#include <stdlib.h>
#include <limits.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <string.h>
#include "syscall.h"

void __procfdname(char*, unsigned);

char* realpath(const char* restrict filename, char* restrict resolved) {
  int fd;
  ssize_t r;
  struct stat st1, st2;
  char buf[15 + 3 * sizeof(int)];
  char tmp[PATH_MAX];

  if (!filename) {
    errno = EINVAL;
    return 0;
  }

  fd = sys_open(filename, O_PATH | O_NONBLOCK | O_CLOEXEC);
  if (fd < 0)
    return 0;
  __procfdname(buf, fd);

  r = readlink(buf, tmp, sizeof tmp - 1);
  if (r < 0)
    goto err;
  tmp[r] = 0;

  fstat(fd, &st1);
  r = stat(tmp, &st2);
  if (r < 0 || st1.st_dev != st2.st_dev || st1.st_ino != st2.st_ino) {
    if (!r)
      errno = ELOOP;
    goto err;
  }

  __syscall(SYS_close, fd);
  return resolved ? strcpy(resolved, tmp) : strdup(tmp);
err:
  __syscall(SYS_close, fd);
  return 0;
}