错误：在 C 中无效使用无效表达式

Question

I'm getting an "invalid use of void expression" error regarding the following function:

void remove_data(char* memory, char* bitmap, int pos)
{
    int i;

    i = (int)test_bit(bitmap, pos-1);

    if(i != 0)
    {
        memory[pos-1] = NULL;
        toggle_bit(bitmap, pos-1);
    }
}

Now from what I've read on other threads regarding this similar issue this error pops up when the programmer uses something within the function that will generate an output. However, I'm failing to see what would cause that in the code I've written.

Edit to add:

The toggle bit function used in it is also type void and doesn't return a value.

void test_bit(char * bitmap, int pos)
{
    int bit;

    bit = bitmap[pos/8] & (1<<(pos%8));
    printf("%d\n", &bit);
}

I commented the printf line in test_bit but the error is still there.

Answer 1

However, I'm failing to see what would cause that in the code I've written.

Me too, because I don't think you have posted the relevant code. Regardless, I think this is probably your problem:

i = (int)test_bit(bitmap, pos-1);

Please show us the signature for test_bit . I'm guessing it returns void or you have forward declared it somewhere and accidentally wrote:

void test_bit(char*, int);

Since the function returns void (ie, nothing) you cannot then proceed to cast the return value (which is nothing) to an int as you are doing, it makes no sense and is illegal.

EDIT: You have verified in the comments that test_bit is in fact declared to return void , so that's your problem.

Answer 2

由于 test_bit 是无效的，你不能使用它的返回值（如i = (int)test_bit(bitmap, pos-1); ）——没有返回值。

Answer 3

You wrote

 i = (int)test_bit(bitmap, pos-1);

Why have a cast?

Seems a bit wrong when the function is

void test_bit(char * bitmap, int pos)
{
    int bit;

    bit = bitmap[pos/8] & (1<<(pos%8));
    printf("%d\n", &bit);
}

You can't convert a void into an integer.

Answer 4

Obvious Solution: Don't assign results of function returning void.

Non-Obvious Solution: Don't nest function calls, and you are less likely to be looking in all of the wrong spots, thinking the compiler is broken.

#include <stdio.h> //:for: printf(...)

void YOUR_MISTAKE_ACTUALLY_HERE();
void  MyFunc( void );
void* WRAPPED_MyFunc( int, int, int );


int main( void ){
    printf("[BEG:main]\n");


    void* HW = NULL;
    HW = WRAPPED_MyFunc(  //:<---So you check this, but it looks fine.
        1
    ,   YOUR_MISTAKE_ACTUALLY_HERE()
    ,   3
    );; //:<--- compiler says "Invalid Use Of Void Expression" Here.

    if( HW ){ /** NOOP **/ };


    printf("[END:main]\n");
} 

    void YOUR_MISTAKE_ACTUALLY_HERE(){ }

    //:Pretend this function is fetched from DLL.
    //:Casting all DLL functions initially to void(*pfn)void
    //:in a lookup table, then re-casting later.
    void MyFunc( void ){

        return;

    }//[;]//

    typedef  
        void* 
        (* MyFunc_TYPEDEF)(            

            int a
        ,   int b
        ,   int c

        );

    void*
    WRAPPED_MyFunc(

        int a
    ,   int b
    ,   int c

    ){


               MyFunc_TYPEDEF
        fun =( MyFunc_TYPEDEF)
               MyFunc;

        void* ret=( fun(

            a
        ,   b
        ,   c

        ));

        return( 
            (void*)
            ret 
        );;

    }//:Func