C中sizeof()返回值的正确格式说明符
我有以下代码:
#include int main() { printf("The 'int' datatype is \t\t %lu bytes\n", sizeof(int)); printf("The 'unsigned int' data type is\t %lu bytes\n", sizeof(unsigned int)); printf("The 'short int' data type is\t %lu bytes\n", sizeof(short int)); printf("The 'long int' data type is\t %lu bytes\n", sizeof(long int)); printf("The 'long long int' data type is %lu bytes\n", sizeof(long long int)); printf("The 'float' data type is\t %lu bytes\n", sizeof(float)); printf("The 'char' data type is\t\t %lu bytes\n", sizeof(char)); }
哪个输出:
The 'int' datatype is 4 bytes The 'unsigned int' data type is 4 bytes The 'short int' data type is 2 bytes The 'long int' data type is 8 bytes The 'long long int' data type is 8 bytes The 'float' data type is 4 bytes The 'char' data type is 1 bytes
但这就是问题,编译器要求我使用%lu
(long unsigned int)而不是%d
(int),正如我所期望的那样。 毕竟,我们这里只讨论单位数字,不是吗? 那么为什么在使用%d
而不是%lu
时会出现以下错误? 是否与我在64位系统(Ubuntu 14.10)上有关?
helloworld.c: In function 'main': helloworld.c:5:5: warning: format '%d' expects argument of type 'int', but argument 2 has type 'long unsigned int' [-Wformat=] printf("The 'int' datatype is \t\t %d bytes\n", sizeof(int)); ^ helloworld.c:6:5: warning: format '%d' expects argument of type 'int', but argument 2 has type 'long unsigned int' [-Wformat=] printf("The 'unsigned int' data type is\t %d bytes\n", sizeof(unsigned int)); ^ helloworld.c:7:5: warning: format '%d' expects argument of type 'int', but argument 2 has type 'long unsigned int' [-Wformat=] printf("The 'short int' data type is\t %d bytes\n", sizeof(short int)); ^ helloworld.c:8:5: warning: format '%d' expects argument of type 'int', but argument 2 has type 'long unsigned int' [-Wformat=] printf("The 'long int' data type is\t %d bytes\n", sizeof(long int)); ^ helloworld.c:9:5: warning: format '%d' expects argument of type 'int', but argument 2 has type 'long unsigned int' [-Wformat=] printf("The 'long long int' data type is %d bytes\n", sizeof(long long int)); ^ helloworld.c:10:5: warning: format '%d' expects argument of type 'int', but argument 2 has type 'long unsigned int' [-Wformat=] printf("The 'float' data type is\t %d bytes\n", sizeof(float)); ^ helloworld.c:11:5: warning: format '%d' expects argument of type 'int', but argument 2 has type 'long unsigned int' [-Wformat=] printf("The 'char' data type is\t\t %d bytes\n", sizeof(char)); ^ Compilation finished successfully.
您正在尝试打印sizeof
运算符的返回值,该值通常为size_t
类型。
看来,在您的情况下, size_t
是long unsigned int
的typedef
,因此它要求使用它的兼容格式说明符%lu
。 这里返回的值无关紧要,问题在于类型不匹配。
注意:要拥有可移植代码,在基于C99和转发标准的编译器上使用%zu
是安全的。
在C中, sizeof
表达式的类型是size_t
。
要使用的printf
说明符是%zu
。 例:
printf("The 'int' datatype is \t\t %zu bytes\n", sizeof(int));
自1999年版C标准以来,已经可以使用它。
从技术上讲,由于格式和数据类型不匹配,您的行为未定义 。
您应该使用%zu
作为与sizeof
相关联的类型(即size_t
)。 例如:
printf("The 'int' datatype is \t\t %zu bytes\n", sizeof(int));
如果您打算同时针对32位和64位平台,这一点尤为重要。
参考: http : //en.cppreference.com/w/c/io/fprintf