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以下是其源代码，那个__ptrlow函数是在哪定义的？我搜遍了整个glibc都没找到其实现，望高手赐教
# define CHECK_BOUNDS_LOW(ARG) \
(((__ptrvalue (ARG) < __ptrlow (ARG)) && BOUNDS_VIOLATED), \
__ptrvalue (ARG))


char *
strcpy (
char *dest,
const char *src
)
{
reg_char c;
char *__unbounded s = (char *__unbounded) CHECK_BOUNDS_LOW (src);
const ptrdiff_t off = CHECK_BOUNDS_LOW (dest) - s - 1;
size_t n;

do
{
c = *s++;
s[off] = c;
}
while (c != '\0');

n = s - src;
(void) CHECK_BOUNDS_HIGH (src + n);
(void) CHECK_BOUNDS_HIGH (dest + n);

return dest;
}