C++标准库中的std::endl究竟做了什么？

﹏ヽ暗。殇╰゛Y 2023-06-13 03:24 65阅读 0赞

先抓出**std::endl**的源代码：

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 <td style="vertical-align:top;width:426.1pt;"> /** &nbsp;* &nbsp;@file &nbsp;ostream &nbsp;* &nbsp;@brief &nbsp;Write a newline and flush the stream. &nbsp;* &nbsp;* &nbsp;This manipulator is often mistakenly used when a simple newline is &nbsp;* &nbsp;desired, leading to poor buffering performance. &nbsp;See &nbsp;* &nbsp;http://gcc.gnu.org/onlinedocs/libstdc++/manual/bk01pt11ch25s02.html &nbsp;* &nbsp;for more on this subject. */ template&lt;typename _CharT, typename _Traits&gt; inline basic_ostream&lt;_CharT, _Traits&gt;&amp; endl(basic_ostream&lt;_CharT, _Traits&gt;&amp; __os) { &nbsp;&nbsp;return flush(__os.put(__os.widen('\n'))); } </td> 
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可以看到endl实际是名字空间std中的一个全局内联**函数**（记住std::endl是一个函数），它做了两件事：

1.  输出一个换行符（**为何要输出一个换行符，如果不输出会怎么样？**）；
2.  调用flush。

继续看flush这个函数（也是std名字空间内的全局内联函数）：

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 <td style="vertical-align:top;width:426.1pt;"> /** &nbsp;* &nbsp;@brief &nbsp;Flushes the output stream. &nbsp;* &nbsp;* &nbsp;This manipulator simply calls the stream's @c flush() member function. */ template&lt;typename _CharT, typename _Traits&gt; inline basic_ostream&lt;_CharT, _Traits&gt;&amp; flush(basic_ostream&lt;_CharT, _Traits&gt;&amp; __os) { &nbsp;&nbsp;return __os.flush(); // 注意这里的os不是操作系统的意思，而是类basic_ostream的缩写 } </td> 
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进一步查看std::basic\_ostream::flush的代码：

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 <td style="vertical-align:top;width:426.1pt;"> /** &nbsp;* &nbsp;@file &nbsp;ostream.tcc */ template&lt;typename _CharT, typename _Traits&gt; basic_ostream&lt;_CharT, _Traits&gt;&amp; basic_ostream&lt;_CharT, _Traits&gt;::flush() { &nbsp;&nbsp;// _GLIBCXX_RESOLVE_LIB_DEFECTS &nbsp;&nbsp;// DR 60. What is a formatted input function? &nbsp;&nbsp;// basic_ostream::flush() is *not* an unformatted output function. &nbsp;&nbsp;ios_base::iostate __err = ios_base::goodbit; &nbsp; &nbsp;&nbsp;__try &nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;// 刷新发生在pubsync，底层调用的是LIBC库函数fflush &nbsp;&nbsp;&nbsp;&nbsp;if (this-&gt;rdbuf() &amp;&amp; this-&gt;rdbuf()-&gt;pubsync() == -1) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;__err |= ios_base::badbit; &nbsp;&nbsp;} &nbsp;&nbsp;__catch(__cxxabiv1::__forced_unwind&amp;) &nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;this-&gt;_M_setstate(ios_base::badbit); &nbsp;&nbsp;&nbsp;&nbsp;__throw_exception_again; &nbsp;&nbsp;} &nbsp;&nbsp;__catch(...) &nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;this-&gt;_M_setstate(ios_base::badbit); &nbsp;&nbsp;} &nbsp;&nbsp;if (__err) &nbsp;&nbsp;&nbsp;&nbsp;this-&gt;setstate(__err); &nbsp;&nbsp;return *this; } </td> 
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下段小代码，可以看到两个帮助释疑的调用栈，：

