while compiling inline assembly in c using gcc with -g flag compiler is throwing "output operand constraint lacks"

calling fucntions from main global declarations int A=0, B=0, C=0;

void load(){        system("clear");        printf("Enter 2 values: ");        scanf("%d %d",&B,&C);        __asm__ __volatile__("movl %1,%%ebx;""movl  %2,%%ecx;"                :: "b" (B) , "c" (C)        );        printf("Loaded Successfully. (press any key to continue.)");        scanf("%d",&temp);}void Add(){    system("clear");    __asm__("movl %ebx,%eax;""addl %ecx,%eax;"        :"=a" (Acc)    );    printf("Answer: %d (press any key to continue)",Acc);    scanf("%d",&temp);}void Sub(){    system("clear");    __asm__("movl %ebx,%eax;""subl %ecx,%eax;"        :"=a" (Acc)    );    printf("Answer: %d (press any key to continue)",Acc);    scanf("%d",&temp);}void Mul(){    system("clear");    __asm__("movl %ebx,%eax;""imull %ecx,%eax;"        :"=a" (Acc)        );    printf("Answer: %d (press any key to continue)",Acc);    scanf("%d",&temp);}void Div(){    int rem;    system("clear");    __asm__("movl %0x0,%edx;""movl %ebx,%eax;""idivl %ecx"        :"=a" (Acc) , "=r" (rem)    );    __asm__("movl %1, %eax"        :: "a" (Acc)    );    printf("Quotient: %d Remainder= %d (press any key to continue)",Acc,rem);    scanf("%d",&temp);}

here's a code using inline assembly in c and while compiling with gcc -g it gives error :

test3.c: In function ‘main’:test3.c:22:13: error: output operand constraint lacks ‘=’             __asm__("movl %1,%ebx;""movl  %2,%ecx;" : "r" (B): "r" (C));             ^~~~~~~test3.c:22:13: error: output operand constraint lacks ‘=’test3.c:22:13: error: invalid lvalue in asm output 0

references from :

https://www.codeproject.com/articles/15971/using-inline-assembly-in-c-c

Source Codes from the site is compiling perfectly but while compiling the above code it gives errors
Also tried asmvolatilebut it again gcc is throwing :

error: bad register name `%%eax'