Question

我从其中一个API找到了以下代码，我正在尝试用它来实时监控vm_stat 1：

BOOL terminated = NO;

-(void)realTimeMonitor
{

NSTask *task = [[NSTask alloc] init];
[task setLaunchPath:@"/bin/bash"];
[task setArguments:[NSArray arrayWithObjects:@"-c", @"/usr/bin/vm_stat 1", nil]]; //works fine

// [task setArguments:[NSArray arrayWithObjects:@"-c", @"/usr/bin/vm_stat 1 | /usr/bin/awk '{print $1}'", nil]]; not working

NSPipe *pipe = [NSPipe pipe];
NSPipe *errorPipe = [NSPipe pipe];
[task setStandardOutput:pipe];
[task setStandardError:errorPipe];

NSFileHandle *outFile = [pipe fileHandleForReading];
NSFileHandle *errFile = [errorPipe fileHandleForReading];


[task launch];
[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(terminated:)
                                             name:NSTaskDidTerminateNotification
                                           object:task];

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(outData:)
                                             name:NSFileHandleDataAvailableNotification
                                           object:outFile];

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(errData:)
                                             name:NSFileHandleDataAvailableNotification
                                           object:errFile];


[outFile waitForDataInBackgroundAndNotify];
[errFile waitForDataInBackgroundAndNotify];
while(!terminated)
{
    if (![[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]])
    {
        break;
    }
}
}

-(void) outData: (NSNotification *) notification
{
NSFileHandle *fileHandle = (NSFileHandle*) [notification object];
NSData *data = [fileHandle availableData];

if ([data length]) {

    NSString *currentString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
    // do something 
}

[fileHandle waitForDataInBackgroundAndNotify]; //Checks to see if data is available in a background thread.
}


-(void) errData: (NSNotification *) notification
{
NSLog(@"errData");
NSFileHandle *fileHandle = (NSFileHandle*) [notification object];
[fileHandle waitForDataInBackgroundAndNotify];
}

- (void) terminated: (NSNotification *)notification
{
NSLog(@"Task terminated");
[[NSNotificationCenter defaultCenter] removeObserver:self];
terminated =YES;
}

======

请查看setArguments行。第一行完全正常“/ usr / bin / vm_stat 1”，但是第二行不起作用“/ usr / bin / vm_stat 1 | / usr / bin / awk'{print $ 1}'”（没有输出at所有），但如果我在终端上运行它，它完全正常。那是为什么？

Answer 1

也许awk正在缓冲它的stdout流，因为它认为它没有将stdout流写入tty设备。

这可能最终导致完全死锁，因为如果vm_stat的stdin被填满且awk上没有消费者，awk可能无法写入其标准输出。的标准。

因此，请尝试使用其内置的awk函数刷新fflush()的标准输出。

[task setArguments:[NSArray arrayWithObjects:@"-c", @"/usr/bin/vm_stat 1 | /usr/bin/awk '{print $1; fflush();}'", nil]];

我能够使用（稍加修改）asynctask.m来运行您的代码。

为什么waitForDataInBackgroundAndNotify不能与具有多个参数的实时监视器一起使用？

1 个答案: