我对 C 很陌生，但我注意到如果你减少一个无符号类型，它将被分配最大值.

I'm pretty new to C but I noticed that if you decrement an unsigned type it will be assigned the maximum value.

我包含了 limits.h 只是为了比较结果.

I've included limits.h just to compare the results.

#include <stdio.h>
#include <limits.h>

int main(void)
{   
    unsigned char c;
    unsigned short s;
    unsigned int i;
    unsigned long l;

    // char
    printf("UCHAR_MAX  = %u\n",  UCHAR_MAX);
    printf("UCHAR_MAX  = %u\n", --c);

    // short
    printf("USHRT_MAX  = %u\n",  USHRT_MAX);
    printf("USHRT_MAX  = %u\n",  --s);

    // int
    printf("UINT_MAX   = %u\n",  UINT_MAX);
    printf("UINT_MAX   = %u\n",  --i);

    // long
    printf("ULONG_MAX  = %lu\n",  ULONG_MAX);
    printf("ULONG_MAX  = %lu\n",  --l);
}   

结果:

UCHAR_MAX  = 255
UCHAR_MAX  = 255
USHRT_MAX  = 65535
USHRT_MAX  = 65535
UINT_MAX   = 4294967295
UINT_MAX   = 4294967295
ULONG_MAX  = 18446744073709551615
ULONG_MAX  = 140731079060271

正如你所看到的，除了长时间之外，它对所有人都有效，至于为什么......好吧，也许有人比我更有经验可以解释.

As you can see it works for all except long, as to why.. well maybe someone more experienced than me can explain.