arakiken さんによる https://qiita.com/arakiken/items/626b02cd857d20c12fbc の記事にあるサンプルコードを改変したものです。どのように変更したかは今ちょっと思い出せません。osiire で使ってます

drcssixel-test.c

#include <stdlib.h>

#include "drcssixel.c"

static void pua_to_utf8(unsigned char *dst, unsigned int *src, unsigned int len) {
  unsigned int i;

  for(i = 0; i < len; i++) {
    unsigned int code = src[i];
    *(dst++) = ((code >> 18) & 0x07) | 0xf0;
    *(dst++) = ((code >> 12) & 0x3f) | 0x80;
    *(dst++) = ((code >> 6) & 0x3f) | 0x80;
    *(dst++) = (code & 0x3f) | 0x80;
  }
}

static char charset = '1';
static int quiet = 0;

char** parse_options(int argc, char** argv) {
  int i;
  for (i = 1; i < argc && argv[i] != NULL;) {
    if (strcmp(argv[i], "-c") == 0) {
      if (argv[i + 1] == NULL) {
        printf("missing argument to -c\n");
        exit(1);
      }
      charset = argv[i + 1][0];
      i += 2;
    } else if (strcmp(argv[i], "-d") == 0) {
      if (argv[i + 1] == NULL) {
        printf("missing argument to -d\n");
        exit(1);
      }
      char *p = argv[i+1];
      col_width = atoi(p);
      while (*p) {
        if (*p == 'x') {
          p++;
          break;
        }
        p++;
      }
      if (!*p) { // the string either does not contain 'x' or ends with 'x'
        printf("invalid argument to -d\n");
        exit(1);
      }
      line_height = atoi(p);
      if (col_width <= 0 || line_height <= 0) {
        printf("Warning: invalid argument to -d (range error)\n");
        col_width = line_height = 0;
      }
      i += 2;
    } else if (strcmp(argv[i], "-q") == 0) {
      quiet = 1;
      i++;
    } else {
      return &argv[i];
    }
  }

  // no files specified
  printf("Usage: drcssixel-test [OPTIONS]... [FILES]...\n");
  printf(" -c\t\tspecify (first) character set. Defaults to '1'\n");
  printf(" -d\t\tspecify cell dimensions. (eg. '8x16')\n");
  printf(" -q\t\tdo not show the image.\n");
  exit(1);
}

int main(int argc, char **argv) {
  unsigned int *buf, *buf_p;
  int is_96cs = 0;
  int count;
  int num_cols, num_rows, col, row;
  char **files;

  files = parse_options(argc, argv);

  for (count = 0; files[count]; count++) {
    buf = drcs_sixel_from_file(files[count], &charset, &is_96cs, &num_cols, &num_rows);
    if (buf == NULL) {
      return 1;
    }

    if (!quiet) {
      pua_to_utf8((char*) buf, buf, num_cols * num_rows); /* UTF32 -> UTF8 */
      buf_p = buf;
      for(row = 0; row < num_rows; row++) {
        for (col = 0; col < num_cols; col++) {
          write(STDOUT_FILENO, buf_p++, 4);
        }
        write(STDOUT_FILENO, "\n", 1);
      }
    }
    free(buf);
  }

  return 0;
}