Linux C编程——结构体操作注意事项（深拷贝、偏移量）

在结构体中，需要考虑的有：

1.  内存对齐，下面例子可以看到；
2. 结构体含一级、二级指针，有深拷贝和浅拷贝之分；
3. s.i和s->i是计算偏移量，在CPU中计算，不会操作内存。

有一个宏 offoset 可以计算结构体成员偏移：

SYNOPSIS
       #include <stddef.h>

       size_t offsetof(type, member);

DESCRIPTION
       The macro offsetof() returns the offset of the field member from the start of the structure type.

       This macro is useful because the sizes of the fields that compose a structure can vary across implementa‐
       tions, and compilers may insert different numbers of padding bytes between fields.  Consequently, an ele‐
       ment's offset is not necessarily given by the sum of the sizes of the previous elements.

       A compiler error will result if member is not aligned to a byte boundary (i.e., it is a bit field).

RETURN VALUE
       offsetof() returns the offset of the given member within the given type, in units of bytes.
           

./include/linux/stddef.h文件中有如下定义。

00020: #undef offsetof
00021: #ifdef  compiler_offsetof
00022: #define offsetof(TYPE,MEMBER)  compiler_offsetof(TYPE,MEMBER)
00023: #else
00024: #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
00025: #endif
00026: #endif /* KERNEL  */
00027:
00028: #endif
           

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

int
main(void)
{
    struct s {
		int i;
		char c;
		double d;
		char a[];
    };

/* Output is compiler dependent */

    printf("offsets: i=%ld; c=%ld; d=%ld a=%ld\n",
			    	(long) offsetof(struct s, i),
		    		(long) offsetof(struct s, c),
		    		(long) offsetof(struct s, d),
		    		(long) offsetof(struct s, a));
    printf("sizeof(struct s)=%ld\n", (long) sizeof(struct s));

    exit(EXIT_SUCCESS);
}
           

[[email protected] apue]# ./a.out 
offsets: i=0; c=4; d=8 a=16
sizeof(struct s)=16