poll() catch thread return value - c++

Question

I have a poll() loop with a small socket communication, I want to start an other program by system() or exec() and I need the the return value of the system()/exec() but I don't want to stop the main loop while the child process is running so I thought I start it in a thread but I am not sure how to set up the pollfd to catch the thread when it is done, I am using c/c++
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>
#include <poll.h>
#include <iostream>
#include <string>
#include <thread>
#include <future>
#define SOCKET_NAME "/tmp/9Lq7BNBnBycd6nxy.socket"
int runProgram(const std::string &programName, const std::string &fileName) {
return system((programName + " " + fileName).c_str());
}
int main(int argc, char *argv[]) {
struct sockaddr_un server;
int sock;
char buf[1024];
unlink(SOCKET_NAME);
sock = socket(AF_UNIX, SOCK_STREAM, 0);
if (sock == -1){
perror("socket");
exit(EXIT_FAILURE);
}
memset(&server, 0, sizeof(struct sockaddr_un));
server.sun_family = AF_UNIX;
strncpy(server.sun_path, SOCKET_NAME, sizeof(server.sun_path) - 1);
if (bind(sock, (struct sockaddr *) &server, sizeof(struct sockaddr_un)) < 0) {
perror("bind");
exit(EXIT_FAILURE);
}
if (listen(sock, 3) < -1) {
perror("listen");
exit(EXIT_FAILURE);
}
struct pollfd fds[2];
fds[0].fd = sock;
fds[0].events = POLLIN;
std::future<int> ret = std::async(&runProgram, "cat", "test.txt");
while (true) {
poll(fds, 2, -1);
if(fds[0].revents & POLLIN) {
int new_sd = accept(fds[0].fd, NULL, NULL);
if (new_sd < 0) {
perror("accept");
}
fds[1].fd = new_sd;
}
if (fds[0].revents & POLLIN) {
int rv = recv(fds[1].fd, buf, 1024, 0);
if (rv < 0)
perror("recv");
else if (rv == 0) {
printf("disconnet\n");
close(fds[1].fd);
} else {
printf("%s\n", buf);
send(fds[1].fd, buf, 1024, 0);
}
memset(buf, 0, 1024);
}
}
close(sock);
return(EXIT_SUCCESS);
}
So I want to add one more to the pollfd (fds[ret.get()]) and get a POLLIN on fds[2] when my thread is done and I can get the return value (ret.get()), here I used an exaple command cat but in my final code the command would need mach more time so I cant wait for that to finish

Answer 1

The simplest solution is to create an anonymous pipe (or, since you say that you are on Linux, an eventfd) and write data to one end of the pipe in the runProgram function once the call to system returns. You can then include the read end of the pipe in the set of file descriptors that you are polling.
int process_eventfd = eventfd(0, EFD_CLOEXEC);
if (process_eventfd == -1) exit(1); // change this to handle appropriately
struct pollfd fds[3];
fds[0].fd = sock;
fds[0].events = POLLIN;
fds[1].fd = process_eventfd;
fds[1].events = POLLIN;
// use fds[2] instead of fds[1] for your socket connection, etc.
You can add the eventfd number as an argument to runProgram. It should now look something like:
int runProgram(const std::string &programName, const std::string &fileName, int process_eventfd) {
return system((programName + " " + fileName).c_str());
uint64_t value = 1;
write(process_eventfd, &value, 8);
}
By the way, your current program has a bug: you always pass 2 as the number of file descriptors to poll, even before you have set up the second file descriptor in the array. You should only pass the number of valid descriptors actually present in the array.
However, if you don't need to use system and can use exec, there is no need to create another thread; just perform the following steps:
Mask (but don't ignore) the SIGCHLD signal. (You may need to set up a signal handler, even if it does nothing; I can't remember if this is true for Linux or not).
Create your external process via fork/exec
Use ppoll rather than poll, and include SIGCHLD in the signals to be enabled
If the ppoll call returns an EINTR error, use waitpid to obtain the child status
The child process will run in parallel to your program.