Quick-and-Dirty Human Size

human_size.c

#include <stdio.h>
#include <sys/types.h>

/**
  @brief   Human file size
  @param   size  File size in bytes
  @return  Base 2 prefixed file size string

  This isn't as complete as Gnulib's `human_readable`, but it does the
  job without any allocation or messing around
*/
const char* human_size(ssize_t size) {
  static char prefix[8] = {'k', 'M', 'G', 'T', 'P', 'E', 'Z', 'Y'};
  static char output[10] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0};

  if (size < 0) {
    (void)snprintf(output, 10, "Error");

  } else {
    double quant = (double)size;
    ssize_t i = -1;

    while (quant >= 1024.0) {
      quant /= 1024.0;
      ++i;
    }

    if (i < 0 || i > 7) {
      (void)snprintf(output, sizeof(output), "%ld B", size);
    } else {
      (void)snprintf(output, sizeof(output), "%.1f %ciB", quant, prefix[i]);
    }
  }
  
  return output;
}

human_size.h

#ifndef _HUMAN_SIZE_H
#define _HUMAN_SIZE_H

#include <sys/types.h>

const char* human_size(ssize_t);

#endif