how to insert a string in a linked list inserting function

Question

I want to insert a string into struct data_node in a function called insert. My struct data_node is

struct data_node {
   char name [25];
   int data;
   struct data_node *next;
   };

and my insert function is:

struct data_node * insert (struct data_node **p_first, int elem, char *word ) {

    struct data_node *new_node, *prev, *current;
    current=*p_first;
    while (current != NULL && elem > current->data) {
       prev=current;
       current=current->next;
    } /* end while */

    /* current now points to position *before* which we need to insert */
    new_node = (struct data_node *) malloc(sizeof(struct data_node));
    new_node->data=elem;
    new_node->name=*word;

    new_node->next=current;
    if ( current == *p_first ) /* insert before 1st element */
       *p_first=new_node;
    else                       /* now insert before current */
       prev->next=new_node;
    /* end if current == *p_first */
    return new_node;
};

when I compile , it says that the line 22 incompatible types when assigning to type 'char[25]' from type 'char' that means new_node->name=*word; is wrong. how could I solve this problem?

Beta · Accepted Answer · 2014-03-31 01:35:25Z

1

The linked list structure is irrelevant. This problem boils down to copying one char[] to another. This will work:

strncpy(new_node->name, word, 25);

with some caveats. If word doesn't point to a valid char[], then this could cause undefined behavior. If it points to an array containing ~~more than 25~~ 25 or more (non-null) characters, the operation will copy the first 25 into new_node->name, which means that new_node->name will not be null-terminated, which could cause trouble later if other code assumes that it is. As WhozCraig points out, it's almost always a good idea to terminate the destination string with a null -- after being sure to leave room for it by copying one less character than you could (i.e. 25-1). And you might consider defining a constant NAMELENGTH so that you don't have the magic number 25 appearing here and there in your code.

edited Mar 31, 2014 at 1:35

answered Mar 31, 2014 at 0:50

Beta

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5 Comments

WhozCraig Over a year ago

The no-termination caveat isn't just if the source string is longer than 25 non null chars, its at least 25 chars. Meaning a string exactly 25 chars long, not just longer than 25, will likewise not terminate. This is the reason when coding a truncation acceptance to use one-less than the target buffer length as the length parameter, and always hard-set the last char in the buffer to 0. If the string were shorter, the hard-set is benign (and in fact the entire tail of the buffer is already zeroed out anyway in that case).

Beta Over a year ago

@WhozCraig: 1) You're right, I was off-by-one. 2) You're slightly less right; setting the last char to 0, although not strictly necessary, is a good convention that I should have mentioned. I'll edit...

WhozCraig Over a year ago

If you fail to terminate the string manually and the string being copied is N-length, where N is the third perimeter to strncpy(), it will not terminate the string. Setting the terminator isn't optional; its mandatory, particularly in the OP's case where the buffer is off a fresh malloc and thus filled with indeterminate data. Were the buffer off a calloc it would be optional, of course, but only if 25-1 (i.e. 24) or shorter is passed as the length.

Beta Over a year ago

@WhozCraig: I respectfully disagree. It will not null-terminate the string, but null-termination is not mandatory, just a good convention. As for the destination buffer starting out full of indeterminate data, it will wind up in the non-null-terminated state only if it has been completely overwritten with data from the source array, so I don't think we have to worry.

WhozCraig Over a year ago

ok then, you're right. we will disagree, because without null-termination you don't have a C string, you have a char buffer. Do with that what you will, so long as nothing calls any C string function afterward that expects a termination char to function properly (which would be all of them). And the non-terminated case full-buff-case is precisely why we do worry.

David Andreoletti · Accepted Answer · 2014-03-31 00:41:36Z

0

data_node.data should be void *. This way you can store any type (though you will have to know what type it is when reading it).

answered Mar 31, 2014 at 0:41

David Andreoletti

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Comments

ديناصور الأمة · Accepted Answer · 2014-03-31 00:47:49Z

0

You should use the strcpy method to copy word value in new_node -> name.

Take a look at this link.

strcpy(new_node->name, word);

answered Mar 31, 2014 at 0:47

ديناصور الأمة

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