basic_filebuf :: underflow错误在/ proc / pid / stat中读取ifstream文件

更新2

我甚至试图通过手动设置微小或巨大的缓冲区大小来强制错误：

  std::filebuf fb; // set tiny input buffer char buf[8]; // or huge: 64*1024 fb.pubsetbuf(buf, sizeof(buf)); fb.open(path.c_str(), std::ios::in); std::istream file(&fb); 

我已经验证了读取大小确实很小（7）使用strace

 sudo strace ./test $(sudo ps h -ae -o pid) |& egrep -w 'read|open' | grep -v '= 7' | less -SR 

有趣的是，没有一个失败。

更新

为了回应这些意见，我设计了一个独立的程序来完成OP所描述的内容，但是我不能重现这个问题：

• 看到它住在Coliru再次 http：//coliru.stacked-crooked.com/view？id = 81960c …

 #include <sys/types.h> // For pid_t. #include <fstream> #include <string> // mock up #include <boost/variant.hpp> namespace { struct None {}; struct Error { std::string s; Error(std::string s): s(s){} }; std::ostream& operator<<(std::ostream& os, None const&) { return os << "None"; } std::ostream& operator<<(std::ostream& os, Error const& e) { return os << "Error {" << es << "}"; } template <typename T> using Result = boost::variant<None, Error, T>; } // end mockup namespace proc { // Snapshot of a process (modeled after /proc/[pid]/stat). // For more information, see: // http://www.kernel.org/doc/Documentation/filesystems/proc.txt struct ProcessStatus { pid_t pid; std::string comm; char state; pid_t ppid, pgrp, session; int tty_nr; pid_t tpgid; unsigned int flags; unsigned long minflt, cminflt, majflt, cmajflt; unsigned long utime, stime; long cutime, cstime, priority, nice, num_threads, itrealvalue; unsigned long long starttime; unsigned long vsize; long rss; unsigned long rsslim, startcode, endcode, startstack, kstkeip, signal, blocked, sigcatch, wchan, nswap, cnswap; friend std::ostream& operator<<(std::ostream& os, proc::ProcessStatus const& ps) { return os << "pid: " << ps.pid << "\n" << "comm: " << ps.comm << "\n" << "state: " << ps.state << "\n" << "ppid: " << ps.ppid << "\n" << "pgrp: " << ps.pgrp << "\n" << "session: " << ps.session << "\n" << "tty_nr: " << ps.tty_nr << "\n" << "tpgid: " << ps.tpgid << "\n" << "flags: " << ps.flags << "\n" << "minflt: " << ps.minflt << "\n" << "cminflt: " << ps.cminflt << "\n" << "majflt: " << ps.majflt << "\n" << "cmajflt: " << ps.cmajflt << "\n" << "utime: " << ps.utime << "\n" << "stime: " << ps.stime << "\n" << "cutime: " << ps.cutime << "\n" << "cstime: " << ps.cstime << "\n" << "priority: " << ps.priority << "\n" << "nice: " << ps.nice << "\n" << "num_threads: " << ps.num_threads << "\n" << "itrealvalue: " << ps.itrealvalue << "\n" << "starttime: " << ps.starttime << "\n" << "vsize: " << ps.vsize << "\n" << "rss: " << ps.rss << "\n" << "rsslim: " << ps.rsslim << "\n" << "startcode: " << ps.startcode << "\n" << "endcode: " << ps.endcode << "\n" << "startstack: " << ps.startstack << "\n" << "kstkeip: " << ps.kstkeip << "\n" << "signal: " << ps.signal << "\n" << "blocked: " << ps.blocked << "\n" << "sigcatch: " << ps.sigcatch << "\n" << "wchan: " << ps.wchan << "\n" << "nswap: " << ps.nswap << "\n" << "cnswap: " << ps.cnswap << "\n"; } }; // Returns the process statistics from /proc/[pid]/stat. // The return value is None if the process does not exist. inline Result<ProcessStatus> status(pid_t pid) { std::string path = "/proc/" + std::to_string(pid) + "/stat"; std::ifstream file(path.c_str()); if (!file.is_open()) { #if 1 return Error("Failed to open '" + path + "'"); #else // FIXME reenable // Need to check if file exists AFTER we open it to guarantee // process hasn't terminated (or if it has, we at least have a // file which the kernel _should_ respect until a close). if (!os::exists(path)) { return None(); } return Error("Failed to open '" + path + "'"); #endif } std::string _; // For ignoring fields. // Parse all fields from stat. ProcessStatus ps; if (file >> _ >> ps.comm >> ps.state >> ps.ppid >> ps.pgrp >> ps.session >> ps.tty_nr >> ps.tpgid >> ps.flags >> ps.minflt >> ps.cminflt >> ps.majflt >> ps.cmajflt >> ps.utime >> ps.stime >> ps.cutime >> ps.cstime >> ps.priority >> ps.nice >> ps.num_threads >> ps.itrealvalue >> ps.starttime >> ps.vsize >> ps.rss >> ps.rsslim >> ps.startcode >> ps.endcode >> ps.startstack >> ps.kstkeip >> ps.signal >> ps.blocked >> ps.sigcatch >> ps.wchan >> ps.nswap >> ps.cnswap) { return ps; } else { return Error("Failed to read/parse '" + path + "'"); } } } // namespace proc { int main(int argc, const char *argv[]) { for (auto i=1; i<argc; ++i) std::cout << proc::status(std::stoul(argv[i])) << "\n"; } 

