StringAssign of Zero-Length (Null) String Fails; Basic 7.10 (68225)

   StringAssign(NULL,0,basicstring,0)
   // (The NULL pointer is defined in STDIO.H in Microsoft C.)
				

MORE INFORMATION

The StringAssign routine lets you transfer far or near string data from one language's memory space to another, using the following C syntax:

   StringAssign(sourceaddress&,sourcelength%,destaddress&,destlength%)
				

StringAssign expects to receive a far pointer (sourceaddress&) either to a valid Basic string descriptor or to the beginning of some string text. If the length of the source string (sourcelength%) is 0 (zero), StringAssign assumes that sourceaddress& is a pointer to a valid Basic string descriptor. StringAssign then dereferences the descriptor and assigns the value to the destaddress& supplied.

Each programmer is responsible for making sure that the pointer passed to StringAssign is valid. StringAssign doesn't validate the pointers. Thus, to pass the NULL pointer is an error. If the address passed does not point to a string descriptor, then the results will be unpredictable.

NOTE: Someone might want to do this to set a Basic variable-length string to the empty or null string from some other language. The intention is to simulate the following line of Basic code:

    A$ = ""
				

To assign a null string, you can use the StringRelease routine. StringRelease isn't exactly the same as A$="", but it does result in a string descriptor that will be treated as a null string. It deletes the string data associated with the string descriptor and zeros the string descriptor. A string descriptor that contains 0 (zero) as the length and 0 (zero) as the offset is the same as an uninitialized string, which is treated as null. This method should work around the limitation of StringAssign described above.

Code Example

The following is an example of this workaround in Microsoft C and Basic. To compile and link the program, you can use the following compile and link lines:

   bc test1.bas /Fs /o /Zi ;
   cl /c /AL /Zi test2.c ;
   link /NOE /NOD /CO test1 test2,,,bcl71efr.LIB llibcer.lib;
				

Note that the following program prompts you for input. If you input a string, then the C function will StringAssign the string. However, if you literally type null, then the function will use StringRelease to null the string.

Test1.BAS -- Basic Routine

DECLARE SUB cfunc CDECL (a$)
CLS
locate 10, 1
print "Before call to cfunc"
a$ = "This is a test"
print "A$ = ", a$
PRINT "Len A$: "; LEN(a$)
print
CALL cfunc(a$)    ' CALL the C function
print "After call to cfunc"
print "A$ = ", a$
PRINT "Len A$: "; LEN(a$)
print
				

Test2.C -- C Routine

#include <stdio.h>
#include <stdlib.h>
extern void far pascal StringAssign(char far *,int,char far *, int);
extern void far pascal StringRelease(char far *);
void cfunc(BasicString)
     char far * BasicString;
{
    char name[80] ;
    char NullStr[] = "";
    printf("Input name[]: ");
    scanf("%s",name);
    if(stricmp(name,"null") == 0) {
      name[0] = '\x00'; // Set it to null
    }
    if ((name == NULL) | (name[0] == '\x00')) {
       StringRelease(BasicString);
    } else {
        StringAssign(name, strlen(name), BasicString,0);
    }
}