gcc attribute 之 constructor

The function modified with constructor will be executed before the main function is executed, and the priority can be set, and the destructor can be executed after the main function.

#include <stdlib.h>
#include <malloc.h>
#include <string.h>
#include <unistd.h>
#include <time.h>

int g_num;

int getRand(int max)
{
    int num = 0;
    static int flag = 0;
    if(!flag) {
        srand((unsigned)time(NULL));
        flag = 1;
    }
    num = rand()%max;

    num++;
	printf("getRand %d\n",num);
	return num;
}

#define printh()  printf("in func %s.%d  g_num=%d\n",__func__,__LINE__,g_num)

static  void __attribute__ ((constructor)) run_before_main_1()
{
    printh();
}

static  void __attribute__ ((constructor(101))) init1()
{
    printh();
}

static  void __attribute__ ((constructor(102))) init2()
{
    printh();
    g_num = getRand(10);
    printh();
}

static  void __attribute__ ((constructor(103))) init3()
{
    printh();
}

static  void __attribute__ ((destructor)) run_after_main()
{
    printh();
}

static  void __attribute__ ((constructor)) run_before_main_2()
{
    printh();
}

int main(void)
{
    printh();
    g_num = 20;

	printf("main done!\n");
    return 0;
}

operation result

 

 

If the priority of destructor is set, it will be executed before main according to the priority of the function modified by constructor

#include <stdlib.h>
#include <malloc.h>
#include <string.h>
#include <unistd.h>
#include <time.h>

int g_num;

int getRand(int max)
{
    int num = 0;
    static int flag = 0;
    if(!flag) {
        srand((unsigned)time(NULL));
        flag = 1;
    }
    num = rand()%max;

    num++;
	printf("getRand %d\n",num);
	return num;
}

#define printh()  printf("in func %s.%d  g_num=%d\n",__func__,__LINE__,g_num)

static  void __attribute__ ((constructor)) run_before_main()
{
    printh();
}

static  void __attribute__ ((constructor(101))) init1()
{
    printh();
}

static  void __attribute__ ((constructor(102))) init2()
{
    printh();
    g_num = getRand(10);
    printh();
}

static  void __attribute__ ((constructor(103))) init3()
{
    printh();
}

static  void __attribute__ ((destructor)) run_after_main()
{
    printh();
}

static  void __attribute__ ((constructor(201))) exit_1()
{
    printh();
}

static  void __attribute__ ((constructor(202))) exit_2()
{
    printh();
}

static  void __attribute__ ((constructor(203))) exit_3()
{
    printh();
}

int main(void)
{
    printh();
    g_num = 20;

	printf("main done!\n");
    return 0;
}

operation result