从命令行使用 sizeof 编译 C 代码时出错

Question

我从 Linux 命令行在 Nano 中编写了以下代码，编译时出现错误：

我想知道我的代码需要更改什么才能正确编译。我正在尝试获取列出的每种数据类型中的位数以在一行上打印。

#include<stdio.h>

int main(void){
    char A;
    unsigned char B;
    int a;
    unsigned int b;
    long c;
    unsigned long d;
    float e;
    double f;
    long double g;

    printf(
        "%c %c %i %u %li %lu %f %lf %Lf\n",
         sizeof(char), sizeof(unsigned char),
         sizeof(int), sizeof(unsigned int),
         sizeof(long), sizeof(unsigned long),
         sizeof(float), sizeof(double), sizeof(long double)
    );

    return 0;
}

Answer 1

这是因为 unsigned char 被提升为一个 int（在正常的 C 实现中），因此一个 int 被传递给 printf 作为说明符 %c。但是，%c 需要一个 unsigned int，因此类型不匹配，并且 C 标准没有定义行为。您可以执行以下方法： 1）尝试使用C11（标准版）。 2) 尝试使用 %zu.

Answer 2

sizeof operator returns an integer of type size_t, so you should use the appropriate printf 格式说明符 ("%zu")（假设 C99）：

printf(
    "%zu %zu %zu %zu %zu %zu %zu %zu %zu\n",
     sizeof(char), sizeof(unsigned char),
     sizeof(int), sizeof(unsigned int),
     sizeof(long), sizeof(unsigned long),
     sizeof(float), sizeof(double), sizeof(long double)
);

但是，这会打印每种类型的字节的数量。如果您想要每种类型中的位的数量，请包括 <limits.h> 并将每个结果乘以 CHAR_BIT 以获得：

#include <limits.h>

/* ... */

printf(
    "%zu %zu %zu %zu %zu %zu %zu %zu %zu\n",
     sizeof(char) * CHAR_BIT, sizeof(unsigned char) * CHAR_BIT,
     sizeof(int) * CHAR_BIT, sizeof(unsigned int) * CHAR_BIT,
     sizeof(long) * CHAR_BIT, sizeof(unsigned long) * CHAR_BIT,
     sizeof(float) * CHAR_BIT, sizeof(double) * CHAR_BIT, sizeof(long double) * CHAR_BIT
);

IMO，如果您标记要打印的内容并在其自己的行上打印每个值，它看起来会更清楚，如下所示：

printf("Number of bits in char = %zu\n", sizeof(char) * CHAR_BIT);
printf("Number of bits in unsigned char = %zu\n", sizeof(unsigned char) * CHAR_BIT);
printf("Number of bits in int = %zu\n", sizeof(int) * CHAR_BIT);
printf("Number of bits in unsigned int = %zu\n", sizeof(unsigned int) * CHAR_BIT);
printf("Number of bits in long = %zu\n", sizeof(long) * CHAR_BIT);
printf("Number of bits in unsigned long = %zu\n", sizeof(unsigned long) * CHAR_BIT);
printf("Number of bits in float = %zu\n", sizeof(float) * CHAR_BIT);
printf("Number of bits in double = %zu\n", sizeof(double) * CHAR_BIT);
printf("Number of bits in long double = %zu\n", sizeof(long double) * CHAR_BIT);

并且可以用宏来减少（虽然宏不是最好的，但它们对重复代码很有用）：

#define PRINT_BITS_IN_TYPE(type) \
    printf("Number of bits in " #type " = %zu\n", sizeof(type) * CHAR_BIT)

PRINT_BITS_IN_TYPE(char);
PRINT_BITS_IN_TYPE(unsigned char);
PRINT_BITS_IN_TYPE(int);
PRINT_BITS_IN_TYPE(unsigned int);
PRINT_BITS_IN_TYPE(long);
PRINT_BITS_IN_TYPE(unsigned long);
PRINT_BITS_IN_TYPE(float);
PRINT_BITS_IN_TYPE(double);
PRINT_BITS_IN_TYPE(long double);

从命令行使用 sizeof 编译 C 代码时出错

Errors compiling C code with sizeof from command line

c

linux

command

sizeof

nano