Question

Hee guys,

I have been reading a couple of things about pointers and pointees and started getting curious. The only thing I dont understand is how pointers behave in functions, hence the following code:

#include <stdio.h>

int pointeeChanger(char* writeLocation) {

    writeLocation = "something";
    return 0;
}

int main(void)
{

    char crypted[] = "nothing";
    char* cryptedPointer = crypted;

    pointeeChanger(cryptedPointer);

    printf("The new value is: %s", cryptedPointer);

    return 0;
}

What my intention to do is to adjust the pointee, “crypted” var, through a pointer given to a function. The only thing is that it is not working. Could you please explain me what is going wrong in my thought process. I am fairly new to C so my errors could be fairly basic.

Thanks in advance!

Greetings,

Kipt Scriddy

Answer 1

Short answer: writeLocation is a local variable and is a copy of cryptedPointer. When you modify writeLocation, cryptedPointer is not modified.

If you want to modify cryptedPointer, you have to pass a pointer to it, like so:

#include <stdio.h>

int pointeeChanger(char** writeLocation) {            /* Note: char** */

    *writeLocation = "something";                     /* Note: *writeLocation */
    return 0;
}

int main(void)
{

    char crypted[] = "nothing";
    char* cryptedPointer = crypted;

    pointeeChanger(&cryptedPointer);                  /* Note: &cryptedPointer */

    printf("The new value is: %s", cryptedPointer);

    return 0;
}

There are other issues with this code though. After the call to pointeeChanger(), cryptedPointer no longer points to the crypted array. I suspect you actually wanted to change the contents of that array. This code fails to do that.

To change the value of crypted[] you will need to use strcpy() or (preferably) strncpy(). Also you will need to watch the size of the crypted[] array – "something" is longer than "nothing" and will cause a buffer overflow unless crypted[] is made larger.

This code will modify the original crypted[] array:

#include <stdio.h>
#include <string.h>

#define MAX_STR_LEN 64

/*
 * Only char* required because we are not modifying the
 * original pointer passed in - we are modifying what it
 * points to.
 */
int pointeeChanger(char* writeLocation)
{
    /*
     * In C, you need to use a function like strcpy or strncpy to overwrite a
     * string with another string. Prefer strncpy because it allows you to
     * specify a maximum size to copy, which helps to prevent buffer overruns.
     */
    strncpy(writeLocation, "something", MAX_STR_LEN);

    /* strncpy doesn't automatically add a \0 */
    writeLocation[MAX_STR_LEN] = '\0';

    return 0;
}


int main(void)
{
    /*
     * The +1 is because an extra character is required for the 
     * null terminator ('\0')
     */
    char crypted[MAX_STR_LEN + 1] = "nothing";

    pointeeChanger(crypted);

    printf("The new value is: %s", crypted);

    return 0;

}