写换符符：

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 <td style="vertical-align:top;width:426.1pt;"> #0 &nbsp;0x00007ffff72e0840 in write&nbsp;() from /lib64/libc.so.6 #1 &nbsp;0x00007ffff726cfb3 in _IO_new_file_write () from /lib64/libc.so.6 #2 &nbsp;0x00007ffff726e41c in __GI__IO_do_write () from /lib64/libc.so.6 #3 &nbsp;0x00007ffff726e7f3 in __GI__IO_file_overflow () from /lib64/libc.so.6 #4 &nbsp;0x00007ffff726ac49 in putc&nbsp;() from /lib64/libc.so.6 #5 &nbsp;0x00007ffff7b694c6 in std::ostream::put(char) () from /lib64/libstdc++.so.6 #6 &nbsp;0x00007ffff7b69712 in std::basic_ostream&lt;char, std::char_traits&lt;char&gt; &gt;&amp; std::endl&lt;char, std::char_traits&lt;char&gt; &gt;(std::basic_ostream&lt;char, std::char_traits&lt;char&gt; &gt;&amp;) () &nbsp;&nbsp;&nbsp;from /lib64/libstdc++.so.6 #7 &nbsp;0x0000000000400803 in main () at xxx.cpp:3 </td> 
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刷新流：

实际上，还可以看到更多，如全局对象std::cout、std::cerr和std::clog等的析构：

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 <td style="vertical-align:top;width:426.1pt;"> #0 &nbsp;0x00007ffff7262030 in fflush&nbsp;() from /lib64/libc.so.6 #1 &nbsp;0x00007ffff7b68d5e in std::ostream::flush() () from /lib64/libstdc++.so.6 #2 &nbsp;0x00007ffff7b41bc8 in std::ios_base::Init::~Init() () from /lib64/libstdc++.so.6 #3 &nbsp;0x00007ffff722fa69 in __run_exit_handlers () from /lib64/libc.so.6 #4 &nbsp;0x00007ffff722fab5 in exit&nbsp;() from /lib64/libc.so.6 #5 &nbsp;0x00007ffff7218c0c in __libc_start_main () from /lib64/libc.so.6 #6 &nbsp;0x0000000000400729 in _start () </td> 
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Init析构的同时析构cout等：

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 <td style="vertical-align:top;width:426.1pt;"> /** &nbsp;* &nbsp;@file &nbsp;ios_init.cc */ ios_base::Init::~Init() { &nbsp;&nbsp;// Be race-detector-friendly. &nbsp;For more info see bits/c++config. &nbsp;&nbsp;_GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&amp;_S_refcount); &nbsp;&nbsp;if (__gnu_cxx::__exchange_and_add_dispatch(&amp;_S_refcount, -1) == 2) &nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;_GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&amp;_S_refcount); &nbsp;&nbsp;&nbsp;&nbsp;// Catch any exceptions thrown by basic_ostream::flush() &nbsp;&nbsp;&nbsp;&nbsp;__try &nbsp;&nbsp;&nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Flush standard output streams as required by 27.4.2.1.6 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;cout.flush(); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;cerr.flush(); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;clog.flush(); &nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;#ifdef _GLIBCXX_USE_WCHAR_T &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;wcout.flush(); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;wcerr.flush(); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;wclog.flush(); &nbsp;&nbsp;#endif &nbsp;&nbsp;&nbsp;&nbsp;} &nbsp;&nbsp;&nbsp;&nbsp;__catch(...) &nbsp;&nbsp;&nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;} &nbsp;&nbsp;} } </td> 
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文件ios\_init.cc其它相关函数源码：

