about char* in struct

**ampc** · 09-11-2010

hello everyone,i dont know how to assign the char* in struct,

Code:

struct List
{
    char *data;
    struct List *next;
};

struct List *InputData(struct List *head)
{
    struct List *p = head;
    struct List *pNewNode = NULL;
    char data[50];

    printf("input data: ");
    while(1)
    {
        scanf("%s ", data);
        if (0 != strcmp("#", data))
        {
            pNewNode = (struct List*)malloc(sizeof(struct List));
            pNewNode->data = (char *)malloc(sizeof(char));
            if (NULL == pNewNode || NULL == pNewNode->data)
            {
                printf("error\n");
                return NULL;
            }

            strcpy(pNewNode->data, data);
            pNewNode->next = NULL;
            p->next = pNewNode;
            p = pNewNode;
        }   
        else
        {
            return head;
        }
    }
		
    return NULL;
}

int  main()
{	
    struct List *head = NULL;

    head = InitList(head);			

    head = InputData(head);
	
    return 0;
}

when i run the program,it has an error,but if i debug it ,it well,please help me out?thanks

**Salem** · 09-12-2010

> pNewNode->data = (char *)malloc(sizeof(char));
Right idea, but you need
strlen(data)+1

**ampc** · 09-12-2010

aha.......what an important thing! but i forgot,hehe,thank u very much~~~

**Elysia** · 09-12-2010

You are forgetting to free what you malloc.
Also, do not reading strings with scanf! SourceForge.net: Scanf woes - cpwiki

**ampc** · 09-12-2010

oh,thank you,now i turn it to gets/puts functions, but i want to use unicode version,still error when i run it!

Code:

struct List
{
    TCHAR *data;
    struct List *next;
};

struct List *InputData(struct List *head)
{
    struct List *p = head;
    struct List *pNewNode = NULL;
    TCHAR data[50];

    printf("input data: ");
    while(1)
    {
        _getws(data);
        if (0 != lstrcmp(TEXT("#"), data))
        {
            pNewNode = (struct List*)malloc(sizeof(struct List));
            if (NULL == pNewNode)
            {
                printf("assign memory error!\n");
                return NULL;
            }
            pNewNode->data = (TCHAR *)malloc(lstrlen(data) + sizeof(TCHAR));
            if (NULL == pNewNode->data)
            {
                printf("assign memory error!\n");
                return NULL;
            }
            lstrcpy(pNewNode->data, data);
            pNewNode->next = NULL;
            p->next = pNewNode;
            p = pNewNode;
        }
        else
        {
            return head;
        }
    }

    return NULL;
}

void OutputData(struct List *head)
{
    struct List *p = NULL;
    if (NULL != head)
    {
        p = head->next;
    }
    else
    {
        return;
    }
    while (NULL != p)
    {
        _putws(p->data);
        p = p->next;
    }
    printf("\n");
}

int __cdecl wmain()
{	
    struct List *head = NULL;

    setlocale(LC_ALL, "chs");

    head = InitList(head);
	
    head = InputData(head);
    OutputData(head);
    FreeList(head);

    return 0;
}

**Elysia** · 09-12-2010

Who said you should use gets? It's even worse!

**Syscal** · 09-12-2010

I would use fgets() since it allows for you to specify the sizeof the string.

**~~CommonTater~~** · 09-12-2010

Originally Posted by ampc

oh,thank you,now i turn it to gets/puts functions, but i want to use unicode version,still error when i run it!

[CODE]struct List
{
TCHAR *data; <--- need to assign size
struct List *next;
};

[CODE]

To make life easyier... When using structs like this you should assign a field size for your string.
You might also want to use void pointers since a pointer to a structure is undefined within the structure's own definition.

Code:

typedef struct tList
    { char data[100];
       void*  Next;  }
     List, *pList;

Then in your linking code you would use...

Code:

// list tracking
pList Head;       // start of list
pList Current;   // current item

// init  new chain
Head = malloc(sizeof(List));   
Current = Head;

// add to chain
Current->Next = malloc(sizeof(list));
/* insert data here */

// move to next item
Current = Current->Next;

**Elysia** · 09-12-2010

Using "struct List" inside the struct is fine.
But don't create typedefs for pointers! It serves only to confuse!
It is also unclear why you're creating an array of pointers in the struct.

Code:

typedef struct List
{
    char* data;
    struct List* Next;
} List;

// first in chain
List* Head = malloc(sizeof(List));   

// init pointer to current item
Current = Head;

// add to chain
List* Current->Next = malloc(sizeof(list));
/* insert data here */

// move to next item
Current = Current->Next;

/* Don't forget to free! */

**~~CommonTater~~** · 09-12-2010

Originally Posted by Elysia

Using "struct List" inside the struct is fine.
But don't create typedefs for pointers! It serves only to confuse!
It is also unclear why you're creating an array of pointers in the struct.

The char* data[100]; is C's version of a string, Elysia. I'm merely defining a buffer to hold the string which would later be copied in with something like strcpy(Current->data, newdata); That way the string is right in the struct eliminating the need to malloc each string separately.

Code:

typedef struct List
{
    char* data;
    struct List* Next;
} List;

On any compiler I've used that typedef would return a redifinition error.

Code:

// first in chain
List* Head = malloc(sizeof(List));   

// init pointer to current item
Current = Head;

That would return an unkown type error for Current.

**Elysia** · 09-12-2010

What?
Using the typedef INSIDE the struct is not allowed.
Using it outside is fine.

**~~CommonTater~~** · 09-12-2010

Originally Posted by Elysia

What?
Using the typedef INSIDE the struct is not allowed.
Using it outside is fine.

Ok, I give up... you win.

**Bayint Naung** · 09-12-2010

You might also want to use void pointers since a pointer to a structure is undefined within the structure's own definition.

erm?

**Elysia** · 09-12-2010

That may be related to the fact that a typedef of the structure is not defined within the structure, so...

Code:

typedef struct mystruct
{
    struct mystruct* p; // OK
    mystruct* pp; // Not OK
} mystruct;

**~~CommonTater~~** · 09-12-2010

Originally Posted by Bayint Naung

erm?

I ran into this once... Elysia has one solution, void pointers are another.

Code:

typedef struct tFRED
   { char data[1000];
      FRED* Next;  }           <--- FRED is not yet defined
    FRED, *pFRED;             <--- Now FRED is defined

I found that using void pointers solved the problem for me... so I went with it.

Code:

typedef struct tFRED
   { char data[1000];
      void* Next;  }           <--- void is already defined
    FRED, *pFRED;

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