Confusing pointers, Prints wierd result

This is a discussion on Confusing pointers, Prints wierd result within the C Programming forums, part of the General Programming Boards category; Hi All, Code: #include <stdio.h> #include <string.h> #include <stdlib.h> //First funtion void func(char *p) { p = "Hello"; // Statement ...

  1. #1
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    Confusing pointers, Prints wierd result

    Hi All,

    Code:
    #include <stdio.h>
    #include <string.h>
    #include <stdlib.h>
    
    //First funtion
    void func(char *p)
    {
          p = "Hello";  // Statement 1
    }
    
    int main()
    {
     
     char *p = NULL;
     func(&p,&c);
     printf("Val of p:%s\n",p);
     
    }
    Alternative function declaration: //Function 2

    Code:
    void func(char **p)
    {
        
        *p = "Hello";         // Statement 2
        
    }


    What is confusing to me is the outputs:

    With the function declared as in Function 1:
    <NULL>

    With the function declared as in Function 2:
    Hello

    Doubts:
    1. How come Statement 1 is valid?
    2. Will address be assigned to the pointer variable p in Statement 1?
    3. What is the difference b/w *p="Hello" and p="Hello"?
    4. how is that by calling Function 2 instead of Function 1 it prints Hello?


    Thanks in advance

  2. #2
    CSharpener vart's Avatar
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    replace pointer with int var and to the same exercise

    Code:
    void func(int val)
    {
       val = 5;
    }
    void func2(int* pVal)
    {
       *pVal = 6;
    }
    int main()
    {
       int x = 0;
       func(x);
       printf("Value of x is %d\n",x);
    
       func2(&x);
       printf("Value of x now is %d\n", x);
       return 0;
    }
    After you understand what is going on here - look at your example where int type is replaced with char* type, that's all
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  3. #3
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    Thanks for your time...

    1. How come Statement 1 is valid?
    2. Will address be assigned to the pointer variable p in Statement 1?
    3. What is the difference b/w *p="Hello" and p="Hello"?


    More doubts follow

    Code:
    int main()
    {
     
     char *p = NULL;
     func(&p,&c);
     printf("Val of p:%s\n",p);
    }
    
    Function 3
    void func(char **p)
    {
        char *q = "test"
        *p = &q;         // Statement 3
        
    }
    Ouput :
    Test

    What my doubts are while introducing Function 3:

    1. Now that i have declared char *q which is local to the Function, how come output is "test"?
    2. Is this dangling pointer? As the memory location is cleared once the function comes out?

    Thanks..

  4. #4
    msh
    msh is offline
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    1. How come Statement 1 is valid?
    Why wouldn't it be? You're creating a local variable char *p, and setting it to point to the address of string "Hello" in static storage. Function exits and local variable p is destroyed. Sounds valid. The value of p in main() remains untouched.
    2. Will address be assigned to the pointer variable p in Statement 1?
    Yes. To the address of "Hello" in static storage.

    1. Now that i have declared char *q which is local to the Function, how come output is "test"?
    All string literals (e.g. everything inside double quotes) persist for the duration of the program.
    2. Is this dangling pointer? As the memory location is cleared once the function comes out?
    See above.

  5. #5
    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by sanddune008 View Post
    More doubts follow
    "doubt" is a state of uncertain belief. "question" is a request for information. What you have is a question, not a doubt. Do no use the word "doubt".
    Every once in a while I have to correct people on this. People that are mostly of indian origin are taught this wrong.
    My homepage
    Advice: Take only as directed - If symptoms persist, please see your debugger

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