它在我的机器上运行愉快，打印像这样的东西

 pid: 594590200 comm: (test) state: R ppid: 8123 pgrp: 8123 session: 8123 ... 

即使/当我用酷刑

 sudo ./test $(sudo ps h -ae -o pid) | grep -v : | sort -u ./test $(sudo ps h -ae -o pid) | grep -v : | sort -u 

它只是显示（大概从底层的sudo / ps ）

 Error {Failed to open '/proc/8652/stat'} Error {Failed to open '/proc/8653/stat'} 

我试图从输入流中读取两次信息（强制读取结束类型的情况），但没有运气。

旧的答案文字：

您需要确定在什么样的条件下发生异常。 以下演示代码在我的系统上按预期工作：

 #include <fstream> #include <iostream> int main() { system("ps -o pid,comm,state,ppid,pgrp,session,tty > input.txt"); try { std::ifstream file("input.txt"); file.exceptions(std::ifstream::goodbit); std::string _, comm, state, ppid, pgrp, session, tty_nr; while (file >> _ >> comm >> state >> ppid >> pgrp >> session >> tty_nr) { for (auto&& s : { _, comm, state, ppid, pgrp, session, tty_nr }) std::cout << s << "\t"; std::cout << "\n"; } file.close(); } catch (const std::ifstream::failure& e) { std::cout << "Exception!!!! " << e.what(); } } 

打印，例如：

 PID COMMAND S PPID PGRP SESS TT 20950 bash S 20945 20950 20950 pts/1 21275 vim S 20950 21275 20950 pts/1 21279 bash S 21275 21275 20950 pts/1 21280 test S 21279 21275 20950 pts/1 21281 sh S 21280 21275 20950 pts/1 21282 ps R 21281 21275 20950 pts/1 

更新

我想，在许多系统上，默认的缓冲区大小可能是最大8192字节; 让我们来创造一些愚蠢的长线，而不是！ 通过替换system调用

 system("od /dev/urandom -t x8 -Anone | xargs -n256 | tr -d ' ' | xargs -n7 | head -n100 > input.txt"); 

结果为7列，每行约29kB。 输出毫不犹豫，达到2.8MiB的输出，用

 make -B && ./test | wc 

在coliru上看到它：

• http://coliru.stacked-crooked.com/view?id=00160c8a2b5d4447cbda3866da3913aa-b8c04540b49ee47c13fd3df477ec213b

请注意，在Coliru上，我们无法访问/dev/urandom ，这就是为什么我从二进制文件本身读取的原因。 “随机”就足够了这个目的:)