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。
感觉原理跟PV操作如出一辙。
不用semaphore时code:
@H_502_7@#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;
}
运行结果
@H_502_7@ 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时code:
@H_502_7@//
// 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,^{
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）：
@H_502_7@...
SemophoreTest[9092:877715] lArrM.count:999
SemophoreTest[9092:877730] lArrM.count:1000
SemophoreTest[9092:877696] final delayed lArrM.count:1000
