Thread: Pls clarify the doubt in pointers...

Hi,
I'm not able to understand why the below code is not working properly.

1 #include
2 void f(char *s)
3 {
4 s="woooo";
5 }
6 void main()
7 {
8 char a[] = "Hello";
9 f(a);
10 printf(a);
11 }

In the function f, i'm trying to assig a value to the char pointer passed from main(). I don't understand why this value is not getting reflected in main(). But if i replace the line
4 from
s="woooo";
to strcpy(s,"woooo");

the value is getting reflected in main(). Any help would be appreciated.

Thanks & regards,
S.Guruprasanna

If you declared the string as char* in main, and did this:

Code:

void f(char *s) 
{ 
    s="woooo"; 
}

It won't be reflected in main, because :
You are making the pointer point somewhere else, you aren't changing the pointed-to-data, which is the string.

When you pass a string, always use strcpy() to change the pointed to data.

If you wanted the make the pointer point somewhere else in memory, then do on of the following:

Return a pointer to the new address and assign it.
OR
Pass the pointer by reference.(Pass it's address to the function)

babu...
u should pass by address in-order to change the value of a variable defined in one function , in another function.

but u r passing the variable by value.

instead if u pass by address, u can change the variable in function f.

check out the following code:

#include<stdio.h>

void f(char **c)
{
*c = "chandra";
}


int main()
{
char *c;
f(&c);
printf("%s",c);
getch();
return 0;
}

hope this wil clear ur doubt.

ucsbme, no. You could do it this way but it would still be wrong. Listen to what the Dog said. Here is what the function should look like

Code:

void f(char *s, int max_size) {
     strncpy(s, "whooo", max_size);
}

The reason you want to have the second parameter is because you have no other way of knowing the size of the buffer that is pointed to by "s." Also remember

Code:

void f(char *s) {
    s = "whooo";
}
//and
void f(char **s) {
    *s = "whooo";
}

both point s to a temporary array. In other words "s" is only valid until the function terminates. Use strcpy or something like this function.