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);
}
```

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