1.  类std::ios\_base::Init的构造函数

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 <td style="vertical-align:top;width:426.1pt;"> ios_base::Init::Init() { &nbsp;&nbsp;if (__gnu_cxx::__exchange_and_add_dispatch(&amp;_S_refcount, 1) == 0) &nbsp;// &nbsp;防止重复初始化 &nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;// Standard streams default to synced with "C" operations. &nbsp;&nbsp;&nbsp;&nbsp;_S_synced_with_stdio = true; &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;new (&amp;buf_cout_sync) stdio_sync_filebuf&lt;char&gt;(stdout); &nbsp;&nbsp;&nbsp;&nbsp;new (&amp;buf_cin_sync) stdio_sync_filebuf&lt;char&gt;(stdin); &nbsp;&nbsp;&nbsp;&nbsp;new (&amp;buf_cerr_sync) stdio_sync_filebuf&lt;char&gt;(stderr); &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;// The standard streams are constructed once only and never destroyed. &nbsp;&nbsp;&nbsp;&nbsp;new (&amp;cout) ostream(&amp;buf_cout_sync); &nbsp;&nbsp;&nbsp;&nbsp;new (&amp;cin) istream(&amp;buf_cin_sync); &nbsp;&nbsp;&nbsp;&nbsp;new (&amp;cerr) ostream(&amp;buf_cerr_sync); &nbsp;&nbsp;&nbsp;&nbsp;new (&amp;clog) ostream(&amp;buf_cerr_sync); &nbsp;&nbsp;&nbsp;&nbsp;cin.tie(&amp;cout); &nbsp;&nbsp;&nbsp;&nbsp;cerr.setf(ios_base::unitbuf); &nbsp;&nbsp;&nbsp;&nbsp;// _GLIBCXX_RESOLVE_LIB_DEFECTS &nbsp;&nbsp;&nbsp;&nbsp;// 455. cerr::tie() and wcerr::tie() are overspecified. &nbsp;&nbsp;&nbsp;&nbsp;cerr.tie(&amp;cout); &nbsp; #ifdef _GLIBCXX_USE_WCHAR_T &nbsp;&nbsp;&nbsp;&nbsp;new (&amp;buf_wcout_sync) stdio_sync_filebuf&lt;wchar_t&gt;(stdout); &nbsp;&nbsp;&nbsp;&nbsp;new (&amp;buf_wcin_sync) stdio_sync_filebuf&lt;wchar_t&gt;(stdin); &nbsp;&nbsp;&nbsp;&nbsp;new (&amp;buf_wcerr_sync) stdio_sync_filebuf&lt;wchar_t&gt;(stderr); &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;new (&amp;wcout) wostream(&amp;buf_wcout_sync); &nbsp;&nbsp;&nbsp;&nbsp;new (&amp;wcin) wistream(&amp;buf_wcin_sync); &nbsp;&nbsp;&nbsp;&nbsp;new (&amp;wcerr) wostream(&amp;buf_wcerr_sync); &nbsp;&nbsp;&nbsp;&nbsp;new (&amp;wclog) wostream(&amp;buf_wcerr_sync); &nbsp;&nbsp;&nbsp;&nbsp;wcin.tie(&amp;wcout); &nbsp;&nbsp;&nbsp;&nbsp;wcerr.setf(ios_base::unitbuf); &nbsp;&nbsp;&nbsp;&nbsp;wcerr.tie(&amp;wcout); #endif &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;// NB: Have to set refcount above one, so that standard &nbsp;&nbsp;&nbsp;&nbsp;// streams are not re-initialized with uses of ios_base::Init &nbsp;&nbsp;&nbsp;&nbsp;// besides &lt;iostream&gt; static object, ie just using &lt;ios&gt; with &nbsp;&nbsp;&nbsp;&nbsp;// ios_base::Init objects. &nbsp;&nbsp;&nbsp;&nbsp;__gnu_cxx::__atomic_add_dispatch(&amp;_S_refcount, 1); &nbsp;&nbsp;} } </td> 
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1.  全局函数ios\_base::sync\_with\_stdio

