Using floats with sprintf() in embedded C

Question

Guys, I want to know if float variables can be used in sprintf() function.

Like, if we write:

sprintf(str,"adc_read = %d \n",adc_read);

where adc_read is an integer variable, it will store the string

"adc_read = 1023 \n"

in str (assuming that adc_read = 1023)

How can I use a float variable in place of integer?

Answer 1

Don't expect sprintf (or any other function with varargs) to automatically cast anything. The compiler doesn't try to read the format string and do the cast for you; at runtime, sprintf has no meta-information available to determine what is on the stack; it just pops bytes and interprets them as given by the format string. sprintf(myvar, "%0", 0); immediately segfaults.

So: The format strings and the other arguments must match!

Answer 2

Yes, use %f

Answer 3

Don't do this; integers in C/C++ are always rounded down so there is no need to use the floor function.

char str[100]; 
int d1 = value;

Better to use

int d1 = (int)(floor(value));

Then you won't get rounding up of the integer part (68.9999999999999999 becomes 69.00..). 68.09999847 instead of 68.1 is difficult to avoid - any floating point format has limited precision.

Answer 4

use the %f modifier:

sprintf (str, "adc_read = %f\n", adc_read);

For instance:

#include <stdio.h>

int main (void) 
{
    float x = 2.5;
    char y[200];

    sprintf(y, "x = %f\n", x);
    printf(y);
    return 0;
}

Yields this:

x = 2.500000

Answer 5

Yes you can. However, it depends on the C-library that you are linking against and you need to be aware of the consequences.

Since you are programming for embedded applications, realise that floating-point support is emulated for a lot of embedded architectures. Compiling in this floating-point support will end up increasing the size of your executable significantly.

Answer 6

Similar to paxdiablo above. This code, inserted in a wider app, works fine with STM32 NUCLEO-F446RE.

#include <stdio.h>
#include <math.h>
#include <string.h>
void IntegFract(char *pcIntegStr, char *pcFractStr, double dbValue, int iPrecis);

main()
{
   char acIntegStr[9], acFractStr[9], char counter_buff[30];
   double seconds_passed = 123.0567;
   IntegFract(acIntegStr, acFractStr, seconds_passed, 3);
   sprintf(counter_buff, "Time: %s.%s Sec", acIntegStr, acFractStr);
}

void IntegFract(char *pcIntegStr, char *pcFractStr, double dbValue, int 
iPrecis)
{
    int iIntegValue = dbValue;
    int iFractValue = (dbValue - iIntegValue) * pow(10, iPrecis);
    itoa(iIntegValue, pcIntegStr, 10);
    itoa(iFractValue, pcFractStr, 10);
    size_t length = strlen(pcFractStr);
    char acTemp[9] = "";
    while (length < iPrecis)
    {
        strcat(acTemp, "0");
        length++;
    }
    strcat(acTemp, pcFractStr);
    strcpy(pcFractStr, acTemp);
}

counter_buff would contain 123.056 .