how to use the struct defined in a .c file in another file

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!

struct should be defined in .h file

onemach
– onemach

2012-03-27 02:06:43 +00:00
Commented Mar 27, 2012 at 2:06 — onemach
– onemach, Commented Mar 27, 2012 at 2:06

chrisaycock · Accepted Answer · 2012-03-27 02:05:57Z

5

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.

answered Mar 27, 2012 at 2:05

chrisaycock

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1 Comment

Stefan Falk Over a year ago

Is there a way to not to have the definition inside the header file? Let's say the design enforces me to define the struct inside a .c file? Could I access this struct from another .c file somehow? See: stackoverflow.com/questions/23520877/…

Howard · Accepted Answer · 2012-03-27 03:20:07Z

2

the error message is very clear. ( error: storage size of 'y' isn't known)

you declare a variable as the name 'yy'

struct tt yy;

but you use another variable with the name 'y' (it doesn't even exist in this scope)

y.a=255;

see? yy != y

answered Mar 27, 2012 at 3:20

Howard

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1 Comment

city Over a year ago

It is my mistake, but it is not the problem.

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