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 <td style="vertical-align:top;width:426.1pt;"> bool ios_base::sync_with_stdio(bool __sync) { &nbsp;&nbsp;// _GLIBCXX_RESOLVE_LIB_DEFECTS &nbsp;&nbsp;// 49. &nbsp;Underspecification of ios_base::sync_with_stdio &nbsp;&nbsp;bool __ret = ios_base::Init::_S_synced_with_stdio; &nbsp; &nbsp;&nbsp;// Turn off sync with C FILE* for cin, cout, cerr, clog iff &nbsp;&nbsp;// currently synchronized. &nbsp;&nbsp;if (!__sync &amp;&amp; __ret) &nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;// Make sure the standard streams are constructed. &nbsp;&nbsp;&nbsp;&nbsp;ios_base::Init __init; &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;ios_base::Init::_S_synced_with_stdio = __sync; &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;// Explicitly call dtors to free any memory that is &nbsp;&nbsp;&nbsp;&nbsp;// dynamically allocated by filebuf ctor or member functions, &nbsp;&nbsp;&nbsp;&nbsp;// but don't deallocate all memory by calling operator delete. &nbsp;&nbsp;&nbsp;&nbsp;buf_cout_sync.~stdio_sync_filebuf&lt;char&gt;(); &nbsp;&nbsp;&nbsp;&nbsp;buf_cin_sync.~stdio_sync_filebuf&lt;char&gt;(); &nbsp;&nbsp;&nbsp;&nbsp;buf_cerr_sync.~stdio_sync_filebuf&lt;char&gt;(); &nbsp; #ifdef _GLIBCXX_USE_WCHAR_T &nbsp;&nbsp;&nbsp;&nbsp;buf_wcout_sync.~stdio_sync_filebuf&lt;wchar_t&gt;(); &nbsp;&nbsp;&nbsp;&nbsp;buf_wcin_sync.~stdio_sync_filebuf&lt;wchar_t&gt;(); &nbsp;&nbsp;&nbsp;&nbsp;buf_wcerr_sync.~stdio_sync_filebuf&lt;wchar_t&gt;(); #endif &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;// Create stream buffers for the standard streams and use &nbsp;&nbsp;&nbsp;&nbsp;// those buffers without destroying and recreating the &nbsp;&nbsp;&nbsp;&nbsp;// streams. &nbsp;&nbsp;&nbsp;&nbsp;new (&amp;buf_cout) stdio_filebuf&lt;char&gt;(stdout, ios_base::out); &nbsp;&nbsp;&nbsp;&nbsp;new (&amp;buf_cin) stdio_filebuf&lt;char&gt;(stdin, ios_base::in); &nbsp;&nbsp;&nbsp;&nbsp;new (&amp;buf_cerr) stdio_filebuf&lt;char&gt;(stderr, ios_base::out); &nbsp;&nbsp;&nbsp;&nbsp;cout.rdbuf(&amp;buf_cout); &nbsp;&nbsp;&nbsp;&nbsp;cin.rdbuf(&amp;buf_cin); &nbsp;&nbsp;&nbsp;&nbsp;cerr.rdbuf(&amp;buf_cerr); &nbsp;&nbsp;&nbsp;&nbsp;clog.rdbuf(&amp;buf_cerr); &nbsp; #ifdef _GLIBCXX_USE_WCHAR_T &nbsp;&nbsp;new (&amp;buf_wcout) stdio_filebuf&lt;wchar_t&gt;(stdout, ios_base::out); &nbsp;&nbsp;new (&amp;buf_wcin) stdio_filebuf&lt;wchar_t&gt;(stdin, ios_base::in); &nbsp;&nbsp;new (&amp;buf_wcerr) stdio_filebuf&lt;wchar_t&gt;(stderr, ios_base::out); &nbsp;&nbsp;wcout.rdbuf(&amp;buf_wcout); &nbsp;&nbsp;wcin.rdbuf(&amp;buf_wcin); &nbsp;&nbsp;wcerr.rdbuf(&amp;buf_wcerr); &nbsp;&nbsp;wclog.rdbuf(&amp;buf_wcerr); #endif &nbsp;&nbsp;} &nbsp;&nbsp;return __ret; } </td> 
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前面提到的endl为何要输出一个换行符，这是因为fflush只是将数据刷到标准输出，标准输出本身也是有缓存的，而且默认是**行缓存**（**\_IO****L****BF** line buffered）。

