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char *filename;
char *userbuf;
unsigned int userlen;

struct stat lstat_info;
int fd;
/* ... */
if (lstat(filename, &lstat_info) == -1) {
  /* handle error */
}

if (!S_ISLNK(lstat_info.st_mode)) {
   fd = open(filename, O_RDWR);
   if (fd == -1) {
       /* handle error */
   }
}
write(fd, userbuf, userlen);

char *filename;
char *userbuf;
unsigned int userlen;

struct stat lstat_info
struct stat fstat_info;
int fd;
/* ... */
if (lstat(filename, &lstat_info) == -1) {
  /* handle error */
}

fd = open(filename, O_RDWR);
if (fd == -1) {
  /* handle error */
}

if (fstat(fd, &fstat_info) == -1) {
  /* handle error */
}

if (lstat_info.st_mode == fstat_info.st_mode &&
    lstat_info.st_ino == fstat_info.st_ino  &&
    lstat_info.st_dev == fstat_info.st_dev) {
  write(fd, userbuf, userlen);
}