some space for the string you would like to store and pass a pointer to that space to scanf, or you could use "%ms" to have scanf allocate the space on its own. For the latter you need to pass a pointer to a pointer. In your case that would be &node->word.