Issue
I'm trying to send a message from one thread to another. Each thread knows the thread ID of the other. How can I send a message between them?
I've already seen some proposed solutions (message queue, anonymous pipe, etc.) but honestly I didn't get them to work. Obviously I didn't understand the previous descriptions enough, hence this topic.
So to sum up, just the shortest possible way to send, let's say a message "Hello!" from thread to another thread, make the 2nd thread show it on stderr, and than send back to 1st thread message 'Hello back!'.
It's probably very easy and I didn't do a good job of researching, but I've been stuck for some time now, and can't find decent way to do this.
Solution
An example, it's pretty simple — first make a pipe with pipe(). It creates two file descriptor — one for reading, and the second for writing. Here we calling it two times to have both read and write sides. Then we calling fork (that makes a second thread), and write/read messages through the pipes we created.
#include <poll.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
int wait_n_read(int fd, char* buf, int szBuf) {
struct pollfd pfd = {
.fd = fd,
.events = POLLIN,
.revents = 0
};
poll(&pfd, 1, -1); //wait for an event
int ret = read(fd, buf, szBuf);
if (ret == -1) {
perror("In read()");
}
return ret;
}
main(){
int chan[4];
enum{
thread1_read = 0, //read end for parent
thread2_write = 1, //write end for child
thread2_read = 2, //read end for child
thread1_write = 3 //write end for parent
};
if (pipe(&chan[thread1_read]) == -1 || (pipe(&chan[thread2_read]) == -1)){
perror("In pipe");
return 1;
}
switch(fork()) {
case -1:
perror("In fork()");
return 1;
case 0:{ //it's a child
char buf[256];
memset(buf, 0, sizeof(buf));
if (wait_n_read(chan[thread2_read], buf, sizeof(buf)-1) == -1)
return 1;
fputs(buf, stderr);
const char helloback[] = "Hello back\n";
write(chan[thread2_write], helloback, sizeof(helloback));
return 0;
}
default: { //a parent
const char hello[] = "Hello\n";
write(chan[thread1_write], hello, sizeof(hello));
char buf[256];
memset(buf, 0, sizeof(buf));
if (wait_n_read(chan[thread1_read], buf, sizeof(buf-1)) == -1)
return 1;
fputs(buf, stderr);
}
}
}
Answered By - Hi-Angel Answer Checked By - Gilberto Lyons (WPSolving Admin)
