Passing char pointer in C

Question

Okay so I am trying to pass a char pointer to another function. I can do this with an array of a char but cannot do with a char pointer. Problem is I don't know the size of it so I cannot declare anything about the size within the main() function.

#include <stdio.h>

void ptrch ( char * point) {
    point = "asd";
}

int main() {
    char * point;
    ptrch(point);
    printf("%s\n", point);
    return 0;
}

This does not work however, these two works:

1)

#include <stdio.h>

int main() {
    char * point;
    point = "asd";
    printf("%s\n", point);
    return 0;
}

2)

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

void ptrch ( char * point) {
    strcpy(point, "asd");
}

int main() {
    char point[10];
    ptrch(point);
    printf("%s\n", point);
    return 0;
}

So I am trying to understand the reason and a possible solution for my problem

hmatar · Accepted Answer · 2013-01-15 05:02:06Z

22

This should work since pointer to the char pointer is passed. Therefore any changes to the pointer will be seen outside thereafter.

void ptrch ( char ** point) {
    *point = "asd";
}

int main() {
    char * point;
    ptrch(&point);
    printf("%s\n", point);
    return 0;
}

answered Jan 15, 2013 at 5:02

hmatar

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2 Comments

Karthik T Over a year ago

I would have prefered to return the pointer rather than another level of indirection, but +1 nonetheless

TomSawyer Over a year ago

why it works with integer and doesn't need to pass pointer to pointer? i think because after dereference, char pointer remain is the address of first element of array?

Karthik T · Accepted Answer · 2013-01-15 05:05:18Z

12

void ptrch ( char * point) {
    point = "asd";
}

Your pointer is passed by value, and this code copies, then overwrites the copy. So the original pointer is untouched.

P.S. Point to be noted that when you do point = "blah" you are creating a string literal, and any attempt to modify is Undefined behaviour, so it should really be const char *

To Fix - pass a pointer to a pointer as @Hassan TM does, or return the pointer as below.

const char *ptrch () {
    return "asd";
}

...
const char* point = ptrch();

edited Jan 15, 2013 at 5:05

answered Jan 15, 2013 at 5:00

Karthik T

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4 Comments

Sarp Kaya Over a year ago

Which way is less expensive?

Karthik T Over a year ago

@SarpKaya both are blazingly fast (difference would be minimal if any), but I would prefer the "return the pointer" method for readability.

Zimano Over a year ago

This will trigger a warning of local variables if you do not hardcode the return value as a string literal.

TomSawyer Over a year ago

why we need const here?

Ricky Stewart · Accepted Answer · 2013-01-15 05:01:16Z

3

Here:

int main() { char * point; ptrch(point);

You're passing point by value. Then, ptrch sets its own local copy of point to point to "asd", leaving the point in main untouched.

A solution would be to pass a pointer to main's point:

void ptrch(char **pp) { *pp = "asd"; return; }

answered Jan 15, 2013 at 5:01

Ricky Stewart

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Comments

KamikazeCZ · Accepted Answer · 2013-01-15 05:09:56Z

1

If you change the value of the pointer in a function, it will remain changed only in that one function call. Don't mess your head with pointers and try:

void func(int i){
  i=5;
}
int main(){
  int i=0;
  func(i);
  printf("%d\n",i);
  return 0;
}

The same with your pointer. You do not change the address it points to.

If you assign to a variable passed by value, the variable outside the function will remain unchanged. You could pass it by a pointer (to pointer) and change it by dereferrencing it and it's the same with an int - in this case, it doesn't matter if the type is int or char * .

edited Jan 15, 2013 at 5:09

answered Jan 15, 2013 at 5:02

KamikazeCZ

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Comments

shoaib soudagar · Accepted Answer · 2016-12-13 06:30:44Z

-1

first declare funtion......like this

 #include<stdio.h>
 void function_call(char s)

next write main code.....

void main()
{
    void (*pnt)(char);  //  pointer function declaration....
    pnt=&function_call;  //    assign address of function
    (*pnt)('b');   //   call funtion....
}

answered Dec 13, 2016 at 6:30

shoaib soudagar

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