在 ARM Cortex M0 uart 控制台使用 printf() 打印浮点数

Question

I configured Atmel's ARM Cortex M0's UART for printing strings and integers at the console using std C function printf() in Atmel studio 7.

Case I

I am trying to make printf() type of function print floating point values and for that I followed following suggestions to do so:

arm-none-eabi-gcc : Printing float number using printf

and later on I edited/added the linker flags following texts separately at different time :

-lc -u _printf_float
-lc -lrdimon -u _printf_float 

Case II

Though I could not understand everything they said but I followed some of the suggestions to edit makefile from this forum too.

Printf/Sprintf for float point variables not working

and added following text inside the makefile

ldflags-gnu-y += -lc -u _printf_float 

Makefile Path (Atmel Studio 7, using ASF) : ../src/ASF/sam0/utils/make/Makefile.sam.in

Now in the main.c I used printf() for printing floating point number as :

float a = 345.65412;
char buffr[20];         
/* --- Print Float using printf only --- */
printf("Float Number 1 : %f\r\n", a);   
/* --- Print Float using sprintf ---*/
sprintf(buffr, "Float Number ( Using Sprintf) : %3.3f\r\n", a); 
printf(buffr);

Output on a UART console app.:

Case I :

Float Number 1 : 2.000000
Float Number ( Using Sprintf) : -0.000

Case II :

Float Number 1 : 
Float Number ( Using Sprintf) :

Does anyone know configuring linker to make printf(), sprintf() or vprintf() work for printing floating point number on the console for ARM Cortex M0 (SAM B 11) in Atmel Studio 7 ?

Answer 1

You buffer is too small for your data

char buffr[20];

have to be at least (considering 0.0 as format output)

char buffr[38];

The small buffer causes a "write out of bounds" of the array. It means you have a Undefined Behavior and a variable is polluted.

sprintf is different than printf . It copy all chars to the output buffer and you must grant space for text and for all chars that will be added by format specifiers.

In your specific case:

• Float Number ( Using Sprintf) : --> takes 32 chars
• %3.3f --> takes 7 chars due to the real output of 345.654
• \\r\\n --> takes 2 chars
• a final byte is needed for the null terminator '\\0'

That menas your buffer have to be:

char buffr[42];