C++标准库中的std::endl究竟做了什么？
先抓出std::endl的源代码：
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/** * @file ostream * @brief Write a newline and flush the stream. * * This manipulator is often mistakenly used when a simple newline is * desired, leading to poor buffering performance. See * http://gcc.gnu.org/onlinedocs/libstdc++/manual/bk01pt11ch25s02.html * for more on this subject. */ template<typename _CharT, typename _Traits> inline basic_ostream<_CharT, _Traits>& endl(basic_ostream<_CharT, _Traits>& __os) { return flush(__os.put(__os.widen('\n'))); } |
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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/** * @brief Flushes the output stream. * * This manipulator simply calls the stream's @c flush() member function. */ template<typename _CharT, typename _Traits> inline basic_ostream<_CharT, _Traits>& flush(basic_ostream<_CharT, _Traits>& __os) { 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 } |
进一步查看std::basic_ostream::flush的代码：
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/** * @file ostream.tcc */ template<typename _CharT, typename _Traits> basic_ostream<_CharT, _Traits>& basic_ostream<_CharT, _Traits>::flush() { // _GLIBCXX_RESOLVE_LIB_DEFECTS // DR 60. What is a formatted input function? // basic_ostream::flush() is *not* an unformatted output function. ios_base::iostate __err = ios_base::goodbit;
__try { // 刷新发生在pubsync，底层调用的是LIBC库函数fflush if (this->rdbuf() && this->rdbuf()->pubsync() == -1) __err |= ios_base::badbit; } __catch(__cxxabiv1::__forced_unwind&) { this->_M_setstate(ios_base::badbit); __throw_exception_again; } __catch(...) { this->_M_setstate(ios_base::badbit); } if (__err) this->setstate(__err); return *this; } |
下段小代码，可以看到两个帮助释疑的调用栈，：
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$ cat xxx.cpp #include <iostream> int main() { std::cout << std::endl; return 0; } |
写换符符：
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#0 0x00007ffff72e0840 in write () from /lib64/libc.so.6 #1 0x00007ffff726cfb3 in _IO_new_file_write () from /lib64/libc.so.6 #2 0x00007ffff726e41c in __GI__IO_do_write () from /lib64/libc.so.6 #3 0x00007ffff726e7f3 in __GI__IO_file_overflow () from /lib64/libc.so.6 #4 0x00007ffff726ac49 in putc () from /lib64/libc.so.6 #5 0x00007ffff7b694c6 in std::ostream::put(char) () from /lib64/libstdc++.so.6 #6 0x00007ffff7b69712 in std::basic_ostream<char, std::char_traits<char> >& std::endl<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&) () from /lib64/libstdc++.so.6 #7 0x0000000000400803 in main () at xxx.cpp:3 |
刷新流：
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#0 0x00007ffff7262030 in fflush () from /lib64/libc.so.6 #1 0x00007ffff7b68d5e in std::ostream::flush() () from /lib64/libstdc++.so.6 #2 0x0000000000400803 in main () at xxx.cpp:3 |
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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#0 0x00007ffff7262030 in fflush () from /lib64/libc.so.6 #1 0x00007ffff7b68d5e in std::ostream::flush() () from /lib64/libstdc++.so.6 #2 0x00007ffff7b41bc8 in std::ios_base::Init::~Init() () from /lib64/libstdc++.so.6 #3 0x00007ffff722fa69 in __run_exit_handlers () from /lib64/libc.so.6 #4 0x00007ffff722fab5 in exit () from /lib64/libc.so.6 #5 0x00007ffff7218c0c in __libc_start_main () from /lib64/libc.so.6 #6 0x0000000000400729 in _start () |
Init析构的同时析构cout等：
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/** * @file ios_init.cc */ ios_base::Init::~Init() { // Be race-detector-friendly. For more info see bits/c++config. _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_S_refcount); if (__gnu_cxx::__exchange_and_add_dispatch(&_S_refcount, -1) == 2) { _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_S_refcount); // Catch any exceptions thrown by basic_ostream::flush() __try { // Flush standard output streams as required by 27.4.2.1.6 cout.flush(); cerr.flush(); clog.flush();
#ifdef _GLIBCXX_USE_WCHAR_T wcout.flush(); wcerr.flush(); wclog.flush(); #endif } __catch(...) { } } } |
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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ios_base::Init::Init() { if (__gnu_cxx::__exchange_and_add_dispatch(&_S_refcount, 1) == 0) // 防止重复初始化 { // Standard streams default to synced with "C" operations. _S_synced_with_stdio = true;
new (&buf_cout_sync) stdio_sync_filebuf<char>(stdout); new (&buf_cin_sync) stdio_sync_filebuf<char>(stdin); new (&buf_cerr_sync) stdio_sync_filebuf<char>(stderr);
