每个程序启动后都有标准输入(standard in)，标准输出(standard out)还有标注出错(standard error)。
要说明的是 macOS 里的「终端(Terminal)」应用和标准输入(standard in)和标准输出(standard out)没有什么必然联系。如果你认为他们可能存在什么联系的话，那可能是因为你刚开始学 c 程序的时候经常在终端里运行程序，误以为终端的输出就是你的标准输出。
我们怎么看标准输出呢？
示例程序:
NSFileHandle *stdOutput = NSFileHandle.fileHandleWithStandardOutput;
NSData *outputData = [@"fan's world\n" dataUsingEncoding:NSUTF8StringEncoding];
[stdOutput writeData:outputData];
这段程序可以在不同的环境下运行
- 如果是用 Xcode 调试的话，终端输出默认就是 Xcode 的调试终端。
- 如果是通过「终端(Terminal)」启动编译好的应用的话，那可以在 「终端(Terminal)」里看到终端输出
是不是觉得这么输出很眼熟，跟 printf 和 nslog 啥的输入很类似呢？后续会说到这个问题。
但是我们观察到一个现象就是标准输出(standard out)的东西并不会出现在 macOS 的 「Console.app」 下，也有人问过类似的问题:
- https://discussions.apple.com/thread/2323513?tstart=0
- https://stackoverflow.com/questions/2196302/where-is-stdout-for-a-mac-app
- https://stackoverflow.com/questions/13104588/how-to-get-stdout-into-console-app
答案:
Prior to Mountain Lion, all processes managed by launchd, including regular applications, had their stdout and stderr file descriptors forwarded to the system log. In Mountain Lion and above, stdout and stderr go nowhere for launchd managed applications. Only messages explicitly sent to the system log will end up there.
If you're writing an application and would like some output to show up in the console, then adopt an API built on syslog(3) or asl(3) instead. NSLog is one such API, and it has the advantage of logging to stderr too so you can easily see your output no matter how you've launched your application. If you'd like that functionality but want to use asl or syslog directly then you'll want to look in to the ASL_OPT_STDERR option to asl_open, and the LOG_PERROR option to open log respectively.
看起来是和系统有关系。在 Mountain Lion 后需要采用建立在 syslog 或者 asl(apple system log) 上的 API。NSLog 就是这样的..
那标准输出(standard out)究竟去哪了……
我们可以看下 I/O stream 的官方说明：
The <stdio.h> header provides generic file operation support and supplies functions with narrow character input/output capabilities.
I/O streams are objects of type FILE that can only be accessed and manipulated through pointers of type FILE* (Note: while it may be possible to create a local object of type FILE by dereferencing and copying a valid FILE*, using the address of such copy in the I/O functions is undefined behavior). Each stream is associated with an external physical device (file, standard input stream, printer, serial port, etc).
I/O 流的本质以及其说明，每个输入输出流都关联了一个外部的物理设备…..
I/O streams can be used for both unformatted and formatted input and output. They are locale-sensitive and may perform wide/multibyte conversions as necessary. All streams access the same locale object: the one most recently installed with setlocale.
所以，标准输出应该是打印到屏幕上了。那标准输入呢，这里的话很明显就是键盘了。
再进一步，printf 和 nslog 有啥区别呢?
我们可以观察到 printf 打印的内容并不会输出到 macOS 的「Console.app」里，但是 NSLog 会。
我们看一下 printf 的官方说明
printf : Writes the results to the output stream stdout.
可以看到 printf 和标准输出其实就是一回事儿...
/dev/stdout 是什么呢?
其实是一个设备文件，是指向 /proc/self/fd/1 的链接文件。
参考地址 https://stackoverflow.com/questions/30780780/difference-between-stdout-and-dev-stdout
Device-File :
In Unix-like operating systems, a device file or special file is an interface for a device driver that appears in a file system as if it were an ordinary file.** There are also special files in MS-DOS, OS/2, and Microsoft Windows.
作用: They allow software to interact with a device driver using standard input/output system calls, which simplifies many tasks and unifies user-space I/O mechanisms.
人模狗样儿的像个文件似的，其实不是。只是为了软件和设备驱动打交道而已。
On the other hand, /dev/stdout is a link to a special file on the procfs file system, which represents file descriptor 1 of the process using it1.
参考:
