Thread: Sending String to Function not working

I am trying to pass a string to this function:

Code:

void insert(ListNodePtr *sPtr, char *value);

I am gathering the string from scanf, printing it, and then passing it to the function like so (I am building a linked list):

Code:

struct listNode
{
   char *data;
   struct listNode *nextPtr;
};

typedef struct listNode ListNode;
typedef ListNode *ListNodePtr;

int main(void)
{
   
   ListNodePtr startPtr = NULL;
   char *zztop;

   scanf("%s", &zztop);

   printf("\n%s\n", &zztop);

   insert(&startPtr, zztop);
}

void insert(ListNodePtr *sPtr, char *value)
{
   ListNodePtr newPtr;
   ListNodePtr previousPtr;
   ListNodePtr currentPtr;

   newPtr = malloc(sizeof(ListNode));

   printf("\n %s \n", &value);
}

The printf value at the end of the insert function is merely a checkpoint to see if the data that was given to function insert is correct. However, random garbage is printed. Why?

My apologies if the question is convoluted, hope this makes some sense! Thanks

There's nothing meaningful that can be printed from &value, much as there's nothing meaningful that can be printed from &zztop. In fact if you lose all those & you'll be a lot better off.

The first printf statement inside main seems to work fine. How should I be printing zztop there otherwise?

I removed the ampersand before value in the second printf statement located in the insert function. It still seems to print garbage.

Code:

   char *zztop;

   scanf("%s", &zztop);

You didn't allocate memory to hold your string.

Oh, so does that mean that the string entered will be lost and unreachable outside of the main function? How do I avoid this, or how do I allocate memory for this? I can't imagine I'd be using a malloc......or would I?

Originally Posted by voidpain()

I am trying to pass a string to this function:
I am gathering the string from scanf, printing it, and then passing it to the function like so (I am building a linked list):

Code:

struct listNode
{
   char *data;                     <---- no memory allocated
   struct listNode *nextPtr;
};

typedef struct listNode ListNode;
typedef ListNode *ListNodePtr;

int main(void)
{
   
   ListNodePtr startPtr = NULL;
   char *zztop;                    <--- no memory allocated

   scanf("%s", &zztop);

   printf("\n%s\n", &zztop);

   insert(&startPtr, zztop);
}

void insert(ListNodePtr *sPtr, char *value)
{
   ListNodePtr newPtr;
   ListNodePtr previousPtr;
   ListNodePtr currentPtr;

   newPtr = malloc(sizeof(ListNode));

   printf("\n %s \n", &value);
}

What you need to do is use char arrays not pointers... as in char zztop[100]... and in your struct... char data[100] ... (or whatever the correct size is)

What is happening is that your scanf function is writing to memory you don't own and you are storing your string in memory you don't own.

Step 1 ... forget the list for the moment... get your software to read and display the user input. When you have that working you can move on to creating your first node in the list...

I actually previously implemented this as char zztop[30], and I got it to print just fine within main. But how exactly am I going to pass zztop[30] into the insert function? I can't pass an array like that into a function....or can I? ....or do I use a pointer to the array, or something?

Originally Posted by voidpain()

I actually previously implemented this as char zztop[30], and I got it to print just fine within main. But how exactly am I going to pass zztop[30] into the insert function? I can't pass an array like that into a function....or can I? ....or do I use a pointer to the array, or something?

The array's name decays to a pointer when passed to a function... So test the proposition...

Code:

void PrintStr(char *str)
  { printf("%s", str); }

// call as
PrintStr(zztop);

... and see what happens.

When in doubt... Experiment!

Also, just wanted to mention how bad ass this forum is. I'm in a summer school class right now, and the school's Help Center has been closed all summer. So far, this forum has been extremely helpful...I apreesh, thank you

Works perfectly. Exactly what I was looking for. Thanks to all!