// The standard streams are constructed once only and never destroyed. new (&cout) ostream(&buf_cout_sync); new (&cin) istream(&buf_cin_sync); new (&cerr) ostream(&buf_cerr_sync); new (&clog) ostream(&buf_cerr_sync); cin.tie(&cout); cerr.setf(ios_base::unitbuf); // _GLIBCXX_RESOLVE_LIB_DEFECTS // 455. cerr::tie() and wcerr::tie() are overspecified. cerr.tie(&cout);
#ifdef _GLIBCXX_USE_WCHAR_T new (&buf_wcout_sync) stdio_sync_filebuf<wchar_t>(stdout); new (&buf_wcin_sync) stdio_sync_filebuf<wchar_t>(stdin); new (&buf_wcerr_sync) stdio_sync_filebuf<wchar_t>(stderr);
new (&wcout) wostream(&buf_wcout_sync); new (&wcin) wistream(&buf_wcin_sync); new (&wcerr) wostream(&buf_wcerr_sync); new (&wclog) wostream(&buf_wcerr_sync); wcin.tie(&wcout); wcerr.setf(ios_base::unitbuf); wcerr.tie(&wcout); #endif
// NB: Have to set refcount above one, so that standard // streams are not re-initialized with uses of ios_base::Init // besides <iostream> static object, ie just using <ios> with // ios_base::Init objects. __gnu_cxx::__atomic_add_dispatch(&_S_refcount, 1); } } |
2) 全局函数ios_base::sync_with_stdio
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bool ios_base::sync_with_stdio(bool __sync) { // _GLIBCXX_RESOLVE_LIB_DEFECTS // 49. Underspecification of ios_base::sync_with_stdio bool __ret = ios_base::Init::_S_synced_with_stdio;
// Turn off sync with C FILE* for cin, cout, cerr, clog iff // currently synchronized. if (!__sync && __ret) { // Make sure the standard streams are constructed. ios_base::Init __init;
ios_base::Init::_S_synced_with_stdio = __sync;
// Explicitly call dtors to free any memory that is // dynamically allocated by filebuf ctor or member functions, // but don't deallocate all memory by calling operator delete. buf_cout_sync.~stdio_sync_filebuf<char>(); buf_cin_sync.~stdio_sync_filebuf<char>(); buf_cerr_sync.~stdio_sync_filebuf<char>();
#ifdef _GLIBCXX_USE_WCHAR_T buf_wcout_sync.~stdio_sync_filebuf<wchar_t>(); buf_wcin_sync.~stdio_sync_filebuf<wchar_t>(); buf_wcerr_sync.~stdio_sync_filebuf<wchar_t>(); #endif
// Create stream buffers for the standard streams and use // those buffers without destroying and recreating the // streams. new (&buf_cout) stdio_filebuf<char>(stdout, ios_base::out); new (&buf_cin) stdio_filebuf<char>(stdin, ios_base::in); new (&buf_cerr) stdio_filebuf<char>(stderr, ios_base::out); cout.rdbuf(&buf_cout); cin.rdbuf(&buf_cin); cerr.rdbuf(&buf_cerr); clog.rdbuf(&buf_cerr);
#ifdef _GLIBCXX_USE_WCHAR_T new (&buf_wcout) stdio_filebuf<wchar_t>(stdout, ios_base::out); new (&buf_wcin) stdio_filebuf<wchar_t>(stdin, ios_base::in); new (&buf_wcerr) stdio_filebuf<wchar_t>(stderr, ios_base::out); wcout.rdbuf(&buf_wcout); wcin.rdbuf(&buf_wcin); wcerr.rdbuf(&buf_wcerr); wclog.rdbuf(&buf_wcerr); #endif } return __ret; } |
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（_IOLBF line buffered）。
是否可将标准输出设置为全缓存（_IOFBF fully buffered）了？答案是可以，执行下列代码即可得到结果。
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$ cat eee.cpp #include <stdio.h> int main(void) { char buf[BUFSIZ]; setvbuf(stdout, buf, _IOFBF, BUFSIZ); printf("Hello, world!\n"); getchar(); return 0; } |
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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$ cat eee.cpp #include <fcntl.h> #include <stdio.h> #include <stdlib.h> #include <sys/types.h> #include <sys/stat.h> #include <unistd.h> int main(int argc, char* argv[]) { char buf[BUFSIZ]; int fd = open("/tmp/xxx.txt", O_WRONLY|O_CREAT|O_TRUNC, S_IRWXU); if (fd == -1) { perror("open"); return 1; } else { int fdnew = dup2(fd, STDOUT_FILENO); // 让标准输出指向文件 if (fdnew == -1) { perror("dup2"); return 1; } else { printf("123\n"); getchar(); return 0; } } } |
匿名管道默认是全缓存，可用下列代码验证：
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$ cat xxx.cpp #include <fcntl.h> #include <stdio.h> #include <stdlib.h> #include <sys/types.h> #include <sys/stat.h> #include <sys/wait.h> #include <unistd.h> int main(int argc, char* argv[]) { int fd[2]; char buf[BUFSIZ]; if (pipe(fd) == -1) { perror("pipe"); exit(1); }
pid_t pid = fork(); if (pid == -1) { perror("fork"); exit(1); } else if (pid == 0) { // Child process dup2(fd[1], STDOUT_FILENO); //setvbuf(stdout, buf, _IOLBF, BUFSIZ); printf("hello\n"); getchar(); fflush(stdout); _exit(0); } else { // Parent process char msg[BUFSIZ]; int n = read(fd[0], msg, sizeof(msg)-1); if (n == -1) { perror("Parent read"); } else { msg[n] = '\0'; fprintf(stdout, "(%d)%s\n", n, msg); }
wait(NULL); exit(0); } } |
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？
