有很多初始化方法我们通常都是在applicationDidFinishLaunching里一个个调用，那么有没有办法像__attribute__((constructor))，能够自动调用被修饰的函数？

可以通过指定函数所在section的方式，然后获取section开头去逐个调用。但是这种方法有个问题，你只能获取到第一个函数的开头，但是你不知道它的大小，也就没有办法去依次获取了。

__attribute__((used, section("__TEXT, CustomInit")))
void custom1() {
    NSLog(@"custom1");
}

__attribute__((used, section("__TEXT, CustomInit")))
void custom2() {
    NSLog(@"custom2");
}

- (void)callCustom
{
    Method orginalMethod = class_getClassMethod([self class], _cmd);
    IMP imp = method_getImplementation(orginalMethod);
    Dl_info info;
    if (dladdr((void *)imp, &info)) {
        printf("dli_fname: %s\n", info.dli_fname);
        printf("dli_sname: %s\n", info.dli_sname);
        printf("dli_fbase: %p\n", info.dli_fbase);
        printf("dli_saddr: %p\n", info.dli_saddr);
    } else {
        printf("error: can't find that symbol.\n");
    }
    
    unsigned long funcSize = 0;
    //CustomFunc段的开始
    uint8_t *funcStart = getsectiondata((struct mach_header_64 *)info.dli_fbase, "__TEXT", "CustomInit", &funcSize);
    ((void(*)())funcStart)();
}

所以可以通过声明全局或者静态指针变量，去持有这些函数，把这些指针放到__DATA里自定义的一个段里，因为指针大小都是8字节，每次跳8字节就能访问到下一个指针了。

void custom1() {
    NSLog(@"custom1");
}

void(*customVar1)(void) __attribute__((used, section("__TEXT, CustomInit")))  = custom1;

void custom2() {
    NSLog(@"custom2");
}

void(*customVar2)(void) __attribute__((used, section("__TEXT, CustomInit")))  = custom1;


- (void)callCustom
{
    Method orginalMethod = class_getClassMethod([self class], _cmd);
    IMP imp = method_getImplementation(orginalMethod);
    Dl_info info;
    if (dladdr((void *)imp, &info)) {
        printf("dli_fname: %s\n", info.dli_fname);
        printf("dli_sname: %s\n", info.dli_sname);
        printf("dli_fbase: %p\n", info.dli_fbase);
        printf("dli_saddr: %p\n", info.dli_saddr);
    } else {
        printf("error: can't find that symbol.\n");
    }
    unsigned long size = 0;
    uint8_t *start = getsectiondata((struct mach_header_64 *)info.dli_fbase, "__DATA", "CustomInit", &size);
        int funcCount = size/sizeof(void *);
    for (int i=0;i<funcCount; i++) {
        //这里要用指针的指针
        void(**f)() = (void(**)())start + i;
        (*f)();
    }
}

这样声明函数和变量太麻烦了，我们可以使用宏来帮忙。

#define CustomInitFuncBegin(funcName) \
void funcName() {

#define CustomInitFuncEnd(funcName) }\
static void (*funcName##_var)() __attribute__((used, section("__DATA, CustomInit")))  = funcName;


CustomInitFuncBegin(init1)
NSLog("测试init1\n");
CustomInitFuncEnd(init1)

CustomInitFuncBegin(init2);
NSLog("测试init2\n");
CustomInitFuncEnd(init2);


+ (void)callCustom
{
    Method orginalMethod = class_getClassMethod([self class], _cmd);
    IMP imp = method_getImplementation(orginalMethod);
    Dl_info info;
    if (dladdr((void *)imp, &info)) {
        printf("dli_fname: %s\n", info.dli_fname);
        printf("dli_sname: %s\n", info.dli_sname);
        printf("dli_fbase: %p\n", info.dli_fbase);
        printf("dli_saddr: %p\n", info.dli_saddr);
    } else {
        printf("error: can't find that symbol.\n");
    }
    unsigned long size = 0;
    uint8_t *start = getsectiondata((struct mach_header_64 *)info.dli_fbase, "__DATA", "CustomInit", &size);
    int funcCount = size/sizeof(void *);
    for (int i=0;i<funcCount; i++) {
        void(**f)() = (void(**)())start + i;
        (*f)();
    }
}

参考：

https://everettjf.github.io/2017/03/06/a-method-of-delay-premain-code/