printf does not print float value correctly

lpcware — Wed, 15 Jun 2016 16:56:37 GMT

Content originally posted in LPCWare by jimmcwin on Tue Jun 26 10:08:02 MST 2012
I am trying to find out why printf is having problem printing floating data. Is this because of M4 FPU ?

Jim

Re: printf does not print float value correctly

lpcware — Wed, 15 Jun 2016 16:56:37 GMT

Content originally posted in LPCWare by bavarian on Wed Jun 27 00:33:21 MST 2012
Hello Jim,

not that much information about your problem ;-)

One thing: the behavior of printf is not related to the existence of an FPU. It is related to the library you use (KEIL MicroLib, standard libs, NewLib from GCC etc).

At least in the NewLib (so if you use the free LPCXpresso tools) I remember a problem. Please look at the code snippet below for a workaround, plus an alternative. The alternative saves quite some memory, if you use %f then a lot of library stuff gets linked in, maybe just for one printf instruction.

Example:
========

#ifdef EXTMEMORY      // Functions of the newlib require more memory, so use them if we have external SRAM
char latbuf[128];     // String buffer for latitude value of a GPS coordinate
char longbuf[128];    // String buffer for longitude value of a GPS coordinate
#else                 // Substitute the newlib functions with a more simple implementation
double f1, f2;
unsigned short d1, d2, n1, n2;
#endif

#ifdef EXTMEMORY
// Workaround for a limitation of sprintf in the newlib with %f:
//   Convert the float into a string using char *gcvt(double val, int precision, char *buf)
//   Note: This requires some amount of memory
gcvt(Nav_Data.Latitude, 6, latbuf);     // Nav_Data.Latitude is a floating-point value
gcvt(Nav_Data.Longitude, 6, longbuf);   // Nav_Data.Longitude is a floating-point value
sprintf(buf, "3D fix: %sN %sE\r\n", latbuf, longbuf);
xDebugPutString( buf, strlen(buf) );
#else // workaround for the memory consuming library version
d1 = Nav_Data.Latitude;    // Get the integer part
d2 = Nav_Data.Longitude;   // Get the integer part

f1 = Nav_Data.Latitude - d1;     // Get the fractional part
f2 = Nav_Data.Longitude - d2;    // Get the fractional part

n1 = (u16)(f1 * 10000);    // Turn into integer with 5 digits
n2 = (u16)(f2 * 10000);    // Turn into integer with 5 digits

// Print as parts, note that you need 0-padding for fractional bit.
// Since d1 is 678 and d2 is 123, you get "678.0123".
sprintf (buf, "3D fix: %d.%04dN %d.%04dE\r\n", d1, n1, d2, n2);
xDebugPutString( buf, strlen(buf) );
#endif

Best regards.

Re: printf does not print float value correctly

lpcware — Wed, 15 Jun 2016 16:56:38 GMT

Content originally posted in LPCWare by jimmcwin on Fri Jun 29 16:35:41 MST 2012
Thanks. I found the problem actually very simple: The current stack of any function or context within a printf call needed to be aligned by 8.

Jim

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printf does not print float value correctly

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