C++标准库中的std::endl究竟做了什么？

 

先抓出std::endl的源代码：

/**  *  @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）：

/**  *  @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：

/**  *  @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，：

$ cat xxx.cpp #include <iostream> int main() {   std::cout << std::endl;   return 0; }

 

写换符符：

#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：

#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：

#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：

/**  *  @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

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);   } }

 

1. 全局函数ios_base::sync_with_stdio

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。

$ 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：

$ 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：

$ 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？