gcc __attribute__属性demo

//aligned.c
#include 
#include 

#ifdef ON
//关键字__attribute__也可以对结构体（struct）或共用体（union）进行属性设置。大致有六个参数值可以被设定，
//即：aligned, packed, transparent_union, unused, deprecated 和 may_alias。
struct p
{
    int a;
    char b;
    char c;
}__attribute__((aligned(4))) pp;

struct q
{
    int a;
    char b;
    struct p qn;
    char c;
}__attribute__((aligned(8))) qq;

#else
struct p
{
    int a;
    char b;
    char c;
} pp;

struct q
{
    int a;
    char b;
    struct p qn;
    char c;
} qq;
#endif


//__attribute__修饰变量属性(Variable Attribute)
struct test
{
    char a;
    int x[2] __attribute__ ((packed));
};


//gcc -Wall aligned.c -D ON -o a2
//gcc -Wall aligned.c -o a1
int main()
{
    printf("sizeof(int)=%d,sizeof(short)=%d.sizeof(char)=%d\n",sizeof(int),sizeof(short),sizeof(char));
    printf("pp=%d,qq=%d \n", sizeof(pp),sizeof(qq));
    return 0;
}

//const.c
#include 
#include 


#ifdef ON
    extern int square(int n) __attribute__((const));

#else
    extern int square(int n);
#endif

//编译对比
//gcc -Wall const.c -D ON
//gcc -Wall const.c
int main()
{
    int total=0;
    int i=0;
    for (i = 0; i < 100; i++ )
    {
            total += square(5) + i;
    }
    printf("total=%d\n",total);
    return 0;
}

//constructor_destructor.c
#include 
#include 


#ifdef ON
static void start(void) __attribute__ ((constructor));
static void stop(void) __attribute__ ((destructor));
#else
static void start(void);
static void stop(void);
#endif

void start(void)
{
    printf("hello world!\n");
}
void stop(void)
{
    printf("goodbye world!\n");
}

//编译运行对比
//gcc -Wall noreturn.c -D ON
//gcc -Wall noreturn.c
int main(int argc, char *argv[])
{
    printf("start == %p\n", start);
    printf("stop == %p\n", stop);
    return 0;
}

//format-myprintf.c
#include 
#include 

#ifdef ON
	//告诉编译器以printf的方式去检查该函数
	extern void myprint(const char *format,...) __attribute__((format(printf,1,2)));
#else
	extern void myprint(const char *format,...);
#endif

void test()
{
    myprint("i=%d\n",6);
    myprint("i=%s\n",6);
    myprint("i=%s\n","abc");
    myprint("%s,%d,%d\n",1,2);
}


//gcc -Wall format-myprintf.c -D ON
//gcc -Wall format-myprintf.c 

int main()
{
    test();
    return 0;
}

//noreturn.c
#include 
#include 


#ifdef ON
    extern void myexit() __attribute__((noreturn));
#else
    extern void myexit() ;
#endif

int test(int n)
{
    if ( n > 0 )
    {
        myexit();
        /* 程序不可能到达这里*/
    }
    else
        return 0;
}
//https://gcc.gnu.org/onlinedocs/gcc-4.0.0/gcc/Function-Attributes.html
//__attribute__ 告诉编译器做额外的事情。

//编译对比
//gcc -Wall noreturn.c -D ON
//gcc -Wall noreturn.c
int main()
{
    test(3);
    return 0;
}

#include 
#include 


#ifdef ON

extern "C" int
pthread_create (pthread_t*,
                const pthread_attr_t*,
                void* (*)(void*),
                void*) __attribute__ ((weak));

extern "C" int
pthread_mutex_init (pthread_mutex_t*,
                    const pthread_mutexattr_t*) __attribute__ ((weak));

extern "C" int
pthread_mutex_lock (pthread_mutex_t*) __attribute__ ((weak));

extern "C" int
pthread_mutex_unlock (pthread_mutex_t*) __attribute__ ((weak));

extern "C" int
pthread_mutex_destroy (pthread_mutex_t*) __attribute__ ((weak));

#else
extern "C" int
pthread_create (pthread_t*,
                const pthread_attr_t*,
                void* (*)(void*),
                void*);
extern "C" int
pthread_mutex_init (pthread_mutex_t*,
                    const pthread_mutexattr_t*);

extern "C" int
pthread_mutex_lock (pthread_mutex_t*);

extern "C" int
pthread_mutex_unlock (pthread_mutex_t*);

extern "C" int
pthread_mutex_destroy (pthread_mutex_t*);

#endif

#define __init         __attribute__ ((__section__ (".text.init")))
#define __exit          __attribute__ ((unused, __section__(".text.exit")))
#define __initdata      __attribute__ ((__section__ (".data.init")))
#define __exitdata      __attribute__ ((unused, __section__ (".data.exit")))
#define __initsetup     __attribute__ ((unused,__section__ (".setup.init")))
#define __init_call     __attribute__ ((unused,__section__ (".initcall.init")))
#define __exit_call     __attribute__ ((unused,__section__ (".exitcall.exit")))

extern void foobar (void) __attribute__ ((section ("bar")));


void add(int a,int b) __attribute__ ((visibility ("protected")));

void sub(int a,int b) __attribute__ ((visibility ("hidden")));

//关键字__attribute__ 也可以对结构体（struct ）或共用体（union ）进行属性设置。大致有六个参数值可以被设定，
//即：aligned, packed, transparent_union, unused, deprecated 和 may_alias 。
struct STudent{

    int age;
    char name[10] __attribute__ ((aligned(8)));
};

//https://gcc.gnu.org/onlinedocs/gcc-4.0.0/gcc/Function-Attributes.html#Function-Attributes
int old_fn () __attribute__ ((deprecated));

void f () __attribute__ ((weak, alias ("__f")));

void mul() __attribute__ ((dllexport));
void mul2() __attribute__ ((dllimport));

extern char * my_dgettext (char *my_domain, const char *my_format)  __attribute__ ((format_arg (2)));

void f2 () __attribute__ ((interrupt ("IRQ")));

//Use naked attribute on the ARM, AVR, C4x and IP2K ports to indicate that the specified function does not need prologue/
//epilogue sequences generated by the compiler.It is up to the programmer to provide these sequences.
void mydiv() __attribute__ ((naked));

void mydiv2() __attribute__ ((noinline));


__attribute__ ((noinline,__annotate__(("nofcf"))))
__attribute__ ((noinline,__annotate__(("nocxf"))))
__attribute__ ((noinline,__annotate__(("nosac")))) void demo();


//编译对比
//gcc -Wall weak.c -D ON
//gcc -Wall weak.c
int main()
{

    return 0;
}