lvalue compile error

emanresu · #1 16-Nov-2006, 06:33

In MS C (ver6), I am trying to copy data between two structures, i.e. from record read in to record to be written out.

I have them declared like this

CPP / C++ / C Code:

struct GLD0DATA
   {
   char GlSt	;       /* 0    */
   char	Filler1 [20];	/* 1    */
   char GlAcCC  [2];    /* 21   */
   char GlAcCX  [1];    /* 23   */
<snip>
   char GlMatDt [6];    /* 442  */
   char GlFill5 [1416]; /* 448  */
   char GlDrac  [7];    /* 1864 */
   char GlCrac	[7];    /* 1871 */
   char GlFill6 [170];  /* 1878  fill out till end  */
   };                   /* 2048 */

struct ICRSEX1A
   {
   char ICAcCC  [2];  
   char ICAcCX  [1];  
   char ICAcMM  [2]; 
   char ICAcno  [6];
   char ICAcSS  [2];	
   char ICCrInt [9];    
   char ICDrInt [9];    
   char ICMatDt [6];    
   };      

char WKAcCC  [2];  
char WKAcCX  [1];  
char WKAcMM  [2]; 
char WKAcno  [6];
char WKAcSS  [2];	
char WKCrInt [9];    
char WKDrInt [9];    
char WKMatDt [6];    

int             IN_BUF_LEN;
int             OUT_BUF_LEN;
struct GLD0DATA REC_BUF,  *pi;    /*  initially I did not use pointers   */
struct ICRSEX1A OUT_BUF,  *po;   /*  with pointers still get same error */   
char            IN_POS_BLK[128];
char            OUT_POS_BLK[128];
char            IN_KEY_BUF[2];
char            OUT_KEY_BUF[2];
char            FIL_NAME[60]= "";
char            OUT_NAME[60]="";

main ()
{
   pi = &REC_BUF;
   po = &OUT_BUF;

First I tried directly from structure to structure, then put in intermediary work variables
(as shown)

CPP / C++ / C Code:

	    if (strncmp(str1,"     ",5) != 0)
	    {
		WKAcCC  =  pi->GlAcCC;
		WKAcCX  =  pi->GlAcCX;
		WKAcMM  =  pi->GlAcMM;
		WKAcno  =  pi->GlAcno;
		WKAcSS  =  pi->GlAcSS;
		WKCrInt =  pi->GlCrInt;
		WKDrInt =  pi->GlDrInt;
		WKMatDt =  pi->GlMatDt;


		po->ICAcCC  =  WKAcCC;
		po->ICAcCX  =  WKAcCX;
		po->ICAcMM  =  WKAcMM;
		po->ICAcno  =  WKAcno;
		po->ICAcSS  =  WKAcSS;
		po->ICCrInt =  WKCrInt;
		po->ICDrInt =  WKDrInt;
		po->ICMatDt =  WKMatDt;

but still get same error
"error C2106: '=' : left operand must be lvalue"

Any help appreciated!

Thanks,
Bob

mathematician · #2 16-Nov-2006, 06:57

The same mistake recurs throughout the program, so I will take one line as a "for example".

WKAcCC = pi->GlAcCC;

WKAcCC and pi->GIcCC are both declared as arrays. Which in C means that the variables WKAcCC and pi->GIAcCC, without any subscript, are pointers to those arrays. As it stands, if the above line was compiled, WKAcCC would end up pointing to the same array (same area in the computer's memory) as pi->GIAcCC.

There are two ways you can move data from one to the other. Firstly (since there are only two elements) you can either do:

WKAcCC[0] = (pi->GIAcCC)[0];
WKAcCC[1] = (pi->GIAcCC)[1];

or else:

memcpy( WKAcCC, pi->GIAcCC, 2 );

Either copies data from the area of memory pointed to by pi->GIAcCC to the area of memory pointed to by WKAcCC.

mathematician · #3 16-Nov-2006, 07:05

To underline the point, because they are pointers WKAcCC[1] = (pi->GIAcCC)[1]; is equivalent to *( WKAcCC + 1 ) = *( Pi->GIAcCC + 1 );

emanresu · #4 16-Nov-2006, 07:45

Many thanks for that, and for taking the trouble to explain it.

Using memcpy as per your example, I now get a clean compile and link.

Cheers,
Bob

davekw7x · #5 16-Nov-2006, 09:07

Quote:

Originally Posted by emanresu

In MS C (ver6), I am trying to copy data between two structures, i.e. from record read in to record to be written out.

You can copy structs with an assignment statement:

CPP / C++ / C Code:

#include <stdio.h>
#include <string.h>


struct St {int x;
           char y[100];
};

int main ()
{
    struct St a, b;

    a.x = 1234;
    strcpy(a.y, "Hello"); 
    printf("After first strcpy:\n");
    printf("   a.x = %d, a.y = %s\n\n", a.x, a.y);

    b = a;
    printf("After assignment:\n");
    printf("   b.x = %d, b.y = %s\n\n", b.x, b.y);

    strcpy(a.y, "Goodbye");
    printf("After second strcpy:\n");
    printf("   a.x = %d, a.y = %s\n\n", a.x, a.y);
    printf("   b.x = %d, b.y = %s\n\n", b.x, b.y);


    return 0;
}

Output:

Code:

After first strcpy:
   a.x = 1234, a.y = Hello

After assignment:
   b.x = 1234, b.y = Hello

After second strcpy:
   a.x = 1234, a.y = Goodbye

   b.x = 1234, b.y = Hello

The entire struct is copied.

You can't copy arrays with a single assignment, but you can copy structs. This has been part of the C language specification "forever", and has carried into the current standards for C and C++ languages. (And it works with compilers from Microsoft, Borland, and GNU to which I have access --- including Visual C++ version 6.)

Note that the entire struct is copied (all of its members). Note particularly, that if a struct has a pointer as one of its members, the value of the pointer is copied, not whatever it is that the pointer is pointing to. The example that I showed uses an array, not a pointer, as a member (as does your program, I think).

Regards,

Dave

emanresu · #6 16-Nov-2006, 09:53

More new info, I always learn something on this site.

Would assignment only work if the structures are identical?

davekw7x · #7 16-Nov-2006, 10:02

Quote:

Originally Posted by emanresu

I always learn something on this site

Hey! That's my line. (But I released it as "uncopylefted free" text, so no royalty is required). http://www.gnu.org/copyleft/

Quote:

Originally Posted by emanresu

Would assignment only work if the structures are identical?

Yes.

But don't take my word for it: Try it with structs declared differently but with identical members. Try it for other kinds of structs. Etc.

CPP / C++ / C Code:

struct St {int x;
           char y[100];
};

struct Ss {int x;
           char y[100];
};

.
.
.

    struct St a;
    struct Ss c;
.
.
.
    c = a;

Regards,

Dave

mathematician · #8 16-Nov-2006, 11:22

I will add a postscript of my own. Since memcpy is a generic function it has no idea of the size of the objects it is being asked to copy; therefore it copies bytes. If, for example, you wanted to copy 5 ints from one location to another, in order to ensure that the right number of bytes got copied you would need:

memcpy( intp1, intp2, sizeof( int ) * 5 );

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lvalue compile error

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