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

User · Answer

use the  f modifier   sprintf  str   adc read    f n   adc read     For instance    include  lt stdio h gt   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

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Many embedded systems have a limited snprintf function that doesn t handle floats   I wrote this  and it does the trick fairly efficiently   I chose to use 64-bit unsigned integers to be able to handle large floats  so feel free to reduce them down to 16-bit or whatever needs you may have with limited resources    include  lt stdio h gt       for uint64 t support    int  snprintf fp  char destination    size t available chars  int decimal digits                    char tail    float source number         int   chars used    0        This will be returned        if   available chars  gt  0                    Handle a negative sign          if   source number  lt  0                            Make it positive             source number   0 - source number              destination  0      -                 chars used                        Handle rounding         uint64 t zeros   1          for   int i   decimal digits  i  gt  0  --i               zeros    10           uint64 t source num    uint64 t     source number    float zeros     0 5f                Determine sliding divider max position          uint64 t  div amount   zeros           Give it a head start         while     div amount   10    lt   source num               div amount    10              Process the digits         while   div amount  gt  0                         uint64 t whole number   source num   div amount              if   chars used  lt   int available chars                                 destination  chars used      0     char whole number                    chars used                   if     div amount    zeros    amp  amp    zeros  gt  1                                           destination  chars used                                chars used                                              source num -    whole number   div amount                div amount    10                         Store the zero          destination  chars used     0              See if a tail was specified          size t tail len   strlen  tail             if     tail len  gt  0    amp  amp    tail len   chars used  lt  available chars                           for   size t i   0  i  lt   tail len    i                   destination  chars used   i     tail  i                chars used    tail len                       return chars used     main          define TEMP BUFFER SIZE 30     char temp buffer  TEMP BUFFER SIZE        char  degrees c        char 248   C   0        float  float temperature   26 845f       int len   snprintf fp  temp buffer  TEMP BUFFER SIZE  2  degrees c  float temperature

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Since you re on an embedded platform  it s quite possible that you don t have the full range of capabilities from the printf  -style functions   Assuming you have floats at all  still not necessarily a given for embedded stuff   you can emulate it with something like   char str 100   float adc read   678 0123   char  tmpSign    adc read  lt  0     -        float tmpVal    adc read  lt  0    -adc read   adc read   int tmpInt1   tmpVal                      Get the integer  678   float tmpFrac   tmpVal - tmpInt1          Get fraction  0 0123   int tmpInt2   trunc tmpFrac   10000       Turn into integer  123       Print as parts  note that you need 0-padding for fractional bit   sprintf  str   adc read    s d  04d n   tmpSign  tmpInt1  tmpInt2     You ll need to restrict how many characters come after the decimal based on the sizes of your integers  For example  with a 16-bit signed integer  you re limited to four digits  9 999 is the largest power-of-ten-minus-one that can be represented    However  there are ways to handle this by further processing the fractional part  shifting it by four decimal digits each time  and using subtracting the integer part  until you have the precision you desire     Update   One final point you mentioned that you were using avr-gcc in a response to one of the other answers  I found the following web page that seems to describe what you need to do to use  f in your printf   statements here   As I originally suspected  you need to do some extra legwork to get floating point support  This is because embedded stuff rarely needs floating point  at least none of the stuff I ve ever done   It involves setting extra parameters in your makefile and linking with extra libraries   However  that s likely to increase your code size quite a bit due to the need to handle general output formats  If you can restrict your float outputs to 4 decimal places or less  I d suggest turning my code into a function and just using that - it s likely to take up far less room   In case that link ever disappears  what you have to do is ensure that your gcc command has  -Wl -u vfprintf -lprintf flt -lm   This translates to    force vfprintf to be initially undefined  so that the linker has to resolve it   specify the floating point printf   library for searching  specify the math library for searching

User · Answer

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

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Similar to paxdiablo above  This code  inserted in a wider app  works fine with STM32 NUCLEO-F446RE    include  lt stdio h gt   include  lt math h gt   include  lt string h gt  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  lt  iPrecis                strcat acTemp   0            length              strcat acTemp  pcFractStr       strcpy pcFractStr  acTemp       counter buff would contain 123 056

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Isn t something like this really easier    include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   char str 10   float adc read   678 0123   dtostrf  adc read  3  4  temp    sprintf str  adc read    10s  n   temp   printf temp

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Yes  use  f

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Yes  and no  Despite what some other replies have said  the C compiler is required to perform conversions for sprintf    and all other variadic functions  as follows    char    int short    int float    double    and signed unsigned variants of the above integral types   It does this precisely because sprintf    and the other print  -family functions  would be unusable without it   Of course  they re pretty unusable as it is    But you cannot assume any other conversions  and your code will have undefined behaviour - read  crash  - if you do it

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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

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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

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Yes of course  there is nothing special with floats  You can use the format strings as you use in printf   for floats and anyother datatypes   EDIT I tried this sample code   float x   0 61  char buf 10   sprintf buf   Test   2f   x   printf buf     Output was   Test 0 61

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g can do this    include  lt stdio h gt  int main       float w   234 567    char x   SIZEOF FLOAT       sprintf x    g   w     puts x

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Look in the documentation for sprintf for your platform  Its usually  f or  e  The only place you will find a definite answer is the documentation    if its undocumented all you can do then is contact the supplier   What platform is it  Someone might already know where the docs are

[c] Using floats with sprintf() in embedded C

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