将unsigned int左移至32位，生成警告

Question

为什么左移unsigned int 32次会产生以下警告？

pthr.c: In function ‘main’:
pthr.c:4: warning: left shift count >= width of type

即使sizeof(unsigned int)在我的系统上是4个字节。

代码：

#include<stdio.h> 
int main(void) 
{ 
    unsigned int mask = 1u << 32; 
    printf("sizeof(unsigned int) = %u\n", sizeof(unsigned int)); 
    return 0;  
} 

gcc --version

gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-18)
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.