分类: LINUX
2010-07-31 18:43:42
sizeof Operator
sizeof expression
The sizeof keyword gives the amount of storage, in bytes, associated with a variable or a type
(including aggregate types). This keyword returns a value of type size_t.
The expression is either an identifier or a type-cast expression (a type specifier enclosed in
parentheses).
When applied to a structure type or variable, sizeof returns the actual size, which may include
padding bytes inserted for alignment. When applied to a statically dimensioned array, sizeof
returns the size of the entire array. The sizeof operator cannot return the size of dynamically
allocated arrays or external arrays.
然后再看一下对strlen是如何定义的:
strlen二、由几个例子说开去。
Get the length of a string.
Routine Required Header:
strlen
size_t strlen( const char *string );
Parameter
string:Null-terminated string
Libraries
All versions of the C run-time libraries.
Return Value
Each of these functions returns the number of characters in string, excluding the terminal
NULL. No return value is reserved to indicate an error.
Remarks
Each of these functions returns the number of characters in string, not including the
terminating null character. wcslen is a wide-character version of strlen; the argument of
wcslen is a wide-character string. wcslen and strlen behave identically otherwise.
char* ss = "0123456789";第二个例子:
sizeof(ss) 结果 4 ===》ss是指向字符串常量的字符指针
sizeof(*ss) 结果 1 ===》*ss是第一个字符
char ss[] = "0123456789";
sizeof(ss) 结果 11 ===》ss是数组,计算到\0位置,因此是10+1
sizeof(*ss) 结果 1 ===》*ss是第一个字符
char ss[100] = "0123456789";
sizeof(ss) 结果是100 ===》ss表示在内存中的大小 100×1
strlen(ss) 结果是10 ===》strlen是个函数内部实现是用一个循环计算到\0为止之前
int ss[100] = "0123456789";
sizeof(ss) 结果 400 ===》ss表示再内存中的大小 100×4
strlen(ss) 错误 ===》strlen的参数只能是char* 且必须是以''\0''结尾的
char q[]="abc";
char p[]="a\n";
sizeof(q),sizeof(p),strlen(q),strlen(p);
结果是 4 3 3 2
class X第三个例子:
{
int i;
int j;
char k;
};
X x;
cout<cout<
char szPath[MAX_PATH]如果在函数内这样定义,那么sizeof(szPath)将会是MAX_PATH,但是将szPath作为虚参声明时(void fun(char szPath[MAX_PATH])),sizeof(szPath)却会是4(指针大小)
short f();输出的结果是sizeof(short),即2。
printf("%d\n", sizeof(f()));
char str[20]="0123456789";
int a=strlen(str); //a=10;
int b=sizeof(str); //而b=20;
fun(char [8])都等价于 fun(char *) 在C++里传递数组永远都是传递指向数组首元素的指针,编译器不知道数组的大小 如果想在函数内知道数组的大小, 需要这样做: 进入函数后用memcpy拷贝出来,长度由另一个形参传进去
fun(char [])
fun(unsiged char *p1, int len)有关内容见: C++ PRIMER?
{
unsigned char* buf = new unsigned char[len+1]
memcpy(buf, p1, len);
}
void *malloc(size_t size),
size_t fread(void * ptr,size_t size,size_t nmemb,FILE * stream)。
void * memset(void * s,int c,sizeof(s))