error: incompatible types in assignment

Discussion in 'C Programming' started by Richard Bos, Jul 25, 2006.

  1. Richard Bos

    Richard Bos Guest

    That's not a pointer.

    No, it's not even a "constant pointer".

    RTFFAQFFS: <http://c-faq.com/aryptr/index.html>.

    Richard
     
    Richard Bos, Jul 25, 2006
    #1
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  2. Richard Bos

    fei.liu Guest

    I have the following source code. It seems wierd to me why gca's value
    cannot be reassigned. It's afterall a pointer and has a pointer value.
    I am aware that the standard says it's not allowed. But I cannot
    convince myself why this is prohibited.

    static int ia[4] = {64, 64, 64, 64};
    static char ca[4] = {'a', 'b', 'c', 'd'};
    static int iaa[] = {64, 64, 64, 64};
    static char caa[] = {'a', 'b', 'c', 'd'};
    static char cab[] = "abcd";
    static char * cac = "abcd";

    char gca[7];
    char gcaa[] = "hello!";

    int x[4];
    int * px;
    int main(void){
    char sgca[7];
    char sgcaa[] = "hello!";
    //cab = "xyz";
    cab[0] = 'x';
    cac = "xyz";

    gca = gcaa;
    //sgca = sgcaa;
    // gcaa = gca;

    px = &main;
    }


    gcc -o a ary.c
    ary.c: In function `main':
    ary.c:20: error: incompatible types in assignment
    ary.c:24: warning: assignment from incompatible pointer type
     
    fei.liu, Jul 25, 2006
    #2
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  3. Richard Bos

    Eric Sosman Guest

    An array is not a pointer. Please see Section 6 of
    the comp.lang.c Frequently Asked Questions (FAQ) list

    http://www.c-faq.com/

    .... with special attention to Questions 6.5 and 6.7
    through 6.10.
     
    Eric Sosman, Jul 25, 2006
    #3
  4. Richard Bos

    fei.liu Guest

    Arrays are ``second-class citizens'' in C; one upshot of this prejudice
    is that you cannot assign to them.

    Well, I guess it's just reiterating my confusion. Why should arrays be
    second class citizens? gca does have a memory location. The value of
    gca has to be stored in memory so gca[0] etc can be accessed.
     
    fei.liu, Jul 25, 2006
    #4
  5. Richard Bos

    Simon Biber Guest

    gca is not a pointer. It is an array. An array is an area of memory. The
    area of memory stores 7 char objects. No pointer to char is stored in
    memory associated with gca.

    In most cases when you use gca as an identifier in your program, the
    compiler substitutes the name of the array for a pointer to the first
    element. It doesn't generate code to read that pointer out of memory. It
    simply writes the pointer value into the generated code, since it knows
    where the array will be placed in memory.

    The C standard says that the name of an array is an lvalue, but not a
    modifiable lvalue. That is, you are not allowed to modify gca. You
    cannot use it on the left hand side of an assignment operator.
    px is a pointer to int.
    main is a function (void) returning int.
    &main is a pointer to function (void) returning int.
    &main and px have incompatible types.
    The assignment is a constraint violation.
     
    Simon Biber, Jul 25, 2006
    #5
  6. Richard Bos

    Simon Biber Guest

    Arrays are second class citizens because that's the way they are defined.

    gca does have a memory location, in the same way that an int variable
    has a memory location. The int variable's memory location is known to
    the compiler, but it is not stored in memory.

    You are attempting to do the equivalent of this:

    int foo;
    int bar;

    &foo = &bar; /* constraint violation */

    Why can't you assign to the address of foo? Foo has a (fixed) memory
    location, but &foo does not have its own memory location. It's just a
    value that the compiler calculated for you.

    Similarly, the array gca has a memory location but its location is not
    stored in memory; it is fixed. You cannot change its address.
     
    Simon Biber, Jul 25, 2006
    #6
  7. Richard Bos

    fei.liu Guest

    Thanks, you've made it very clear now. The key is the value of gca as a
    pointer is a compiler calculated value that has no memory storage.
    That's why it cannot be reassigned.

    Fei
     
    fei.liu, Jul 25, 2006
    #7
  8. Right.

    Just to pound on the point a bit harder, gca is an array object, not a
    pointer. Arrays are not pointers, and pointers are not arrays.

    What causes the confusion is that, in most contexts, an expression of
    array type is implicitly converted to a pointer to its first element.
    The resulting pointer is a value, not an object. It might be clearer
    to say that the array expression is converted to *the address of* its
    first element. (This conversion does not occur if the array
    expression is the operand of a unary "&" or "sizeof" operator, or if
    it's a string literal used in an initializer for an array.)

    And, as you know by now, this is well explained in section 6 of the
    FAQ, <http://www.c-faq.com/>.
     
    Keith Thompson, Jul 25, 2006
    #8
  9. NO. Pointers are not arrays, and arrays are not pointers.
    Arrays are blocks of memory.
    Pointers are addresses of memory.

    A house is not an address, or vice versa.

    --
    Mark McIntyre

    "Debugging is twice as hard as writing the code in the first place.
    Therefore, if you write the code as cleverly as possible, you are,
    by definition, not smart enough to debug it."
    --Brian Kernighan
     
    Mark McIntyre, Jul 25, 2006
    #9
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