__attribute__ ((attribute-list))

http://blog.csdn.net/ithomer/article/details/6566739

构造与析构:

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

static  __attribute__((constructor)) void before()
{

    printf("Hello");
}

static  __attribute__((destructor)) void after()
{
    printf(" World!\n");
}

int main(int args,char ** argv)
{
    printf("main");
    return EXIT_SUCCESS;
}
~ 

[root@workstation2017 ~]# ./test.out
Hellomain World!

优先级：

[root@workstation2017 ~]# vi test.c

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

static  __attribute__((constructor())) void begin101()
{

    printf("Hello-101\n");
}

static  __attribute__((constructor())) void begin102()
{

    printf("Hello-102\n");
}

static  __attribute__((destructor())) void end101()
{
    printf(" World!-101\n");
}

static  __attribute__((destructor())) void end102()
{
    printf(" World!-102\n");
}

int main(int args,char ** argv)
{

    return EXIT_SUCCESS;
}

[root@workstation2017 ~]# ./test.out
Hello-
Hello-
 World!-
 World!-

对齐属性：

__attrubte__ ((packed)) 的作用就是告诉编译器取消结构在编译过程中的优化对齐,按照实际占用字节数进行对齐

[root@workstation2017 ~]# vi test.c

#include <stdio.h>
#include <stdlib.h>
#define __u8    unsigned char
#define __u16   unsigned short

struct str_struct{
        __u8    a;
        __u8    b;
        __u8    c;
        __u16   d;
} __attribute__ ((packed));

typedef struct {
        __u8    a;
        __u8    b;
        __u8    c;
        __u16   d;
} __attribute__ ((packed)) str;

typedef struct {
        __u8    a;
        __u8    b;
        __u8    c;
        __u16   d;
}str_temp __attribute__ ((packed));

typedef struct {
        __u8    a;
        __u8    b;
        __u8    c;
        __u16   d;
}str_nopacked;

int main(void)
{
        printf("sizeof str = %d\n", sizeof(str));
        printf("sizeof str_struct = %d\n", sizeof(struct str_struct));
        printf("sizeof str_temp = %d\n", sizeof(str_temp));
        printf("sizeof str_nopacked = %d\n", sizeof(str_nopacked));
        return ;
}

[root@workstation2017 ~]# ./test.xx
sizeof str =
sizeof str_struct =
sizeof str_temp =
sizeof str_nopacked = 

变量占内存大小 x64 x32差别：

 变量的长度，在不同的系统之间会有差别，64位Linux系统和32位Linux系统中，几种常见C语言变 量的长度：
              short    int    long    long long    ptr    time_t
32位
64位