Warning while passing array by reference to function

This is a discussion on Warning while passing array by reference to function within the C Programming forums, part of the General Programming Boards category; Hello All, I'm a novice to C programming and I am working on a program that keeps giving me a ...

  1. #1
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    Question Warning while passing array by reference to function

    Hello All,

    I'm a novice to C programming and I am working on a program that keeps giving me a warning that I can't seem to get rid of. But rather than posting the chunks of code from the original program, I managed to create a simplified example of what I was doing that still causes the same Warning. First, here's my code:

    Code:
    #include <stdio.h>
    
    int myFunc (char* inputArray);
    
    int main()
    {
    	char myArray[4];
    
    	myFunc(&myArray);
    	return(0);
    }
    
    int myFunc (char *inputArray)
    {
    	inputArray[3] = 'A';
    	return(0);
    }
    When attempting to compile this in Visual Studio (it is saved as a .c file) I get the following two warnings:

    1>.\Code.c(9) : warning C4047: 'function' : 'char *' differs in levels of indirection from 'char (*)[4]'
    1>.\Code.c(9) : warning C4024: 'myFunc' : different types for formal and actual parameter 1

    I know the issue has something to do with passing the array by reference to the function but I'm not sure how to fix the issue. Any help would be greatly appreciated.

    God Bless,
    Jason O

  2. #2
    Technical Lead QuantumPete's Avatar
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    You don't need the ampersand (&), myArray is already a pointer.

    QuantumPete
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  3. #3
    C++まいる!Cをこわせ! Elysia's Avatar
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    I would like to point out that it's better to say that you're passing the array via pointer, not by reference (that's a C++ term).
    Also note that when passing an array, you pass a pointer to its first element, so using the ampersand (&) on it, will make it a pointer to pointer, which isn't what you want.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    instead of

    Code:
    myFunc(&myArray);
    
    
    myFunc(myArray);

    would work


    myArray ==> myArray[0]

    so first element address will be passed in above case.

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    Pointers and arrays in C are *almost* the same thing. You can even use array index notation (the []'s) on a pointer! The array name is a varable containing (pointing to) the address of the first array element in memory. So, you don't need to use the & operator:
    Code:
    myFunc(myArray);
    You could also (but i'm not sure why you would want to) do this:
    Code:
    myFunc(&myArray[0]);
    The similarity between pointers and arrays should become obvious inside myFunc(). inputArray is declared as a pointer to a character, yet you are using array notation on it here:
    Code:
    inputArray[3] = 'A';
    If you wanted to do this in "pointer notation", I think the correct code would look like:
    Code:
    *(inputArray + 3) = 'A';
    But, either method works, and the former is probably easier to read.

  6. #6
    and the hat of wrongness Salem's Avatar
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    > Also note that when passing an array, you pass a pointer to its first element, so using the ampersand (&) on it, will make it a pointer to pointer, which isn't what you want.
    No, it's type would be pointer to array.

    As in
    int myFunc ( char (*inputArray)[4] );
    would be called with
    myFunc(&myArray);

    Having
    int myFunc ( char **inputArray );
    cannot be called with either
    myFunc(myArray);
    or
    myFunc(&myArray);

    Instead, you would need
    char *temp = myArray;
    myFunc( &myArray );
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by Salem View Post
    No, it's type would be pointer to array.
    Which I classified as a pointer-to-pointer, which is close to the truth, but not 100%.
    Although I haven't really tested it myself, so I'm not 100% sure of the behavior either...
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  8. #8
    and the hat of wrongness Salem's Avatar
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    pointer to array != pointer to pointer.
    I strongly suggest you try it
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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    C++まいる!Cをこわせ! Elysia's Avatar
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    Indeed... it's a pointer with set bounds.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  10. #10
    Technical Lead QuantumPete's Avatar
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    Quote Originally Posted by Elysia View Post
    Indeed... it's a pointer with set bounds.
    Though nothing stops you from reading and writing data outside those bounds...
    "No-one else has reported this problem, you're either crazy or a liar" - Dogbert Technical Support
    "Have you tried turning it off and on again?" - The IT Crowd

  11. #11
    C++まいる!Cをこわせ! Elysia's Avatar
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    Indeed... it's like a suggestion more than set dimensions.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  12. #12
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    Smile

    Hi Everyone,

    Thank you very much for the fast replies! To solve my problem, I simply used Jason_m's suggestion of removing the ampersand from in front of the array in the function call and that fixed everything. Again thanks for all the great input!

    God Bless,
    Jason O

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