Using floats with sprintf() in embedded C

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 < 0) ? "-" : "";
float tmpVal = (adc_read < 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.

Related Posts:

  1. Correct format specifier for double in printf
  2. Printf width specifier to maintain precision of floating-point value
  3. Floating point exception (core dumped)
  4. What is the difference between C and embedded C?
  5. Strings and character with printf
  6. printf() formatting for hexadecimal
  7. What is the argument for printf that formats a long?
  8. ARM Assembler – How do I use CMP, BLT and BGT?
  9. Is there a printf converter to print in binary format?
  10. What does “%.*s” mean in printf?
  11. How to print a char array in C through printf?
  12. How can one print a size_t variable portably using the printf family?
  13. error: aggregate value used where an integer was expected
  14. What does this GCC error “… relocation truncated to fit…” mean?
  15. Which of sprintf/snprintf is more secure?
  16. Does C have a string type?
  17. Execution of printf() and Segmentation Fault
  18. What are .axf files?
  19. printf format specifiers for uint32_t and size_t
  20. How to convert string to float?
  21. What primitive data type is time_t? [duplicate]
  22. warning: implicit declaration of function
  23. pthread_join() and pthread_exit()
  24. What is the difference between float and double?
  25. Why should we typedef a struct so often in C?
  26. What is the difference between ++i and i++?
  27. Undefined reference to pthread_create in Linux
  28. Stack smashing detected
  29. Why am I getting “void value not ignored as it ought to be”?
  30. Pointer Arithmetic
  31. dereferencing pointer to incomplete type
  32. Openssl : error “self signed certificate in certificate chain”
  33. How do I create an array of strings in C?
  34. How do function pointers in C work?
  35. “Expected expression before ‘ { ‘ token”
  36. How to generate a random int in C?
  37. How do I use valgrind to find memory leaks?
  38. C pointers and arrays: [Warning] assignment makes pointer from integer without a cast
  39. How to print the array?
  40. How to solve the error: assignment to expression with array type
  41. segmentation fault : 11
  42. What does “1e” mean?
  43. how to use uint64_t in C [duplicate]
  44. munmap_chunk(): invalid pointer
  45. How do I solve the following errors: “Undefined reference to WinMain”, “[Error] Id returned 1 exit status”?
  46. Warning comparison between pointer and integer
  47. What is the LD_PRELOAD trick?
  48. How do I properly compare strings in C?
  49. Connect: Socket operation on non-socket
  50. Using %s in C correctly – very basic level
  51. How to correctly use the extern keyword in C
  52. How do you make an array of structs in C?
  53. How do we check if a pointer is NULL pointer?
  54. how to use wait in C
  55. How do I calculate MB/s & MiB/s?
  56. In C programming, what is `undefined reference`error, when compiling?
  57. double free or corruption (fasttop)
  58. Returning string from C function
  59. error: expected declaration or statement at end of input in c
  60. Why do I get clang: error: linker command failed with exit code 1?
  61. Char Comparison in C
  62. How to convert integer to char in C?
  63. Program received signal SIGPIPE, Broken pipe
  64. How to read from stdin with fgets()?
  65. Why am I getting “array initializer must be an initializer list or string literal”?
  66. #31 expression must have integral type
  67. How do you pass a function as a parameter in C?
  68. getopt_long() — proper way to use it?
  69. C read file line by line
  70. C read file line by line
  71. Copying a part of a string (substring) in C
  72. Incompatible implicit declaration of built-in function ‘malloc’
  73. Does C have a “foreach” loop construct?
  74. What is a bus error? Is it different from a segmentation fault?
  75. What is the cause of flexible array member not at end of struct error?
  76. How can I get argv[] as int?
  77. Why I do get “Cannot find bound of current function” when I overwrite the ret address of a vulnerable program?
  78. valgrind – Address —- is 0 bytes after a block of size 8 alloc’d
  79. How to printf a memory address in C
  80. Data argument not used by format strings in C
  81. Writing binary number system in C code
  82. C convert floating point to int
  83. How to print an unsigned char in C?
  84. How does one represent the empty char?
  85. Removing trailing newline character from fgets() input
  86. “Multiple definition”, “first defined here” errors
  87. How do you allow spaces to be entered using scanf?
  88. Allocating char array using malloc
  89. Implementation of strtok() function
  90. switch case: error: case label does not reduce to an integer constant
  91. warning: incompatible implicit declaration of built-in function ‘printf’ [enabled by default]
  92. warning: return makes pointer from integer without a cast but returns integer as desired
  93. The difference between char * and char[] [duplicate]
  94. How to clear input buffer in C?
  95. How to format strings using printf() to get equal length in the output
  96. Usage of \b and \r in C
  97. How to normalize a mantissa
  98. Parsing command-line arguments in C
  99. Compiler warning – suggest parentheses around assignment used as truth value
  100. lvalue required as increment operand

Leave a Comment