是否可将标准输出设置为**全缓存**（**\_IO****F****BF** fully buffered）了？答案是可以，执行下列代码即可得到结果。

是否行缓存和全缓存，与是否为字符设备（Character Device，只能顺序读取）或块设备（Block Device，支持随机存取）无关。

将标准输出定向到普通文件，则缓存类型同普通文件，即默认全缓存：

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 <td style="vertical-align:top;width:426.1pt;"> $ cat eee.cpp #include &lt;fcntl.h&gt; #include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;sys/types.h&gt; #include &lt;sys/stat.h&gt; #include &lt;unistd.h&gt; int main(int argc, char* argv[]) { &nbsp;&nbsp;char buf[BUFSIZ]; &nbsp;&nbsp;int fd = open("/tmp/xxx.txt", O_WRONLY|O_CREAT|O_TRUNC, S_IRWXU); &nbsp;&nbsp;if (fd == -1) { &nbsp;&nbsp;&nbsp;&nbsp;perror("open"); &nbsp;&nbsp;&nbsp;&nbsp;return 1; &nbsp;&nbsp;} else { &nbsp;&nbsp;&nbsp;&nbsp;int fdnew = dup2(fd, STDOUT_FILENO); // 让标准输出指向文件 &nbsp;&nbsp;&nbsp;&nbsp;if (fdnew == -1) { &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;perror("dup2"); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return 1; &nbsp;&nbsp;&nbsp;&nbsp;} else { &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;printf("123\n"); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;getchar(); &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return 0; &nbsp;&nbsp;&nbsp;&nbsp;} &nbsp;&nbsp;} } </td> 
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匿名管道默认是全缓存，可用下列代码验证：

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 <td style="vertical-align:top;width:426.1pt;"> $ cat xxx.cpp #include &lt;fcntl.h&gt; #include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;sys/types.h&gt; #include &lt;sys/stat.h&gt; #include &lt;sys/wait.h&gt; #include &lt;unistd.h&gt; int main(int argc, char* argv[]) { &nbsp;&nbsp;int fd[2]; &nbsp;&nbsp;char buf[BUFSIZ]; &nbsp;&nbsp;if (pipe(fd) == -1) { &nbsp;&nbsp;&nbsp;&nbsp;perror("pipe"); &nbsp;&nbsp;&nbsp;&nbsp;exit(1); &nbsp;&nbsp;} &nbsp; &nbsp;&nbsp;pid_t pid = fork(); &nbsp;&nbsp;if (pid == -1) { &nbsp;&nbsp;&nbsp;&nbsp;perror("fork"); &nbsp;&nbsp;&nbsp;&nbsp;exit(1); &nbsp;&nbsp;} else if (pid == 0) { // Child process &nbsp;&nbsp;&nbsp;&nbsp;dup2(fd[1], STDOUT_FILENO); &nbsp;&nbsp;&nbsp;&nbsp;//setvbuf(stdout, buf, _IOLBF, BUFSIZ); &nbsp;&nbsp;&nbsp;&nbsp;printf("hello\n"); &nbsp;&nbsp;&nbsp;&nbsp;getchar(); &nbsp;&nbsp;&nbsp;&nbsp;fflush(stdout); &nbsp;&nbsp;&nbsp;&nbsp;_exit(0); &nbsp;&nbsp;} else { // Parent process &nbsp;&nbsp;&nbsp;&nbsp;char msg[BUFSIZ]; &nbsp;&nbsp;&nbsp;&nbsp;int n = read(fd[0], msg, sizeof(msg)-1); &nbsp;&nbsp;&nbsp;&nbsp;if (n == -1) { &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;perror("Parent read"); &nbsp;&nbsp;&nbsp;&nbsp;} else { &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;msg[n] = '\0'; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;fprintf(stdout, "(%d)%s\n", n, msg); &nbsp;&nbsp;&nbsp;&nbsp;} &nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;wait(NULL); &nbsp;&nbsp;&nbsp;&nbsp;exit(0); &nbsp;&nbsp;} } </td> 
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有名管道和套接字（包含UNIX套接字也是全缓存），附问题：如何在GDB中调试跟踪std::cout？