Strip away preview feature minor version from `.class` file

remove-preview-minor-version.c

#include <stdio.h>
#include <fcntl.h>
#include <stdint.h>
#include <sys/mman.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>

struct Head {
  uint32_t magic;
  uint16_t minor;
  uint16_t major;
};

uint32_t toBE32(uint32_t le) {
  unsigned char *p = (unsigned char *) &le;
  return ((p[0] << 24) | (p[1] << 16) | (p[2] << 8) | (p[3]));
}

int main(int argc, const char **argv) {
  --argc; ++argv;
  if (argc != 1) {
    printf("Usage: %s: <class-file>\n", argv[-1]);
    return 1;
  }

  int f = open(argv[0], O_RDWR);
  if (f < 0) {
    perror("open");
    return 1;
  }

  char *cf = mmap(
    NULL,
    sizeof(struct Head),
    PROT_READ | PROT_WRITE,
    MAP_SHARED,
    f,
    0
  );

  if (cf == MAP_FAILED) {
    perror("mmap");
    return 1;
  }

  struct Head *h = (struct Head *) cf;
  if (h->magic == toBE32(0xcafebabe) && h->minor == 0xffff) {
    printf("%s has preview feature bit set, clearing\n", argv[0]);
    h->minor = 0;
  }
  // printf("magic: %x\n", h->magic);
  // printf("minor: %u\n", h->minor);
  // printf("major: %u\n", h->major);

  munmap(cf, sizeof(struct Head));
  close(f);
}