I am trying to learn programming on and I don’t understand why I can't get child 2 to print hello. I have also closed both read and write ends of the pipe for the parent because I don’t want the parent to communicate. Any help is much appreciated.
int main ()
{
int fd[2];
pid_t child_pid, child_pid1;
if (pipe(fd) < 0)
{
printf("Pipe error");
}
child_pid = fork (); // child 1
if (child_pid != 0)
{
printf("im child 1");
dup2 (fd[1], STDIN_FILENO);
printf("greetings");
child_pid1 = fork (); //child 2
if (child_pid1 != 0)
{
printf("im child 2");
dup2(fd[0], STDOUT_FILENO);
printf("hello");
}
else if (child_pid1 > 0) //Parent Code
{
close (fd[0]);
close (fd[1]);
}
}
return 0;
}
Your first if clause is wrong.
child_pid = fork (); // child 1
if (child_pid != 0)
{
printf("im child 1");
}
This is not the child but the parent. If it is the child, then pid will be 0, only on the parent you have child_pid != 0.
The same happens in the second if clause as well.
You should do if(child_pid == 0) in both cases, if you want that code to be executed in the child process.
Related
This is my code and I'm trying to write a pid in a file, I'm using fprintf but it doesn't work
This is my code :
int write_pid_in_file(struct config *conf, pid_t pid)
{
//conf->global->pid_file = /tmp/HTTPd.pid
FILE *file_opened = fopen(conf->global->pid_file,"w+");
if (!file_opened)
return 2;
printf("%d\n",pid); //check if it's the good pid
fprintf(file_opened, "%d\n",pid);
fclose(file_opened);
return 0;
}
In this function I call the write function and I get the pid from the demone
int daemon_start(struct config *conf)
{
pid_t cpid;
cpid = fork();
if (cpid == -1)
{
perror("fork");
exit(0);
}
write_pid_in_file(conf,cpid);
if (cpid > 0)
{
close(STDIN_FILENO);
close(STDOUT_FILENO);
close(STDERR_FILENO);
exit(0);
}
serv(conf->vhosts); //infinite loop
return 0;
}
I'm doing this bc later I need to make a function who stop this deamon ^^
Given the code
int daemon_start(struct config *conf)
{
pid_t cpid;
cpid = fork();
if (cpid == -1)
{
perror("fork");
exit(0);
}
write_pid_in_file(conf,cpid); <-- both parent and child will call this
.
.
.
There will be two processes that call write_pid_in_file() - the parent process will write the pid of the child process, and the child process will write 0.
int main()
{
int pipefd[2];
char buf;
int pid, pid1;
pid = fork();
if (pipe(pipefd) == -1) {
perror("pipe");
exit(EXIT_FAILURE);
}
if(pid == 0){ // CHILD 1
close(pipefd[1]);
while(read(pipefd[0],&buf,1) > 0){ // THIS DOESNT WORK
printf("FIRST CHILD WRITES: %s\n",&buf); // THIS DOESNT WORK
} // THIS DOESNT WORK
close(pipefd[0]);
_exit(EXIT_SUCCESS);
}else{
pid1 = fork();
if(pid1 == 0){ // CHILD 2
close(pipefd[1]);
// while(read(pipefd[0],&buf,1) > 0){ // ONLY THIS (WOULD) WORK
// printf("SECOND CHILD WRITES: %s\n",&buf); // ONLY THIS (WOULD) WORK
// } // ONLY THIS (WOULD) WORK
close(pipefd[0]);
_exit(EXIT_SUCCESS);
}else{ // PARENT
close(pipefd[0]);
char* s = "Write To Pipe";
write(pipefd[1],s,strlen(s));
close(pipefd[1]);
wait(NULL); // WAIT FOR CHILD TO TERMINATE
wait(NULL); // WAIT FOR CHILD TO TERMINATE
}
}
return 0;
}
Whenever I try to run the program only the 2ND CHILD can read from the pipe, the 1ST CHILD never. So I tried commenting the second child's pipe reading, however the first child still can't read from the pipe to which the parent wrote into.
Why can't the 1ST CHILD read from the pipe?
Thanks for the help!
The order is wrong. Your code is
pid = fork();
if (pipe(pipefd) == -1) {
perror("pipe");
exit(EXIT_FAILURE);
}
You need to create the pipe before you fork. You would probably catch this type of error if you check for errors on the close and/or the read.
How can i send signal from parent process to the child?
