Thread: Beginner pointers/arrays question

Hi all - apologies if this is a tired old question. I couldn't find anything on first glance in the forums or tutorials.
I'm new to C, and I'm trying to find out from inside a function how many characters have been allocated in memory to a string array. The catch is that the function only has a pointer to the array that it takes as an argument. I've tried something like this:

Code:

#include <stdlib.h>

int get_free_chars(char *ptr)
{
    return sizeof(ptr)/sizeof(char);
}

main()
{
    char foo[50] = "";
    int a = sizeof(foo) / sizeof(char);
    int b = get_free_chars(foo);
}

(Obviously there would be no point in the get_free_chars function alone - I'm trying to do something similar within a string manipulation function I'm coding)

'a' takes the value I'd expect - 50 - but 'b' comes out to be 4 (obviously because the program is measuring the size of the pointer (4 bytes = 32 bits, which is the size for my system) rather than the array itself). Doing sizeof(*ptr) obviously wouldn't work as that would point directly to the first character only.

Is there a nice way to get this information from within the function?

Thanks in advance!

Originally Posted by whiteandnerdy

I'm trying to find out from inside a function how many characters have been allocated in memory to a string array. The catch is that the function only has a pointer to the array that it takes as an argument.
(...)
Is there a nice way to get this information from within the function?

No, at least not one that is portable. Just provide another parameter for the size.

Typically you can make a struct like

Code:

struct String
{
    char* data;
    unsigned size;
}

and any functions you like to help you.

Alternatively, you use a terminating character like '\0' c-strings use. So you would have

Code:

int get_free_chars(char *ptr)
{
    while(*ptr != '\0')
       ptr++;
}

which does what strlen() does in this matter for a c-string. But I guess you want the first option which is faster in terms of performance