strlen返回错误的值

Question

I'm building a text editor using doubly linked lists. These are my structs:

#define N 4
typedef struct node
{
    char data[N];
    int size;
    struct node* next;
    struct node* prev;
}node;

typedef struct text
{
    struct node* head;
    struct node* tail;
    int count;
    int size;
}text;

This is the piece of code that I use to fill the first node.

void push_text (text * t, char * s)
{
    int i;
    int len = strlen(s);
    node *newnode, *nextnode, *head;

    newnode = create_node();
    t->count++;
    head = newnode;
    for (i=0; i<len; i++)
    {
        if (newnode->size < 4)
        {
            newnode->data[newnode->size] = s[i];
            newnode->size++;
        }
    .
    .
    .

When I print the node through printf or through the debugger the output is 4 chars long, as expected. Note that, I print it as soon as the first node is filled so the problem lies in this piece of code. However, when I use strlen(newnode->data) I get an output of 5. This is causing me many problems later on.

What's wrong here?

Answer 1

You are not copying the nul terminator, ac string needs a '\\0' at the end, so if it has 4 characters it uses 5 bytes, the last one being '\\0' which is not copied in your loop.

You should however use strcpy() instead of copying they bytes one by one in a loop.

The strlen() function scans the bytes until it finds the '\\0' , so the missing '\\0' is causing the Wrong Value !, also that's a reason not to call strlen() in a loop, which is a very common mistake.

Answer 2

You cannot put a four-character C string into a four-element array of char , because you need space for null terminator. Change all declarations of data to

char data[N+1];

You should also use N in place of constant 4 in expressions that expect the length to be less than N (eg newnode->size < N instead of newnode->size < 4 ).

Answer 3

All the answers explained before were correct but no one explain the mistake.

With an example you will see better the mistake

node.data[0] = 'a';
node.data[1] = 'b';
node.data[2] = 'c';
node.data[3] = 'd';
node.datasize = 4;

In a little endian machine the memory will be filled in that way:

Hex value of 'a', 'b', 'c','d' coded in a byte and four bytes to code the int

Memory: 'a''b''c''d'4000

Then you will obtain...

strlen(node.data) --> 5
printf(node.data) --> abcd (plus 4 ascii code).