#include <iostream>
using namespace std;

int main()
{
	int a;
	char *name = NULL;
	a = strlen(name);	
	return 0;
}

以上程序编译没问题，运行将会报错。

原因就是name为NULL，strlen参数不能为NULL，为探究原因，我查看了glibc对strlen函数的实现代码如下：

#include <string.h>
#include <stdlib.h>

#undef strlen

#ifndef STRLEN
# define STRLEN strlen
#endif
size_t STRLEN(const char *str)
{
	const char *char_ptr;
	const unsigned long int *longword_ptr;
	unsigned long int longword, himagic, lomagic;

	// Handle the first few characters by reading one character at a time.
	//Do this until CHAR_PTR is aligned on a longword boundary.  
	for (char_ptr = str; ((unsigned long int) char_ptr & (sizeof(longword) - 1)) != 0; ++char_ptr)
		if (*char_ptr == '\0') return char_ptr - str;
	longword_ptr = (unsigned long int *) char_ptr;

	/* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
	the "holes."  Note that there is a hole just to the left of
	each byte, with an extra at the end:

	bits:  01111110 11111110 11111110 11111111
	bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD

	The 1-bits make sure that carries propagate to the next 0-bit.
	The 0-bits provide holes for carries to fall into.  */
	himagic = 0x80808080L;
	lomagic = 0x01010101L;
	if (sizeof(longword) > 4)
	{
		/* 64-bit version of the magic.  */
		/* Do the shift in two steps to avoid a warning if long has 32 bits.  */
		himagic = ((himagic << 16) << 16) | himagic;
		lomagic = ((lomagic << 16) << 16) | lomagic;
	}
	if (sizeof(longword) > 8)
		abort();

	/* Instead of the traditional loop which tests each character,
	we will test a longword at a time.  The tricky part is testing
	if *any of the four* bytes in the longword in question are zero.  */
	for (;;)
	{
		longword = *longword_ptr++;   //strlen函数为什么不能传入空指针原因！  

		if (((longword - lomagic) & ~longword & himagic) != 0)
		{
			/* Which of the bytes was the zero?  If none of them were, it was
			a misfire; continue the search.  */

			const char *cp = (const char *)(longword_ptr - 1);

			if (cp[0] == 0)
				return cp - str;
			if (cp[1] == 0)
				return cp - str + 1;
			if (cp[2] == 0)
				return cp - str + 2;
			if (cp[3] == 0)
				return cp - str + 3;
			if (sizeof(longword) > 4)
			{
				if (cp[4] == 0)
					return cp - str + 4;
				if (cp[5] == 0)
					return cp - str + 5;
				if (cp[6] == 0)
					return cp - str + 6;
				if (cp[7] == 0)
					return cp - str + 7;
			}
		}
	}
}

从第47行可见，如果传入NULL，则

longword = *longword_ptr++;

longword_ptr为NULL，从而*longword_ptr是不可访问的。

注：*longword_ptr++等效于*(longword_ptr++)，即先求*longword_ptr，再对longword_ptr+1。