How to compare strings in C language and how to calculate the length of strings

/*************************************** ******************************
* Author: Samson
* Date: 01/09/2012
* Test Platform:
* GNU Linux version 2.6.29.4
* GCC version 4.4.0 20090506 (Red Hat 4.4.0-4) (GCC)

**************************************** ****************************/

/* Compare two strings */
Int _ strcmp (const char * S1, const char * S2)
{
While (* S1! = '\ 0' & * S2! = '\ 0 ')
{
If (* S1! = * S2)
{
Return * S1-* S2;
}
S1 ++;
S2 ++;
}
Return 0;
}

/* Compare two strings */
Int _ strncmp (const char * S1, const char * S2, int N)
{
While (* S1! = '\ 0') & (* S2! = '\ 0') & (n> 0 ))
{
If (* S1! = * S2)
{
Return * S1-* S2;
}
S1 ++;
S2 ++;
N --;
}
Return 0;
}

/* Calculate the length of a string */
Int _ strlen (const char * s)
{
Int Len = 0;

While ('\ 0 '! = * S ++)
{
Len ++;
}
Return Len;
}