Trouble with Unions in C program

Question

I am working on a C program that uses a Union. The union definition is in FILE_A header file and looks like this...

// FILE_A.h****************************************************
xdata union  
{
long position;
char bytes[4];
}CurrentPosition;

If I set the value of CurrentPosition.position in FILE_A.c and then call a function in FILE_B.c that uses the union, the data in the union is back to Zero. This is demonstrated below.

// FILE_A.c****************************************************
int main.c(void)
{
    CurrentPosition.position = 12345;
    SomeFunctionInFileB();
}

// FILE_B.c****************************************************
void SomeFunctionInFileB(void)
{
    // After the following lines execute I see all zeros in the flash memory.
    WriteByteToFlash(CurrentPosition.bytes[0];
    WriteByteToFlash(CurrentPosition.bytes[1];
    WriteByteToFlash(CurrentPosition.bytes[2];
    WriteByteToFlash(CurrentPosition.bytes[3];
}

Now, If I pass a long to SomeFunctionInFileB(long temp) and then store it into CurrentPosition.bytes within that function, and finally call WriteBytesToFlash(CurrentPosition.bytes[n]... it works just fine.

It appears as though the CurrentPosition Union is not global. So I tried changing the union definition in the header file to include the extern keyword like this...

extern xdata union  
{
long position;
char bytes[4];
}CurrentPosition;

and then putting this in the source (.c) file...

xdata union  
{
    long position;
    char bytes[4];
}CurrentPosition;

but this causes a compile error that says:

C:\SiLabs\Optec Programs\AgosRot\MotionControl.c:76: error 91: extern definition for 'CurrentPosition' mismatches with declaration. C:\SiLabs\Optec Programs\AgosRot\/MotionControl.h:48: error 177: previously defined here

So what am I doing wrong? How do I make the union global?

Answer 1

Is FILE_A.h really MotionControl.h? If so I think the fix is to define a union type in the header:

typedef
union xdata
{
    long position;
    char bytes[4];
} xdata;

And declare a global variable of that type elsewhere in a header file (maybe the same one):

extern xdata CurrentPosition;   // in a header file

Finally define the global variable in a C file exactly once. Maybe in file_a.c:

xdata CurrentPosition;

Of course a better fix might be to pass the xdata variable you want to write out to flash to SomeFunctionInFileB() so you don't have to depend on a global variable, which are well known to be problematic when not very, very carefully used. And there seems to be no good reason to not pass the data as a parameter:

// in a header file
void SomeFunctionInFileB( xdata const* pPosition);


void SomeFunctionInFileB( xdata const* pPosition)
{
    // After the following lines execute I see all zeros in the flash memory.
    WriteByteToFlash(pPosition->bytes[0];
    WriteByteToFlash(pPosition->bytes[1];
    WriteByteToFlash(pPosition->bytes[2];
    WriteByteToFlash(pPosition->bytes[3];
}

And call it like so:

int main.c(void)
{
    CurrentPosition.position = 12345;
    SomeFunctionInFileB( &CurrentPosition);
}

Answer 2

Ideally you need a typedef for the union and an extern declaration in FILE_A.h and the actual definition of the union in FILE_A.c.

-

// FILE_A.h

typedef union  
{
    long position;
    char bytes[4];
} Position;

extern Position CurrentPosition; // declaration

-

// FILE_A.c

#include "FILE_A.h"

Position CurrentPosition; // definition

int main(void)
{
    CurrentPosition.position = 12345;
    SomeFunctionInFileB();
    return 0;
}

-

// FILE_B.c

#include "FILE_A.h"

void SomeFunctionInFileB(void)
{
    // now there will be valid data in the flash memory.
    WriteByteToFlash(cp.bytes[0];
    WriteByteToFlash(cp.bytes[1];
    WriteByteToFlash(cp.bytes[2];
    WriteByteToFlash(cp.bytes[3];
}

-

Answer 3

You haven't instantiated the union.
You need :

// FILE_A.c****************************************************

#include "File_a.h"
CurrentPosition cp;
int main(void)
{
    cp.position = 12345;
    SomeFunctionInFileB();
}

// FILE_B.c****************************************************
#include "File_a.h"
extern CurrentPosition cp;
void SomeFunctionInFileB(void)
{
    // now there will be valid data in the flash memory.
    WriteByteToFlash(cp.bytes[0];
    WriteByteToFlash(cp.bytes[1];
    WriteByteToFlash(cp.bytes[2];
    WriteByteToFlash(cp.bytes[3];
}

Answer 4

If sizeof(long) is not 4, then endianess comes into play...

consider

union{
   long position
   char bytes[sizeof long];
}