I have the following function:
Qt Code: 
  1. string ldecimal(double x,double toler)
  2. {
  3.   double x2;
  4.   int h,i,iexp,chexp;
  5.   size_t zpos;
  6.   char *dotpos,*epos,*pLcNumeric;
  7.   string ret,s,m,antissa,exponent,saveLcNumeric;
  8.   char buffer[32],fmt[8];
  9.   assert(toler>=0);
  10.   pLcNumeric=getenv("LC_NUMERIC");
  11.   if (pLcNumeric)
  12.     saveLcNumeric=pLcNumeric;
  13.   setenv("LC_NUMERIC","C",true);
  14.   if (toler>0 && x!=0)
  15.   {
  16.     iexp=floor(log10(fabs(x/toler))-1);
  17.     if (iexp<0)
  18.       iexp=0;
  19.   }
  20.   else
  21.     iexp=DBL_DIG-1;
  22.   h=-1;
  23.   i=iexp;
  24.   while (true)
  25.   {
  26.     sprintf(fmt,"%%.%de",i);
  27.     sprintf(buffer,fmt,x);
  28.     x2=atof(buffer);
  29.     if (h>0 && (fabs(x-x2)<=toler || i>=DBL_DIG+3))
  30.       break;
  31.     if (fabs(x-x2)>toler || i<=0)
  32.       h=1;
  33.     i+=h;
  34.   }
  35.   dotpos=strchr(buffer,'.');
  36.   epos=strchr(buffer,'e');
  37.   if (epos && !dotpos) // e.g. 2e+00 becomes 2.e+00
  38.   {
  39.     memmove(epos+1,epos,buffer+31-epos);
  40.     dotpos=epos++;
  41.     *dotpos=='.';
  42.   }
  43.   if (dotpos && epos)
  44.   {
  45.     m=string(buffer,dotpos-buffer);
  46.     antissa=string(dotpos+1,epos-dotpos-1);
  47.     exponent=string(epos+1);
  48.     if (m.length()>1)
  49.     {
  50.       s=m.substr(0,1);
  51.       m.erase(0,1);
  52.     }
  53.     iexp=atoi(exponent.c_str());
  54.     zpos=antissa.find_last_not_of('0');
  55.     antissa.erase(zpos+1);
  56.     iexp=stoi(exponent);
  57.     if (iexp<0 && iexp>-5)
  58.     {
  59.       antissa=m+antissa;
  60.       m="";
  61.       iexp++;
  62.     }
  63.     if (iexp>0)
  64.     {
  65.       chexp=iexp;
  66.       if (chexp>antissa.length())
  67. 	chexp=antissa.length();
  68.       m+=antissa.substr(0,chexp);
  69.       antissa.erase(0,chexp);
  70.       iexp-=chexp;
  71.     }
  72.     while (iexp>-5 && iexp<0 && m.length()==0)
  73.     {
  74.       antissa="0"+antissa;
  75.       iexp++;
  76.     }
  77.     while (iexp<3 && iexp>0 && antissa.length()==0)
  78.     {
  79.       m+='0';
  80.       iexp--;
  81.     }
  82.     sprintf(buffer,"%d",iexp);
  83.     exponent=buffer;
  84.     ret=s+m;
  85.     if (antissa.length())
  86.       ret+='.'+antissa;
  87.     if (iexp)
  88.       ret+='e'+exponent;
  89.   }
  90.   else
  91.     ret=buffer;
  92.   setenv("LC_NUMERIC",saveLcNumeric.c_str(),true);
  93.   return ret;
  94. }
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The function is used to format numbers when writing to files, so it has to use '.' for the decimal point regardless of locale. I added the saving and restoring LC_NUMERIC code to fix this bug. It didn't work.

I am using it in at least two programs: bezitest, which is a non-Qt test program, and viewtin, which is a Qt GUI program. Until I implement hit-testing, viewtin is displaying the coordinates of the cursor in world space. "bezitest ldecimal" outputs, among others, "3.141592653589793". viewtin displays a tooltip saying something like "3,141592653589793,2,718281828459045". I ran it in gdb and found that the comma is put there by sprintf.

locale outputs the following:
Qt Code: 
  1. LANG=en_DK.UTF-8
  2. LANGUAGE=en_US:es
  3. LC_CTYPE="en_DK.UTF-8"
  4. LC_NUMERIC="en_DK.UTF-8"
  5. LC_TIME="en_DK.UTF-8"
  6. LC_COLLATE="en_DK.UTF-8"
  7. LC_MONETARY="en_DK.UTF-8"
  8. LC_MESSAGES="en_DK.UTF-8"
  9. LC_PAPER="en_DK.UTF-8"
  10. LC_NAME="en_DK.UTF-8"
  11. LC_ADDRESS="en_DK.UTF-8"
  12. LC_TELEPHONE="en_DK.UTF-8"
  13. LC_MEASUREMENT="en_DK.UTF-8"
  14. LC_IDENTIFICATION="en_DK.UTF-8"
  15. LC_ALL=
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LC_NUMERIC is actually unset.