Question

I have an odd problem. When I try to compile the code below, it works without a failure as expected:

#include <iostream>
#include <Windows.h>

int main(){
       FILE *f = fopen("trystl.geo","w");
       fprintf(f,"Merge \"trystl.stl\";");
       fprintf(f,"\n");
       fprintf(f,"Surface Loop(2) = {1};");
       fprintf(f,"\n");
       fprintf(f,"Volume(3) = {2};");
       fclose(f);
       return 0;
}

But when I try to connect this program to a button with FLTK user interface, it gives me an assertion runtime error. The segment of my code:

void UserInterface::cb_m_BtnSTLToGEOConverter_i(Fl_Button*, void*){
    //OnSTLToGEOConvert();
       FILE *f = fopen("trystl.geo","w");
       fprintf(f,"Merge \"trystl.stl\";");
       fprintf(f,"\n");
       fprintf(f,"Surface Loop(2) = {1};");
       fprintf(f,"\n");
       fprintf(f,"Volume(3) = {2};");
       fclose(f);
}
void UserInterface::cb_m_BtnSTLToGEOConverter(Fl_Button* o, void* v){
    ((UserInterface*)(o->parent()->parent()->parent()->parent()->parent()->parent()->parent()->user_data()))->cb_m_BtnSTLToGEOConverter_i(o,v);   
}

When the user presses the button, I want the program to create a file called trystl.geo and perform the operations shown. But when compile and open the program and click the button, it says:

Debug Assertion Failed!

Program: *…….\src\fprintf.c Line 55:

Expression: (str! NULL)

abort retry or ignore…

I’m using Visual Studio 2010.

Answer 1

The error is simple: Line 55 in fprintf.c in VC++ is _VALIDATE_RETURN( (str != NULL), EINVAL, -1); and str is the FILE* parameter (I’ve seen better named variables though).

For the curious (I was) _VALIDATE_RETURN is defined as follows:

#define _VALIDATE_RETURN( expr, errorcode, retexpr )                           \
    {                                                                          \
        int _Expr_val=!!(expr);                                                \
        _ASSERT_EXPR( ( _Expr_val ), _CRT_WIDE(#expr) );                       \
        if ( !( _Expr_val ) )                                                  \
        {                                                                      \
            errno = errorcode;                                                 \
            _INVALID_PARAMETER(_CRT_WIDE(#expr) );                             \
            return ( retexpr );                                                \
        }                                                                      \
    }

So better check if your fopen() call succeeds before trying to write to a nonexistant filedescriptor.