int Strlen(const char *str)
{
   assert(*str != NULL);
   int icount = 0;
   while(*str++)
   {
      icount++;
   }
   return icount;
}

试一下这个
size_t strlen(register const char *str)
{
  size_t cnt=0;
  while(*str!='\0')
     ++cnt;
  return cnt;
}

忘了，少写了一句话。
size_t  str_len( register const char *str)
{
  size_t  cnt=0;
  for( ; *str!='\0' ; ++str )
      ++cnt ;
  return  cnt ;
}

assert(*str != NULL);//应该为assert(str != NULL);传入的不能使NULL指针，但是可以*str可以是NULL

int mystrlen(char *mystr)
{
    int sum=0;
    while(mystr[sum++])
    ;
    return --sum;
}

看看BSD的实现

 1 /*-
    2  * Copyright (c) 1990, 1993
    3  *      The Regents of the University of California.  All rights reserved.
    4  *
    5  * Redistribution and use in source and binary forms, with or without
    6  * modification, are permitted provided that the following conditions
    7  * are met:
    8  * 1. Redistributions of source code must retain the above copyright
    9  *    notice, this list of conditions and the following disclaimer.
   10  * 2. Redistributions in binary form must reproduce the above copyright
   11  *    notice, this list of conditions and the following disclaimer in the
   12  *    documentation and/or other materials provided with the distribution.
   13  * 3. All advertising materials mentioning features or use of this software
   14  *    must display the following acknowledgement:
   15  *      This product includes software developed by the University of
   16  *      California, Berkeley and its contributors.
   17  * 4. Neither the name of the University nor the names of its contributors
   18  *    may be used to endorse or promote products derived from this software
   19  *    without specific prior written permission.
   20  *
   21  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
   22  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   23  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   24  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
   25  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   26  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   27  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   28  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   29  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   30  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   31  * SUCH DAMAGE.
   32  *
   33  * $FreeBSD: src/sys/libkern/strlen.c,v 1.6 1999/08/28 00:46:37 peter Exp $
   34  * $DragonFly: src/sys/libkern/strlen.c,v 1.5 2004/01/26 11:09:44 joerg Exp $
   35  */
   36
   37 #include <sys/libkern.h>
   38
   39 size_t
   40 strlen(const char *str)
   41 {
   42         const char *s;
   43
   44         for (s = str; *s; ++s);
   45         return(s - str);
   46 }