temperature.c

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#include <errno.h>
#include <math.h>

#if !defined(__NetBSD__)
#include <values.h>
#include <float.h>
#endif

#define VARIANCE 0.00004

void usage(char *fname) {
    printf("Usage: %s n C|F|K\n", fname);
}

void handleRangeError(double value, int error, char *fname) {
  printf("Error: %d\tValue: %g\n", error, value);
  if (error == ERANGE) {
    /* We've already established that there was a range error. */
    errno = 0;

    printf("Double is out of range.  ");
    if ((value == HUGE_VAL)||(value == (-HUGE_VAL))) {
      printf("Absolute value cannot be greater than %.0g.\n", DBL_MAX);
    } else {
      printf("Absolute value cannot be smaller than %g.\n", DBL_MIN);
    }
  } else {
    printf("Missing value!\n\n");
    usage(fname);
  }
  return;
}

int main(int argc, char **argv) {
  double c, f, k, v;
  char scale;
  char *scale_ptr;
  char value[255];
  int okay = EXIT_SUCCESS;
  
  if (argc<2||argc>3) {
    usage(argv[0]);
    return EXIT_FAILURE;
  }

  errno=0;
  if (argc==3) {
    strncpy(value, argv[1], 255);
    v = strtod(value, &scale_ptr);
    if (errno) {
      handleRangeError(v, errno, argv[0]);
      return EXIT_FAILURE;
    }
    scale = tolower(argv[2][0]);
  } else {
    strncpy(value, argv[1], 255);
    v = strtod(value, &scale_ptr);
    if (errno) {
      handleRangeError(v, errno, argv[0]);
      return EXIT_FAILURE;
    }
    scale = tolower(*scale_ptr);
  }
  /* If scale_ptr == value then no numeric value was present */
  if (value==scale_ptr) {
    handleRangeError(v, 0, argv[0]);
    return EXIT_FAILURE;
  }
  
  switch (scale) {
  case 'c':
    c = v;
    f = c*9/5+32;
    k = c+273.15;
    break;
  case 'k':
    k = v;
    c = k-273.15;
    f = c*9/5+32;
    break;
  case 'f':
    f = v;
    c = (f-32)*5/9;
    k = c+273.15;
    break;
  default:
    usage(argv[0]);
    return EXIT_FAILURE;
    break;
  }

  /* 
     Limited precision can result in rounding to -0.00.  Rounding up 
     to avoid this.  VARIANCE must be adjusted if we adjust precision.
  */
  k += VARIANCE;
  c += VARIANCE;
  f += VARIANCE;

  if (k<0.0) {
    printf("The temperature cannot be below absolute zero!\n");
    return EXIT_FAILURE;
  }
  
  printf("%+10.4fF\n%+10.4fC\n%+10.4fK\n", f, c, k);
  return EXIT_SUCCESS;
}
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Saturday, 13 December 2025 Michael J. Chappell Contact me at: mcsuper5@freeshell.org
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