__attribute__ ((attribute-list))

構造與析構:      

#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!      

優先級：

[root@workstation2017 ~]# vi test.c

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


static  __attribute__((constructor(101))) void begin101()
{

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

static  __attribute__((constructor(102))) void begin102()
{

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


static  __attribute__((destructor(150))) void end101()
{
    printf(" World!-101\n");
}

static  __attribute__((destructor(151))) void end102()
{
    printf(" World!-102\n");
}

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

    return EXIT_SUCCESS;
}      

[root@workstation2017 ~]# ./test.out
Hello-101
Hello-102
 World!-102
 World!-101      

對齊屬性：

__attrubte__ ((packed)) 的作用就是告訴編譯器取消結構在編譯過程中的優化對齊,按照實際占用位元組數進行對齊

[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 0;
}      

[root@workstation2017 ~]# ./test.xx
sizeof str = 5
sizeof str_struct = 5
sizeof str_temp = 6
sizeof str_nopacked = 6      

變量占記憶體大小 x64 x32差别：

變量的長度，在不同的系統之間會有差别，64位Linux系統和32位Linux系統中，幾種常見C語言變 量的長度：
              short    int    long    long long    ptr    time_t
32位           2         4       4          8      4        4
64位           2         4       8          8      8        8