Pointer to a pointer to an array (?)

Question

Please forgive me if the title is not 100% correct. I have a function which takes two inputs - a pointer to a string, and a pointer to a pointer to an array (?). The function I am writing is
int string_parser(char *inp, char **array_of_words[])
What I want to be doing is taking these two arguments and the function should return

the amount of words in the string array (string array is char *inp)
a pointer to an array of pointers char **array_of_words[] - each element in the array pointing to the address of the first character in each word (I apologise if that is wordy)
I have created the pointer to an array of pointers, and allocated space to this array
char **startOfWords_ptr = (char **) malloc(amountOfWords * sizeof(char*));
and have been manipulating the contents just fine. I now want to pass the array at *start_of_words back to array_of_words - but I don't understand how to do it

With *array_of_words = *(startOfWords_ptr); am I saying: a pointer to point at the starting address of the array of pointers?
Surely I am not because I don't get the same values when printing...

printf("%p\n", (void *) *array_of_words); // 0x401f2c, o
printf("%p\n", (void *) *startOfWords_ptr); // 0x401f2c, o

printf("%p\n", (void *) array_of_words[1]); // 0x401f2c, o
printf("%p\n", (void *) startOfWords_ptr[1]); // 0x401f30, t

Suggestions with comments as output

*array_of_words =  (startOfWords_ptr); // This currently does not work

printf("%p, %c\n", (void *) array_of_words[0], *array_of_words[0]); // 0xcbbdd0, ;
printf("%p, %c\n", (void *) startOfWords_ptr[0], *startOfWords_ptr[0]); // 0x401f3b, o

printf("%p, %c\n", (void *) array_of_words[1], *array_of_words[1]); // 0x401f2c, o
printf("%p, %c\n", (void *) startOfWords_ptr[1], *startOfWords_ptr[1]); // 0x401f3f, t

In C++ one would normally use std::string and std::vector and avoid raw arrays and memory allocation. So no, not the same when using natural constructs of the language. — crashmstr
– crashmstr, Commented Mar 24, 2017 at 20:35
It looks like you are looking for *array_of_words = startOfWords_ptr;. — n. m. could be an AI
– n. m. could be an AI, Commented Mar 24, 2017 at 20:53
@fOrceez: Let me make sure I understand your requirement - given an input string like "This is a test" (inp), you want to create an array of pointers to the first letter of each word? — John Bode
– John Bode, Commented Mar 24, 2017 at 21:35

John Bode · Accepted Answer · 2017-03-24 22:05:39Z

1

Okay, so let's look at this from the perspective of the caller of this function:

int main( void )
{
  char my_input[] = "This is a test";
  char **my_output = NULL;

  int count = string_parser( my_input, &my_output );
  for ( int i = 0; i < count; i++ )
    printf( "pointer #%d: %p\n", i, (void *) my_output[i] );
}

We know that after calling string_parser, my_output will point to the first of a sequence of pointers to char. Since we need to modify the value of my_output, we must pass a pointer to it in the call (&my_output).

This means that the prototype for string_parser needs to be

int string_parser( const char *inp, char ***array_of_words ) 
{
   ...
}

In the context of a function parameter declaration, T a[N] and T a[] are both treated as T *a, so char ***array_of_words is the same as char **array_of_words[] - both are ultimately char ***.

So given

I have created the pointer to an array of pointers, and allocated space to this array char **startOfWords_ptr = (char **) malloc(amountOfWords * sizeof(char*)); and have been manipulating the contents just fine. I now want to pass the array at *start_of_words back to array_of_words - but I don't understand how to do it

you have it backwards - you'd assign the startOfWords_ptr to *array_of_words:

*array_of_words = startOfWords_ptr; // char ** = char **

array_of_words has one more level of indirection than startOfWords_ptr, so we need to dereference it to get the types to match. After this assignment,

*array_of_words[i] == startOfWords_ptr[i] // char * == char *

answered Mar 24, 2017 at 22:05

John Bode

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1 Comment

fOrceez Over a year ago

Thanks, helped me get an understanding of what I needed. Turns out even when I had the correct assignment, I was printing incorrectly. Dereferencing when I didn't need to, not dereferencing when I needed to. Answer has been accepted :)

user3629249 · Accepted Answer · 2017-03-24 21:35:53Z

1

this parameter in the function signature:

char **array_of_words[]

can be re-written as:

char ***array_of_words

I.E. either write it as:

char **array_of_words

or as:

char *array_of_words[]

This is very similar to the different ways the second parameter to main() can be written.

answered Mar 24, 2017 at 21:35

user3629249

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