After i write to the pipe i want to send signal to the child process.
pid_t pid;
int filds[2];
pipe(filds);
char *args[150] = {"./draw.out", NULL};
char buff = '\0';
if ((pid = fork()) < 0) { // fork a child process/
printf("*** ERROR: forking child process failed\n");
exit(1);
} else if (pid == 0) {
execvp(args[0], args); // execute the command
} else { // for the parent
char btnPressed = getch();
while (btnPressed != 'q'){
btnPressed = getch();
write(filds[1],buff, BUFF_SIZE);
//signal
}
// signal finish game.
}
kill(PID, SIGwhatever);
however this is probably a poor choice; the better solution is
close(filds[1]);
and handle close of input in the child. I think you're missing a
dup2(files[0],0);
in the child path as well.
I'm struggling to understand this concept. Let's say I want to run 3 concurrent processes (threads are not an option, this is an assignment).
I would do:
int main(){
fork();
if (getPid() == -1) { //this is the parent
fork(); //make a third process
} else if (getPid() == 0) { //child process
//do child things
}
So from what I've learned the parent pid is -1. And there's two children, both with PID 0?
So then the "parent" can spawn as many children as possible, correct? They will all do child things.
What if I want to do 3 different things? How do I track the PID's so that I have 3 unique
ones?
as per the comments - is this how it's done?
pid_t child2Pid;
pid_t child1Pid = fork();
switch /*if getPid is wrong what do I put here?*/ {
case -1: //parent
child2Pid = fork(); //create another child
case child1Pid :
//do what child1 would do
case child2Pid :
//do what child2 would do
pid_t child1, child2, child3;
if ((child1 = fork()) == 0)
{
// first child stuff goes here
_exit(0);
}
if ((child2 = fork()) == 0)
{
// second child stuff goes here
_exit(0);
}
if ((child3 = fork()) == 0)
{
// third child stuff goes here
_exit(0);
}
// we are in the parent, we have the PIDs of our three
// children in child1, child2, and child3
The whole idea is that you enter fork() once, but leave it twice - once in the parent, once in the child. In the parent, fork() returns the child's PID, in the child, fork() returns 0. -1 means error and there is no child.
So when you call fork(), look at the return value. Like this:
if(fork() > 0) //We're in the parent, and a child was spawned
fork(); //Spawn another.
This will create three processes. Error handling omitted.
**
int main()
{
pid_t pid;
int i;
for(i = 0;i < 3;i++)
{
pid = fork();
}
if(pid == 0)
{
printf("child\n");
}
else if(pid > 0)
{
printf("parent\n");
}
return 0;
}
you can use pid_t = fork() to record the value,and select what you do by the value.**
Hello i have such problem
pid_t pid1;
pid_t pid2;
void switch_files(int sig_type)
{
printf("%d %d\n", pid1, pid2);
}
int main(int argc, char **argv)
{
pid_t lpid1,lpid2;
if ((lpid1 = fork()) == 0)
{
signal(SIGUSR1, switch_files);
//Some work
} else {
pid1 = lpid1;
}
if ((lpid2 = fork()) == 0)
{
signal(SIGUSR2, switch_files);
//Some work
} else {
pid2 = lpid2;
}
while(scanf("%s", input) > 0)
{
write(pipe1[1], input, strlen(input) + 1);
kill(pid1, SIGUSR1);
}
waitpid(pid1, 0, 0);
waitpid(pid2, 0, 0);
}
So i need to have value of pid1 and pid2 in my callback for signals, and at printf i have 0 0 but in main i have normal value of pids. How i can fix this, thank you for any help.
If you want the child to have the pid, simply ask for it:
if ((lpid1 = fork()) == 0)
{
pid1 = getpid();
pid2 = -1; // other child doesn't even exist yet
signal(SIGUSR1, switch_files);
//Some work
exit(0); // you don't want the child to go executing parent code, do you?
}
And
if ((lpid2 = fork()) == 0)
{
//pid1 already set with pid of 1st child
pid2 = getpid();
signal(SIGUSR2, switch_files);
//Some work
exit(0); // you don't want the child to go executing parent code, do you?
}
If you need to the 1st child to have the pid of the 2nd child, then you need to use some form of IPC, so you can communicate to the the 1st child after also 2nd child has been started and its pid is known.
If you want the children to have information, you have to communicate that information to them somehow. You can use a file, a pipe, shared memory, or any mechanism you want. But you have to pass the information somehow.