pointers question

**Tool** · 03-31-2010

Code:

int strlen(char *s)
{
     int len = 0;
     while(*s) 
        s++, len++;
     return len;
}

This is my own strlen function...

Now i was wandering why do some people do;

char *p = s;
and then use the p pointer in while iteration instead of s itself.

Code:

int strlen(char *s)
{
     int len = 0;
     char *p = s;
     while(*p) 
        p++, len++;
     return len;
}

What's the diffrence?

**laserlight** · 03-31-2010

Perhaps because that way they only need to increment one variable in the loop, and after the loop perform a subtraction.

**Tool** · 03-31-2010

Also one more question... on theory.

When this function is invoked in main:

Code:

int strlen(char *s)
{
     int len = 0;
     while(*s) 
        s++, len++;
     return len;
}

Does the function itself create a new pointer (in this case s) on stack which then points to the passed adress?

**hk_mp5kpdw** · 03-31-2010

BTW, argument "s" should probably be pointer to const char instead of just pointer to char.

Just a nit...

**laserlight** · 03-31-2010

Originally Posted by Tool

Does the function itself create a new pointer (in this case s) on stack which then points to the passed adress?

I would not say "the function itself", but roughly: yes. Parameters are local variables.

**Tool** · 03-31-2010

And those parameter variables are declared (created) at each call of the function?

**laserlight** · 03-31-2010

Originally Posted by Tool

And those parameter variables are declared (created) at each call of the function?

Space is allocated on the stack for them on each function call.

**Elysia** · 03-31-2010

It is also possible to implement the function as...

Code:

int strlen(const char* s)
{
    const char* p = s;
    while(*p++) ;
    return p - s - 1;
}

...showing why it might be useful to declare an extra variable that can holds s's value.
In your examples, however, they are merely style.

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