在C中将char转换为int

Question

Just trying to print the square of a number using macros. Self explanatory C program :

#include <stdio.h>
#define  numb(a)  \
   printf(#a * #a)

int main(void) {
   int num;
   printf("Enter a num: ");
   scanf("%d",&num);
   numb(num);
   return 0;
}

Getting the error :

main.c: In function ‘main’:
main.c:4:14: error: invalid operands to binary * (have ‘char *’ and ‘char *’)
    printf(#a * #a)
              ^
main.c:10:14:
    square(num);
              ~
main.c:10:4: note: in expansion of macro ‘square’
    square(num);
    ^~~~~~

How can I convert that char to int to sucessfully use the binary operator?

Answer 1

Your macro needs to be c code so just ditch the # and write the formatting string for an integer

#include <stdio.h>
#define  square(a)  \
   printf("%d", (a) * (a))

int main(void) {
   int num;
   printf("Enter a num: ");
   scanf("%d",&num);
   square(num);
   return 0;
}

Answer 2

通过使用“ cc -E main.c -o main.i”检查预处理器阶段的输出，并检查预处理器如何将宏转换为指令。