The sethostname() system call is not standardized by POSIX, only by the LSB:

In particular, it should fail with EINVAL if
"len is negative or larger than the maximum allowed size".

In Cygwin 3.5.3, a too long hostname is not rejected.

Witness: The test program below, when run on the Cygwin 3.5.3 machines
on GitHub (under a user account that is in the Administrators group),
fails with the message "setting a too long hostname succeeded."

The implementation is in winsup/cygwin/ line 773.


====================== Test program essentially =====================
#include <assert.h>
#include <unistd.h>

/* for HOST_NAME_MAX */
#include <limits.h>
/* for strlen */
#include <string.h>

#include <errno.h>
#include <stdio.h>

#define TESTHOSTNAME "gnulib-hostname"

/* mingw and MSVC 9 lack geteuid, so setup a dummy value.
   On Cygwin, geteuid() may return non-zero even for user accounts with
   administrator privileges, so use a dummy value as well.  */
#if defined __CYGWIN__
# define geteuid() 0

main (int argc, char *argv[])
  char origname[HOST_NAME_MAX];
  char newname[HOST_NAME_MAX];
  char longname[HOST_NAME_MAX + 2];
  int rcs, i;

  /* skip the tests if we don't have root privilege.  this does not
     consider things like CAP_SYS_ADMIN (linux) or PRIV_SYS_ADMIN
     (solaris), etc.  systems without a working geteuid (mingw, MSVC
     9) will always skip this test. */
  if (geteuid () != 0)
      fprintf (stderr, "Skipping test: insufficient permissions.\n");
      return 77;

  /* we want to ensure we can do a get/set/get check to ensure the
     change is accepted. record the current name so it can be restored
     later */
  assert (gethostname (origname, sizeof (origname)) == 0);

  /* try setting a valid hostname.  if it fails -1/ENOSYS, we will
     skip the test for long names as this is an indication we're using
     the stub function that doesn't do anything on this platform. */
  rcs = sethostname (TESTHOSTNAME, strlen (TESTHOSTNAME));

  if (rcs != 0)
      if (rcs == -1 && errno == ENOSYS)
          fprintf (stderr,
                   "Skipping test: sethostname is not really implemented.\n");
          return 77;
      else if (rcs == -1
               && (errno == EPERM
                   || errno == EACCES)) /* Cygwin */
          fprintf (stderr, "Skipping test: insufficient permissions.\n");
          return 77;
          fprintf (stderr, "error setting valid hostname.\n");
          return 1;
      assert (gethostname (newname, sizeof (newname)) == 0);

      /* On Windows, a hostname change becomes effective only after
         a reboot.  */
#if !(defined _WIN32 || defined __CYGWIN__)

      /* if we don't get back what we put in, there is no need to
         restore the original name as we will assume it was not
         properly changed. */
      if (strcmp (newname, TESTHOSTNAME) != 0)
          fprintf (stderr, "set/get comparison failed.\n");
          return 1;

  /* glibc does allow setting a zero length name, so the lower bound
     needs no test. validate that we are constrained by
  for (i = 0; i < (HOST_NAME_MAX + 1); i++)
    longname[i] = 'a';

  longname[i] = '\0';

  rcs = sethostname (longname, (HOST_NAME_MAX + 1));

  if (rcs != -1)
      /* attempt to restore the original name. */
      assert (sethostname (origname, strlen (origname)) == 0);
      fprintf (stderr, "setting a too long hostname succeeded.\n");
      return 1;

  /* restore the original name. */
  assert (sethostname (origname, strlen (origname)) == 0);

  return 0;

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