sprintf() without trailing null space in C

Question

Is there a way to use the C sprintf() function without it adding a \'\\0\' character at the end of its output? I need to write formatted text in the middle of a fixed width stri

Answer 1

Here's an option for memory constrained devices. It trades off speed for using less RAM. I sometimes have to do this to update the middle of a string that gets printed to a LCD.

The idea is that you first call snprintf with a zero sized buffer to determine which index will get clobbered by the null terminator.

You can run the below code here: https://rextester.com/AMOOC49082

#include 
#include 

int main(void)
{
  char buf[100] = { 'a', 'b', 'c', 'd', 'e' };
  const size_t buf_size = sizeof(buf);
  const int i = 123;

  int result = snprintf(buf, 0, "%i", i);
  if (result < 0)
  {
    printf("snprintf error: %i\n", result);
    return -1;
  }

  int clobbered_index = result; //this index will get the null term written into it

  if (result >= buf_size)
  {
    printf("buffer not large enough. required %i chars\n", result + 1);
    return -1;
  }

  char temp_char = buf[clobbered_index];
  result = snprintf(buf, buf_size, "%i", i); //add result error checking here to catch future mistakes
  buf[clobbered_index] = temp_char;

  printf("buf:%s\n", buf);

  return 0;
}

Prints buf:123de