更新 2
我什至试图通过手动设置微小或巨大的缓冲区大小来强制出错：
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，这一切都没有失败。
更新
作为对评论的回应，我设计了一个独立程序，该程序完全符合 OP 的描述，但我无法重现该问题：
#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 {" << e.s << "}";
}
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";
}
它在我的机器上愉快地运行，打印类似的东西
pid: 594590200
comm: (test)
state: R
ppid: 8123
pgrp: 8123
session: 8123
...
即使/当我用
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（大概是sudo/ps来自子shell）
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（以强制读取结束类型的情况），但没有运气。
旧答案文本：
您需要确定发生异常的条件。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，默认缓冲区大小可能是最大 8192 字节；让我们创建一些愚蠢的长线吧！system将呼叫替换为
system("od /dev/urandom -t x8 -Anone | xargs -n256 | tr -d ' ' | xargs -n7 | head -n100 > input.txt");
产生 7 列的行，每行占用约 29kB。输出毫不犹豫地达到 2.8MiB 的输出，用
make -B && ./test | wc
在 coliru 上看到它：
请注意，在 Coliru 上，我们无法访问/dev/urandom，这就是我从二进制文件本身读取的原因。“随机”足以达到这个目的:)