strcmp and strncmp implementation in glibc

What is the strcmp and strncmp implementation in glibc?

strcmp implementation in glibc 2.16:

string/strcmp.c

/* Copyright (C) 1991, 1996, 1997, 2003 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <<a href="http://www.gnu.org/licenses/" rel="nofollow">http://www.gnu.org/licenses/</a>>.  */

#include <string.h>
#include <memcopy.h>

#undef strcmp

/* Compare S1 and S2, returning less than, equal to or
   greater than zero if S1 is lexicographically less than,
   equal to or greater than S2.  */
int
strcmp (p1, p2)
     const char *p1;
     const char *p2;
{
  register const unsigned char *s1 = (const unsigned char *) p1;
  register const unsigned char *s2 = (const unsigned char *) p2;
  unsigned char c1, c2;

  do
    {
      c1 = (unsigned char) *s1++;
      c2 = (unsigned char) *s2++;
      if (c1 == '')
        return c1 - c2;
    }
  while (c1 == c2);

  return c1 - c2;
}

libc_hidden_builtin_def (strcmp)

See strcmp in glibc.

string/strncmp.c:

/* Copyright (C) 1991, 1996, 1997, 2003 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <<a href="http://www.gnu.org/licenses/" rel="nofollow">http://www.gnu.org/licenses/</a>>.  */

#include <string.h>
#include <memcopy.h>

#undef strncmp

#ifndef STRNCMP
#define STRNCMP strncmp
#endif

/* Compare no more than N characters of S1 and S2,
   returning less than, equal to or greater than zero
   if S1 is lexicographically less than, equal to or
   greater than S2.  */
int
STRNCMP (const char *s1, const char *s2, size_t n)
{
  unsigned char c1 = '';
  unsigned char c2 = '';

  if (n >= 4)
    {
      size_t n4 = n >> 2;
      do
      {
        c1 = (unsigned char) *s1++;
        c2 = (unsigned char) *s2++;
        if (c1 == '' || c1 != c2)
          return c1 - c2;
        c1 = (unsigned char) *s1++;
        c2 = (unsigned char) *s2++;
        if (c1 == '' || c1 != c2)
          return c1 - c2;
        c1 = (unsigned char) *s1++;
        c2 = (unsigned char) *s2++;
        if (c1 == '' || c1 != c2)
          return c1 - c2;
        c1 = (unsigned char) *s1++;
        c2 = (unsigned char) *s2++;
        if (c1 == '' || c1 != c2)
          return c1 - c2;
      } while (--n4 > 0);
      n &= 3;
    }

  while (n > 0)
    {
      c1 = (unsigned char) *s1++;
      c2 = (unsigned char) *s2++;
      if (c1 == '' || c1 != c2)
        return c1 - c2;
      n--;
    }

  return c1 - c2;
}

libc_hidden_builtin_def (STRNCMP)

See strncmp in glibc.

The BSD implementation of strcmp and strncmp:

strcmp:

int
strcmp(s1, s2)
        register const char *s1, *s2;
{
        while (*s1 == *s2++)
                if (*s1++ == 0)
                        return (0);
        return (*(const unsigned char *)s1 - *(const unsigned char *)(s2 - 1));
}

strncmp:

int
strncmp(s1, s2, n)
        register const char *s1, *s2;
        register size_t n;
{

        if (n == 0)
                return (0);
        do {
                if (*s1 != *s2++)
                        return (*(const unsigned char *)s1 -
                                *(const unsigned char *)(s2 - 1));
                if (*s1++ == 0)
                        break;
        } while (--n != 0);
        return (0);
}