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Passing variables to functions

This is a discussion on Passing variables to functions within the C Programming forums, part of the General Programming Boards category; Hi, I'm trying to pass a variable to testVars(), have testVars() change the variable, and then make that variable available ...

  1. #1
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    Passing variables to functions

    Hi,

    I'm trying to pass a variable to testVars(), have testVars() change the variable, and then make that variable available to main().

    Passing the variable from main() to testVars() is working--getting it back to main() is not.

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    
    char testVars(char *a, char *b);
    
    main()
    {
        char * testChar = "We just passed";
        char * testCharTwo = "a main() variable!";
        testVars(testChar, testCharTwo);
    
        printf("%s %s - Back to main()!\n", testChar, testCharTwo);
    
        return 0;
    }
    
    char testVars(char *a, char *b)
    {
        b = "a variable!";
        printf("%s %s\n", a, b);
    }
    The expected result is that the program will print the following two lines:

    We just passed a variable!
    We just passed a variable! - Back to main()!

    But what I'm getting is:

    We just passed a variable!
    We just passed a main() variable! - Back to main()!

    I'm new to C and am still in the early stages of learning it, but it's my understanding that if I pass a pointer variable from main(), in my case, to testVars() that testVars() is actually working with the dereferenced location of testCharTwo (since this is the only one I wish to change in testVars()) and any modifications it makes to testCharTwo should be immediately available to main().

    Am I missing something here? Or is my understanding of this not correct?

    ~ Tom

  2. #2
    Registered User hk_mp5kpdw's Avatar
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    Within your function testVars, the variable b is local to the function and changes to it disappear once the function ends. This is the same as when passing anything to a function, if you need to have it changed by that function when it ends, you need to pass a pointer. In this case what you are passing in is already a pointer to begin with but to have that changed by the function what you need to pass is still the address of that variable (a pointer to a pointer).
    Code:
    void test(const char** testp);
    
    int main()
    {
        const char * mytestptr = "Hello World!\n";
    
        printf("%s",mytestptr);
        test(&mytestptr);  // Pass address of mytestptr, a pointer to a pointer
        printf("%s",mytestptr);
    
        return 0;
    }
    
    void test(const char** testp)
    {
        *testp = "Goodbye World\n";
    }
    Last edited by hk_mp5kpdw; 02-06-2012 at 10:21 AM.
    theitsmith likes this.
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  3. #3
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    You are working in C.
    b is a char *.
    In the function you assign to this "a variable!" this means that the address of this string (that is somewhere in the memory) is used in testVars.
    When the function returns, b points again the initial address.
    You can try with a strcpy(b,"a variable!"); this copies the content in the actual address b. But be sure that the new string is shorter than the first!

  4. #4
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    You need to use the dereference symbol at b = "a variable"

  5. #5
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    Quote Originally Posted by hk_mp5kpdw View Post
    Within your function testVars, the variable b is local to the function and changes to it disappear once the function ends. This is the same as when passing anything to a function, if you need to have it changed by that function when it ends, you need to pass a pointer. In this case what you are passing in is already a pointer to begin with but to have that changed by the function what you need to pass is still the address of that variable (a pointer to a pointer).
    Thanks, this worked!

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    
    void testVars(char *a, char** b);
    
    int main()
    {
        char * testChar = "We just passed";
        char * testCharTwo = "a main() variable!";
        testVars(testChar, &testCharTwo);
    
        printf("%s %s - Back to main()!\n", testChar, testCharTwo);
    
        return 0;
    }
    
    void testVars(char *a, char** b)
    {
        *b = "a variable!";
        printf("%s %s\n", a, *b);
    }
    Last edited by theitsmith; 02-06-2012 at 11:22 AM.

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