How can I access a global pointer outside of a C function?

Question

I am trying to access the data of*tkn within a different function in my program for example: putchar(*tkn); It is a global variable but its not working correctly. Any ideas?

#define MAX 20
// globals
char *tkn;
char array[MAX];
...


void tokenize()
{

int i = 0, j = 0;
char *delim = " ";


tkn = strtok (str," ");         // get token 1
if (tkn != NULL) {
    printf("token1: ");

    while ((*tkn != 0) && (tkn != NULL))
    {
        putchar(*tkn);
        array[i] = *tkn;
        *tkn++;
        i++;
    }
   }                
}

Answer 1

You should use

tkn++

rather than

*tkn++

Answer 2

Just use strlcpy(3) instead of hand-coding the copy (hint - you are forgetting string zero terminator):

strlcpy( array, tkn, MAX );

Answer 3

In this line:

    while ((*tkn != 0) && (tkn != NULL))

you need to reverse the conditions. If tkn is a null pointer, you will crash when the first term is evaluated. If you need to check a pointer for validity, do so before dereferencing it.

    while (tkn != NULL && *tkn != '\0')

The extra parentheses you added do no harm but are not necessary. And although 0 is a perfectly good zero, the '\0' emphasizes that *tkn is a character. Of course, given the prior tkn != NULL condition in the if statement, there is no real need to repeat the check in the while loop.

---

Working code based on yours - some work left to do for subsequent tokens in the string, for example...

#include <stdlib.h>
#include <string.h>

enum { MAX = 20 };
char *tkn;
char array[MAX];
char str[2*MAX];

void tokenize(void)
{
    int i = 0;

    array[0] = '\0';

    tkn = strtok(str, " ");         // get token 1
    if (tkn != NULL)
    {
        printf("token1: ");
        while (tkn != NULL && *tkn != '\0' && i < MAX - 1)
        {
            putchar(*tkn);
            array[i++] = *tkn++;
        }
        *tkn = '\0';
        putchar('\n');
    }
}

int main(void)
{
    strcpy(str, "abc def");
    tokenize();
    printf("token = <<%s>>\n", array);
    strcpy(str, "abcdefghijklmnopqrstuvwxyz");
    tokenize();
    printf("token = <<%s>>\n", array);
    return(0);
}

Sample output:

token1: abc
token = <<abc>>
token1: abcdefghijklmnopqrs
token = <<abcdefghijklmnopqrs>>

---

Asked:

But what if I am taking in a string 'abc 3fc ghi' and I want to use just '3fc' in another function that say converts it from ascii to hex? How do I just use say tkn2 for 3fc and get that only using a pointer? – patrick 9 mins ago

That's where it gets trickier because strtok() has a moderately fiendish interface.

Leaving tokenize() unchanged, let's redefine str:

char *str;

Then, we can use (untested):

int main(void)
{
    char buffer[2*MAX];
    strcpy(buffer, "abc 3fc ghi");
    str = buffer;
    tokenize();
    printf("token = <<%s>>\n", array); // "abc"
    str = NULL;
    tokenize();
    printf("token = <<%s>>\n", array); // "3fc"
    str = NULL;
    tokenize();
    printf("token = <<%s>>\n", array); // "ghi"
    return(0);
}

Clearly, this relies on knowing that there are three tokens. To generalize, you'd need the tokenizer to tell you when there's nothing left to tokenize.

Note that having tkn as a global is really unnecessary - indeed, you should aim to avoid globals as much as possible.

Answer 4

Although tkn itself is a global variable, you also have to make sure that what it points to (ie. *tkn) is still around when you try to use it.

When you set tkn with a line like:

tkn = strtok (str," ");

Then tkn is pointing to part of the string that str points to. So if str was pointing to a non-static array declared in a function, for example, and that function has exited - then *tkn isn't allowed any more. If str was pointing to a block of memory allocated by malloc(), and you've called free() on that memory - then accessing *tkn isn't allowed after that point.