sizeof是一个单目运算符，它的运算对象是变量或数据类型，运算结果为一个整数。运算的一般形式如下:
sizeof(<类型或变量名>)
它只针对数据类型，而不针对变量！
若运算对象为变量，则所求的结果是这个变量占用的内存空间字节数；若运算对象是数据类型，则所求结果是这种数据类型的变量占用的内存空间字节数。
sizeof是一个使用频率很高的操作符，经常用来获取变量或数据类型所占用的内存空间的大小，下面的程序显示了sizeof的用法。
#include <stdio.h>
struct Student
{
int number;
char name[8];
};
enum season{
spring,s ummer, fall, winter
};
int main()
{
int a = 10;
float b = 3.5;
struct Student s1 = {1, “zhangsan”};
enum season myseason;
printf ("the size of char is %d bytes ",sizeof(char));
printf ("the size of short is %d bytes ",sizeof(short));
printf ("the size of int is %d bytes ",sizeof(int));
printf ("the size of a is %d bytes ",sizeof(a));
printf ("the size of long is %d bytes ",sizeof(long));
printf ("the size of long long is %d bytes ",sizeof(long long));
printf ("the size of float is %d bytes ",sizeof(float));
printf ("the size of b is %d bytes ",sizeof(b));
printf ("the size of double is %d bytes ",sizeof(double));
printf ("the size of struct Student is %d bytes ",sizeof(struct Student));
printf ("the size of enum season is %d bytes ", sizeof (enum season));
printf ("the size of myseason is %d bytes ", sizeof (myseason));
return 0;
}
程序执行结果如下：
linux@ubuntu:~/book/ch4$ cc test.c –o test -Wall
linux@ubuntu:~/book/ch4$./test
the size of char is 1 bytes
the size of short is 2 bytes
the size of int is 4 bytes
the size of a is 4 bytes
the size of long is 4 bytes
the size of long long is 8 bytes
the size of float is 4 bytes
the size of b is 4 bytes
the size of double is 8 bytes
the size of struct Student is 12 bytes
the size of enum season is 4 bytes
the size of myseason is 4 bytes
从该结果中，可以清楚地看到不同数据类型及变量所占的字节数，读者应该熟悉这些结果。还可以看到，变量所占用的空间，由其数据类型决定，与变量的值没有关系。