Semaphore回顧

用途

在多線程訪問可變變量時，是非線程安全的。可能致使程序崩潰。此時，能夠經過使用信號量（semaphore）技術，保證多線程處理某段代碼時，後面線程等待前面線程執行，保證了多線程的安全性。使用方法記兩個就好了，一個是wait（dispatch_semaphore_wait），一個是signal（dispatch_semaphore_signal）。根據dispatch_semaphore_wait的參數類型提示去建立信號量（dispatch_semaphore_t）便可。
感受原理跟PV操做一模一樣。

不用semaphore時code:

#import <Foundation/Foundation.h>

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;
}

運行結果

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

用semaphore時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);//參數1，表明只能有1個線程同時訪問被限制的代碼。
    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使信號量的值-1，若是結果爲0，其餘線程來訪問時，須要等待。直到信號量的值大於0
            [lArrM addObject:@(i)];
            NSLog(@"lArrM.count:%zd",lArrM.count);
            dispatch_semaphore_signal(lSema);///wait使信號量的值+1，必須和dispatch_semaphore_wait配對使用。
        });
    }
    
    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

運行結果（用commandlineTool運行代碼時，log存在不完整問題）：

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