I just tested a toy Excel add-in project, cross building the XLL with mingw32 tool chains.
Here is my code:
//testXLL.c
#include "windows.h"
#include "xlcall.h"
#define MEMORYSIZE 65535000
char vMemBlock[MEMORYSIZE];
int vOffsetMemBlock =0;
LPSTR GetTempMemory(int cBytes){
LPSTR lpMemory;
if(vOffsetMemBlock + cBytes > MEMORYSIZE)
return 0;
else{
lpMemory = (LPSTR) &vMemBlock + vOffsetMemBlock;
vOffsetMemBlock += cBytes;
if(vOffsetMemBlock & 1) vOffsetMemBlock++;
return lpMemory;
}
}
LPXLOPER TempStr(LPSTR lpstr){
LPXLOPER lpx;
int chars;
lpx = (LPXLOPER)GetTempMemory(sizeof(XLOPER));
if(!lpx) return 0;
chars = lstrlen(lpstr);
if(chars>255) chars=255;
lpx->val.str=(char*)GetTempMemory((sizeof(char)*chars+1));
if(!lpx->val.str) return 0;
strncpy(lpx->val.str, lpstr,chars);
lpx->val.str[0]=(BYTE) chars;
//lpx->val.str[chars]='\0';
lpx->xltype = xltypeStr;
return lpx;
}
#ifdef __cplusplus
extern "C" {
#endif
__declspec(dllexport) double __stdcall myadd2(double a1,double a2){
return a1+a2;
}
static char functionTable[11][255] =
{" myadd2", // procedure
" BBB", // type_text
" add", // function_text
" add1,add2", // argument_text
" 1", // macro_type
" category", // category
" ", // shortcut_text
" some help topic", // help_topic
" Adds toy", // function_help
" 1st.", // argument_help1
" 2nd" // argument_help2
};
__declspec(dllexport) int __stdcall xlAutoOpen(){
LPXLOPER pxDLL;
Excel4(xlGetName,pxDLL,0);
XLOPER xlRegArgs[11];
for(int i = 0; i < 11; i++){
xlRegArgs[i] = *TempStr(functionTable[i]);
}
Excel4(xlfRegister, 0, 12,
pxDLL,
&xlRegArgs[0], &xlRegArgs[1], &xlRegArgs[2],
&xlRegArgs[3], &xlRegArgs[4], &xlRegArgs[5],
&xlRegArgs[6], &xlRegArgs[7], &xlRegArgs[8],
&xlRegArgs[9], &xlRegArgs[10]);
return 1;
}
__declspec(dllexport) LPXLOPER __stdcall xlAddInManagerInfo(LPXLOPER xlAction) {
static XLOPER xlReturn, xlLongName, xlTemp;
xlTemp.xltype = xltypeInt;
xlTemp.val.w = xltypeInt;
Excel4(xlCoerce, &xlReturn, 2, xlAction, &xlTemp);
if(1 == xlReturn.val.w) {
xlLongName = *TempStr(" xll-name");
} else {
xlLongName.xltype = xltypeErr;
xlLongName.val.err = xlerrValue;
}
return &xlLongName;
}
#ifdef __cplusplus
}
#endif
I built this testXLL.c file in Ubuntu:
>i686-w64-mingw32-gcc -shared -Wl,--kill-at testXLL.c -o win.xll -L. -lxlcall32
This generates the "win.xll" successfully but, when loading this win.xll, Excel crashes.
In Windows 10, I tried to use gdb to debug it, but I can't catch break point in the xll file – it got disabled automatically when loading. But I can see in the gdb output, it is a segmentation fault when Excel crashes.
XLOPER xlRegArgs[11];
for(int i = 0; i < 11; i++){
xlRegArgs[i] = *TempStr(functionTable[i]);
}
What's weird is that, if I substitute the above for loop with the following line-by-line assignments in the xlAutoOpen function, the compiled XLL file works fine in Excel:
XLOPER xlRegArgs[11];
xlRegArgs[0] = *TempStr(functionTable[0]);
xlRegArgs[1] = *TempStr(functionTable[1]);
xlRegArgs[2] = *TempStr(functionTable[2]);
xlRegArgs[3] = *TempStr(functionTable[3]);
xlRegArgs[4] = *TempStr(functionTable[4]);
xlRegArgs[5] = *TempStr(functionTable[5]);
xlRegArgs[6] = *TempStr(functionTable[6]);
xlRegArgs[7] = *TempStr(functionTable[7]);
xlRegArgs[8] = *TempStr(functionTable[8]);
xlRegArgs[9] = *TempStr(functionTable[9]);
xlRegArgs[10] = *TempStr(functionTable[10]);
Please enlighten me. What's the difference between these two assignment approaches?
Although I don't (yet) have a full explanation for this behaviour, I'm posting this as a possible 'workaround', which I have used in a very similar case I encountered in one of my projects.
The issue appears to be some form of 'stack corruption' caused by the use of the function-local variable (i) used as the loop index; converting this to a global/static variable will likely fix the issue. The following code snippet is a suggested fix (I have changed the name of the index variable to avoid possible name clashes elsewhere in the code):
///...
static int regloop; // Used as the loop index, below...
__declspec(dllexport) int __stdcall xlAutoOpen(){
LPXLOPER pxDLL;
Excel4(xlGetName,pxDLL,0);
XLOPER xlRegArgs[11];
for(regloop = 0; regloop < 11; regloop++){
xlRegArgs[regloop] = *TempStr(functionTable[regloop]);
}
Here's the section of code from my aforementioned project (but note this is C++/MFC) that exhibits the same sort of behaviour – but only in x86 builds (x64 builds work without issue):
static int plin; // NOTA BENE:- We use this in the two functions below, as the use of
// a local 'plin' loop index is prone to induce stack corruption (?),
// especially in MSVC 2017 (MFC 14) builds for x86.
void BasicApp::OnUpdatePICmd(uint32_t nID, void *pUI)
{
//! for (int plin = 0; plin < Plugin_Number; ++plin) { // Can cause problems - vide supra
for (plin = 0; plin < Plugin_Number; ++plin) {
BOOL mEbl = FALSE; int mChk = -1;
if ((Plugin_UDCfnc[plin] != nullptr) && Plugin_UDCfnc[plin](nID, &mEbl, &mChk)) {
CommandEnable(pUI, mEbl ? true : false);
if (mChk >= 0) CmdUISetCheck(pUI, mChk);
return;
}
}
CommandEnable(pUI, false);
return;
}
(The Plugin_UDCfnc is a static array member of the BasicApp class.)
I have, in the years since the above code was written, had occasional 'fleeting insights' into why this is happening but, as of now, I can't offer a more robust fix. I shall revisit the issue and update this post if I should stumble upon a resolution. In the meantime, others are welcome to take this as a 'clue' and post their own explanations/solutions.