为什么if(fork(）==0）{getpid(）}和popen(）进程返回相同的进程id

据我所知，当fork((中的getpid((被认为是与popen((产生的进程不同的进程时，我想知道为什么两个进程id匹配。

有人告诉我，我的代码之所以有效，是因为我认为基于Ubuntu的发行版(如Xubuntu、Lubuntu和KDE-neon(可能存在漏洞(这些是我迄今为止测试过的发行版(。您可以从这里轻松编译和测试代码：https://github.com/time-killer-games/XTransientFor如果你不信任该链接中提供的x64二进制文件，请忽略它。特别欢迎Arch、RedHat等测试人员在这种情况下提供反馈。

这里有一个更简单的方法来证明这个问题：

// USAGE: xprocesstest [command]
#include <X11/Xlib.h>
#include <X11/Xatom.h>
#include <X11/Xutil.h>
#include <unistd.h>
#include <thread>
#include <chrono>
#include <iostream>
#include <string>
using std::string;
static inline Window XGetActiveWindow(Display *display) {
unsigned long window;
unsigned char *prop;
Atom actual_type, filter_atom;
int actual_format, status;
unsigned long nitems, bytes_after;
int screen = XDefaultScreen(display);
window = RootWindow(display, screen);
filter_atom = XInternAtom(display, "_NET_ACTIVE_WINDOW", True);
status = XGetWindowProperty(display, window, filter_atom, 0, 1000, False, AnyPropertyType, &actual_type, &actual_format, &nitems, &bytes_after, &prop);
unsigned long long_property = prop[0] + (prop[1] << 8) + (prop[2] << 16) + (prop[3] << 24);
XFree(prop);
return (Window)long_property;
}
static inline pid_t XGetActiveProcessId(Display *display) {
unsigned long window = XGetActiveWindow(display);
unsigned char *prop;
Atom actual_type, filter_atom;
int actual_format, status;
unsigned long nitems, bytes_after;
filter_atom = XInternAtom(display, "_NET_WM_PID", True);
status = XGetWindowProperty(display, window, filter_atom, 0, 1000, False, AnyPropertyType, &actual_type, &actual_format, &nitems, &bytes_after, &prop);
unsigned long long_property = prop[0] + (prop[1] << 8) + (prop[2] << 16) + (prop[3] << 24);
XFree(prop);
return (pid_t)(long_property - 1);
}
int main(int argc, const char **argv) {
if (argc == 2) {
char *buffer = NULL;
size_t buffer_size = 0;
string str_buffer;
FILE *file = popen(argv[1], "r");
if (fork() == 0) {
Display *display = XOpenDisplay(NULL);
Window window;
unsigned i = 0;
while (i < 10) {
std::this_thread::sleep_for(std::chrono::milliseconds(200));
if (XGetActiveProcessId(display) == getpid()) {
window = XGetActiveWindow(display);
break;
}
i++;
}
if (window == XGetActiveWindow(display)) 
std::cout << "process id's match!" << std::endl;
else std::cout << "process id's don't match!" << std::endl;
XCloseDisplay(display);
exit(0);
}
while (getline(&buffer, &buffer_size, file) != -1)
str_buffer += buffer;
std::cout << str_buffer;
free(buffer);
pclose(file);
}
}

编译使用：

cd "${0%/*}"
g++ -c -std=c++17 "xprocesstest.cpp" -fPIC -m64
g++ "xprocesstest.o" -o "xprocesstest" -fPIC -lX11

运行方式：

cd "${0%/*}"
./xprocesstest "kdialog --getopenfilename"

您可以将引号中的命令替换为任何设置_NET_WM_PID原子的可执行文件。