Thread: get “error :lvalue required as left operand of assignment ” when set value for regist

  1. #1
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    get “error :lvalue required as left operand of assignment ” when set value for regist

    I can not solve this problem. I don't understand why. Here is my simple code


    Code:
    mjpeg.h:
    
        #define START_REG                 (*(volatile unsigned int*) BASE+0x000)  
        #define MJPEG_PIC_START_RD                                  (0x00000001)


    Code:
    mjpeg.c:
        void encStreamInit(void){
        START_REG |= MJPEG_PIC_START_RD;
        }


    Code:
    test.c:
        int main(void){
         encStreamInit();
         return 0;
        }

    It get "error: lvalue required as left operand of assignment" when compiling. Please help me. Thank for your help.

  2. #2
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    From what I can see it looks as if an lvalue is required as the left operand of your assignment .

    START_REG is a macro definition, not a variable. You cannot assign to a macro as it is literally replaced with the expression that you define it as before compilation begins. If BASE is a variable (hopefully it's not because upper case variable names aren't particularly good practice) then it is still wrong because you're dereferencing a variable and then adding something to it which makes it an expression. You can't assign to expressions.

    I'm guessing BASE is probably a defined address and maybe this is like embedded programming or something. I'm fairly sure the same reasoning as above applies. Remember that dereference operator has higher precedence than addition.

  3. #3
    and the Hat of Guessing tabstop's Avatar
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    "*(volatile unsigned int*) BASE+0x000" is not a thing that can be assigned to, since it is an expression. Did you mean to cast "BASE+0x000" to a pointer, then dereference it? If so, your parentheses start in the wrong place for that -- as it is now, BASE is being dereferenced, and then 0x000 is being added to that dereferenced value.

  4. #4
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    Thank you all, I solved the problem.

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