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if (!fgets(file namefilename, sizeof(file namefilename), stdin)) {
  /* handle error */
}

if (open(file namefilename, O_WRONLY) == -1) {
  /* handle error */
}

#ifdef O_NOFOLLOW
  #define OPEN_FLAGS O_NOFOLLOW | O_NONBLOCK
#else
  #define OPEN_FLAGS O_NONBLOCK
#endif

/* ... */

struct stat orig_st;
struct stat open_st;
int fildes;
int flags;

if (!fgets(file namefilename, sizeof(file namefilename), stdin)) {
  /* handle error */
}

if ((lstat(file namefilename, &orig_st) != 0) 
 || (!S_ISREG(orig_st.st_mode))) 
{
  /* handle error */
}

/* A TOCTOU race condition exists here, see below */

if ((fildes = open(file namefilename, OPEN_FLAGS | O_WRONLY)) == -1) {
  /* handle error */
}

if (fstat(fildes, &open_st) != 0) {
  /* handle error */
}

if ((orig_st.st_mode != open_st.st_mode) ||
    (orig_st.st_ino  != open_st.st_ino) ||
    (orig_st.st_dev  != open_st.st_dev)) {
  /* file was tampered with */
}

/* Optional: drop the O_NONBLOCK now that we are sure 
 * this is a good file */
if ((flags = fcntl(fildes, F_GETFL)) == -1) {
  /* handle error */
}

if ((fcntl(fildes, F_SETFL, flags & ~O_NONBLOCK) != 0) {
  /* handle error */
}

/* operate on file */