whats th difference between *ptr and **ptr in a signature..

**mutandis** · 12-14-2008

But, would you only use **ptr when dereferencing?

I ask because I *think* I've seen it used as an argument in a function prototype (do you guys call them prototypes or declarations here?)

I've confused myself with these double asterisks myself... Could anyone give an example of when you would want a pointer to a pointer? Why wouldn't you just change where the first pointer pointed to?

-Dave

**tabstop** · 12-14-2008

Originally Posted by mutandis

But, would you only use **ptr when dereferencing?

I ask because I *think* I've seen it used as an argument in a function prototype (do you guys call them prototypes or declarations here?)

I've confused myself with these double asterisks myself... Could anyone give an example of when you would want a pointer to a pointer? Why wouldn't you just change where the first pointer pointed to?

-Dave

Well, for the obvious reason that you can't change where the first pointer points to, just like you can't change any parameter passed in to a function. If you search the board, you will find 374 examples of people trying to create a new linked list like so:

Code:

void create_list(Node *head, Data start_data) {
    head = malloc(sizeof Node);
    head.data = start_data;
    head.next = NULL;
}

and wondering why they didn't have a list when they got back to main.

**esbo** · 12-14-2008

Originally Posted by mutandis

But, would you only use **ptr when dereferencing?

I ask because I *think* I've seen it used as an argument in a function prototype (do you guys call them prototypes or declarations here?)

I've confused myself with these double asterisks myself... Could anyone give an example of when you would want a pointer to a pointer? Why wouldn't you just change where the first pointer pointed to?

-Dave

Code:

// this might be a bit 'old style' C

main( int argv, char **argc) {

.....etc....

eg programname parameter1 parameter2 etc....eg print file1 file2 etc...

argv is the number of parameters to a program
**argc are the parameters, a pointer to an array of strings, or expressed in another
way a pointer to a pointer to a character.

Code:

in print file1 file2 
argc[0]="file1"
argc[0][1]='i'
argc[0][4]='1'

argc[1]="file2"
argc[1][0]='f'
argc[1][4]='2'


char words[2][6];
ptr=words;
strcpy(*ptr,"word1");
ptr++;
strcpy(*ptr,"word2");

If you printed
printf("&#37;s\n",words[0]);
printf("%s\n",words[1]);

it would probably say

word1
word2


or
ptr=words;
printf("%s\n",*ptr);
ptr++
printf("%s\n",*ptr);

Should do the same.

Then again it might fall over

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