每個(gè)程序啟動(dòng)后都有標(biāo)準(zhǔn)輸入(standard in)，標(biāo)準(zhǔn)輸出(standard out)還有標(biāo)注出錯(cuò)(standard error)辨嗽。
要說(shuō)明的是 macOS 里的「終端(Terminal)」應(yīng)用和標(biāo)準(zhǔn)輸入(standard in)和標(biāo)準(zhǔn)輸出(standard out)沒(méi)有什么必然聯(lián)系世落。如果你認(rèn)為他們可能存在什么聯(lián)系的話，那可能是因?yàn)槟銊傞_始學(xué) c 程序的時(shí)候經(jīng)常在終端里運(yùn)行程序糟需，誤以為終端的輸出就是你的標(biāo)準(zhǔn)輸出屉佳。
我們?cè)趺纯礃?biāo)準(zhǔn)輸出呢？
示例程序:
NSFileHandle *stdOutput = NSFileHandle.fileHandleWithStandardOutput;
NSData *outputData = [@"fan's world\n" dataUsingEncoding:NSUTF8StringEncoding];
[stdOutput writeData:outputData];
這段程序可以在不同的環(huán)境下運(yùn)行
- 如果是用 Xcode 調(diào)試的話洲押，終端輸出默認(rèn)就是 Xcode 的調(diào)試終端武花。
- 如果是通過(guò)「終端(Terminal)」啟動(dòng)編譯好的應(yīng)用的話，那可以在 「終端(Terminal)」里看到終端輸出
是不是覺(jué)得這么輸出很眼熟杈帐，跟 printf 和 nslog 啥的輸入很類似呢体箕？后續(xù)會(huì)說(shuō)到這個(gè)問(wèn)題。
但是我們觀察到一個(gè)現(xiàn)象就是標(biāo)準(zhǔn)輸出(standard out)的東西并不會(huì)出現(xiàn)在 macOS 的 「Console.app」 下挑童，也有人問(wèn)過(guò)類似的問(wèn)題:
- 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.
看起來(lái)是和系統(tǒng)有關(guān)系累铅。在 Mountain Lion 后需要采用建立在 syslog 或者 asl(apple system log) 上的 API。NSLog 就是這樣的..
那標(biāo)準(zhǔn)輸出(standard out)究竟去哪了……
我們可以看下 I/O stream 的官方說(shuō)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 流的本質(zhì)以及其說(shuō)明站叼，每個(gè)輸入輸出流都關(guān)聯(lián)了一個(gè)外部的物理設(shè)備…..
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.
所以娃兽，標(biāo)準(zhǔn)輸出應(yīng)該是打印到屏幕上了。那標(biāo)準(zhǔn)輸入呢尽楔，這里的話很明顯就是鍵盤了换薄。
再進(jìn)一步，printf 和 nslog 有啥區(qū)別呢?
我們可以觀察到 printf 打印的內(nèi)容并不會(huì)輸出到 macOS 的「Console.app」里翔试，但是 NSLog 會(huì)。
我們看一下 printf 的官方說(shuō)明
printf : Writes the results to the output stream stdout.
可以看到 printf 和標(biāo)準(zhǔn)輸出其實(shí)就是一回事兒...
/dev/stdout 是什么呢?
其實(shí)是一個(gè)設(shè)備文件复旬，是指向 /proc/self/fd/1 的鏈接文件垦缅。
參考地址 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.
人模狗樣兒的像個(gè)文件似的，其實(shí)不是驹碍。只是為了軟件和設(shè)備驅(qū)動(dòng)打交道而已壁涎。
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.
參考:
