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用途

在多线程访问可变变量时，是非线程安全的。可能导致程序崩溃。此时，可以通过使用信号量（semaphore）技术，保证多线程处理某段代码时，后面线程等待前面线程执行，保证了多线程的安全性。使用方法记两个就行了，一个是wait（dispatch_semaphore_wait），一个是signal（dispatch_semaphore_signal）。根据dispatch_semaphore_wait的参数类型提示去创建信号量（dispatch_semaphore_t）即可。

不用semaphore时code:

#import 
   
    int main(int argc, const char * argv[]) {    @autoreleasepool {        // insert code here...        NSLog(@"Hello, World!");                NSMutableArray *lArrM = [NSMutableArray array];        for (int i = 0; i < 100; ++i) {            dispatch_async(dispatch_get_global_queue(0, 0), ^{                [lArrM addObject:@(i)];            });        }    }    return 0;}
   

运行结果

SemephoreTest[3665:130800] Hello, World!SemephoreTest(3665,0x700001daa000) malloc: *** error for object 0x10077b2e0: pointer being freed was not allocatedSemephoreTest(3665,0x700001daa000) malloc: *** set a breakpoint in malloc_error_break to debugSemephoreTest(3665,0x700001e2d000) malloc: *** error for object 0x102b00340: pointer being freed was not allocatedSemephoreTest(3665,0x700001e2d000) malloc: *** set a breakpoint in malloc_error_break to debugSemephoreTest[3665:130800] lArrM.count:0(lldb)

用semaphore时code:

////  ViewController.m//  SemophoreTest////  Created by LongMa on 2019/8/22.//  Copyright © 2019 . All rights reserved.//#import "ViewController.h"@interface ViewController ()@end@implementation ViewController- (void)viewDidLoad {    [super viewDidLoad];        NSMutableArray *lArrM = [NSMutableArray array];    dispatch_semaphore_t lSema = dispatch_semaphore_create(1);//参数1，代表只能有1个线程同时访问被限制的代码。    for (int i = 0; i < 1000; ++i) {        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{            dispatch_semaphore_wait(lSema, DISPATCH_TIME_FOREVER);//wait使信号量的值-1，如果结果为0，其他线程来访问时，需要等待。直到信号量的值大于0            [lArrM addObject:@(i)];            NSLog(@"lArrM.count:%zd",lArrM.count);            dispatch_semaphore_signal(lSema);///wait使信号量的值+1，必须和dispatch_semaphore_wait配对使用。        });    }        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{        NSLog(@"final  delayed lArrM.count:%zd",lArrM.count);    });}@end

运行结果（用commandlineTool运行代码时，log存在不完整问题）：

... SemophoreTest[9092:877715] lArrM.count:999 SemophoreTest[9092:877730] lArrM.count:1000 SemophoreTest[9092:877696] final  delayed lArrM.count:1000