Question

my test code is below:

main1.c:

#include <stdio.h>
extern struct tt ;
int main()
{
    struct tt y;
    y.a=255;
    y.b=0;
    printf("tt->a=%#x   ,tt->b=%#x \n",y.a,y.b);
}

main2.c:

#include<stdio.h>

 struct tt
{
    int a;
    int b;
};

makefile:

main: main1.o
    gcc -o main main1.o
main1.o:  main2.c main1.c

but the compiler reports:

cc    -c -o main1.o main1.c
main1.c:2: warning: useless storage class specifier in empty declaration
main1.c: In function ‘main’:
main1.c:5: error: storage size of ‘y’ isn’t known
make: *** [main1.o] Error 1

how do I write code to in a .c file use the struct defined in another .c file ???

thx for your help!

Answer 1

You need to define the struct in a header file that both .c files include. For example:

#ifndef __INCLUDE_GUARD_HERE__
#define __INCLUDE_GUARD_HERE__

struct tt {
  int a;
  int b;
};

#endif

Now both .c files can #include the header file.