使用 %d 打印 unsigned long long [英] Printing unsigned long long using %d

问题描述

为什么我打印以下内容时得到 -1?

Why do I get -1 when I print the following?

unsigned long long int largestIntegerInC = 18446744073709551615LL;

printf ("largestIntegerInC = %d\n", largestIntegerInC);

我知道我应该使用 llu 而不是 d，但是为什么我得到 -1 而不是 18446744073709551615LL?

I know I should use llu instead of d, but why do I get -1 instead of 18446744073709551615LL?

是因为溢出吗?

推荐答案

在C(99)中，LLONG_MAX，long long int类型的最大值保证为至少为 9223372036854775807.unsigned long long int 的最大值保证至少为 18446744073709551615，即 2⁶⁴−1 (0xffffffffffffffff).

In C (99), LLONG_MAX, the maximum value of long long int type is guaranteed to be at least 9223372036854775807. The maximum value of an unsigned long long int is guaranteed to be at least 18446744073709551615, which is 2⁶⁴−1 (0xffffffffffffffff).

所以，初始化应该是:

unsigned long long int largestIntegerInC = 18446744073709551615ULL;

(注意 ULL.)因为 largestIntegerInC 是 unsigned long long int 类型，你应该用正确的格式说明符打印它，这是 "%llu":

(Note the ULL.) Since largestIntegerInC is of type unsigned long long int, you should print it with the right format specifier, which is "%llu":

$ cat test.c
#include <stdio.h>

int main(void)
{
    unsigned long long int largestIntegerInC = 18446744073709551615ULL;
    /* good */
    printf("%llu\n", largestIntegerInC);
    /* bad */
    printf("%d\n", largestIntegerInC);
    return 0;
}
$ gcc  -std=c99 -pedantic test.c
test.c: In function ‘main’:
test.c:9: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘long long unsigned int’

上面的第二个printf()是错误的，它可以打印任何东西.您正在使用 "%d"，这意味着 printf() 需要一个 int，但得到一个 unsigned long long int，它(很可能)与 int 的大小不同.您得到 -1 作为输出的原因是由于(坏)运气，以及在您的机器上，数字使用二进制补码表示的事实.

The second printf() above is wrong, it can print anything. You are using "%d", which means printf() is expecting an int, but gets a unsigned long long int, which is (most likely) not the same size as int. The reason you are getting -1 as your output is due to (bad) luck, and the fact that on your machine, numbers are represented using two's complement representation.

要看看这会如何糟糕，让我们运行以下程序:

To see how this can be bad, let's run the following program:

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

int main(int argc, char *argv[])
{
    const char *fmt;
    unsigned long long int x = ULLONG_MAX;
    unsigned long long int y = 42;
    int i = -1;
    if (argc != 2) {
        fprintf(stderr, "Need format string\n");
        return EXIT_FAILURE;
    }
    fmt = argv[1];
    printf(fmt, x, y, i);
    putchar('\n');
    return 0;
}

在我的 Macbook 上，使用 "%d %d %d" 运行程序给我 -1 -1 42，在 Linux 机器上，相同的程序使用相同的格式给我 -1 42 -1.糟糕.

On my Macbook, running the program with "%d %d %d" gives me -1 -1 42, and on a Linux machine, the same program with the same format gives me -1 42 -1. Oops.

事实上，如果你想在你的 largestIntegerInC 变量中存储最大的 unsigned long long int 数字，你应该包括 limits.h 并使用 ULLONG_MAX.或者您应该将 assing -1 存储到您的变量中:

In fact, if you are trying to store the largest unsigned long long int number in your largestIntegerInC variable, you should include limits.h and use ULLONG_MAX. Or you should store assing -1 to your variable:

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

int main(void)
{
    unsigned long long int largestIntegerInC = ULLONG_MAX;
    unsigned long long int next = -1;
    if (next == largestIntegerInC) puts("OK");
    return 0;
}

在上面的程序中，largestIntegerInC 和 next 都包含 unsigned long long int 类型的最大可能值.

In the above program, both largestIntegerInC and next contain the largest possible value for unsigned long long int type.

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