On my system(Opensuse) by default the child process always executes first, after fork. There are also ways to force the child process to run first. I would like to know if there is any way to force the parent process to run first?
You can use this method
pid_t pid = fork();
if (pid == -1)
abort();
else if (pid == 0)
{
raise(SIGSTOP); // stop the child
}
else
{
waitpid(pid, NULL, WUNTRACED); // wait until the child is stopped
kill(pid, SIGCONT); // resume the child
}
Related
Can I be running in the same child process two times in the same fork? Ex.
pid_t pid;
pid = fork();
if (pid == 0){
some code here
}else{
some code here
}
wait(NULL)
if (pid ==0){
some code here
}else{
some core here
}
When you leave the if/else, the code runs in both the parent and child. The next if/else again switches to different code in each.
pid_t pid;
pid = fork();
if (pid == 0){
// child code here
}else{
// parent code here
}
wait(NULL) // runs in both
if (pid ==0){
// more child code here
}else{
// more parent code here
}
Unless the child forks another child, wait(NULL) in the child will return immediately (it will return -1 and set errno to ECHILD to indicate that it has no children to wait for). But in the parent process it will wait for the child to exit. So the second if/else block will run immediately in the child, while it will not run in the parent until the child exits.
I'm trying to change the pgrp of the processes to that of the child's so i can setsid on the parent process. The only thing is I keep getting an EPERM error code. Both processes have the same session group, according to htop.
I'm basing this off of this blog post, so I can change which terminal output gets directed to.
void sig_exit(int signum)
{
_Exit(0);
}
pid_t change_process_group()
{
pid_t child_pid;
if ((child_pid = fork()) < 0)
{
perror("fork failed while attaching to term");
exit(1);
}
if (child_pid == 0)
{
pid_t parent = getppid();
setpgid(0, getpid());
signal(SIGUSR1, sig_exit); // wait till parent tells child to exit
//sleep(5);
//kill(parent, SIGUSR2);
pause();
printf("Shouldn't reach this\n");
}
//sleep(5);
//signal(SIGUSR2, sig_wait);
//pause();
int parent_pid = getpid();
int code = setpgid(parent_pid, child_pid); // need child process group
printf("%s\n", strerror(errno));
setsid();
return child_pid;
}
main()
{
pid_t child = change_process_group();
kill(child, SIGUSR1);
}
The commented out lines were from me thinking the processes might not be executing in the correct order, but those don't appear to fix the problem.
How may I correctly use setpgid to change the pgrp of the parent process to the child's?
This is a race condition and it works if you uncomment the sleep(5) line in the parent. When you call setpgid(parent_pid, child_pid), the child_pid process group must exist. It isn't enough that there exists a process with the PID child_pid: setpgid needs an existing process group unless the process is putting itself into its own group. If setpgid(parent_pid, child_pid) in the parent runs after setpgid(0, getpid()) in the child, it works.
Sleeping is both inefficient and fragile, so instead of that the parent should wait for a notification from the child. Signals are fragile because there aren't many different signals and they could come from anywhere. A good way to communicate between related processes is a pipe. Since all you need here is a one-time notification, you can set up a pipe and read from it in the parent (with the write end closed in the parent). The parent will wait until the child writes to the pipe or closes it. In the child, just close the write end of the pipe when you've finished the preparations. The parent's read call (or select if you need to do other things at the same time) will return.
Proof-of-concept code:
pid_t change_process_group()
{
pid_t child_pid;
int child_ready_pipe[2];
if (pipe(child_ready_pipe) < 0)
{
perror("pipe");
exit(1);
}
if ((child_pid = fork()) < 0)
{
perror("fork failed while attaching to term");
exit(1);
}
if (child_pid == 0)
{
close(child_ready_pipe[0]);
sleep(1); // mimic slow start of the child
if (setpgid(0, 0))
perror("child setpgid to create group");
close(child_ready_pipe[1]);
signal(SIGUSR1, sig_exit); // wait till parent tells child to exit
pause();
printf("Shouldn't reach this\n");
}
close(child_ready_pipe[1]);
int parent_pid = getpid();
char ignored;
read(child_ready_pipe[0], &ignored, 1);
close(child_ready_pipe[0]);
if (setpgid(parent_pid, child_pid) < 0) // need child process group
perror("parent setpgid");
if (setsid() < 0)
perror("parent setsid");
return child_pid;
}
I wanna create spare process (child?) in specific function called eg. void process(). I want just to create that child process and do nothing with it. I just wanna it alive and do nothing while main() of my app will be working as I want.
In some point of my app's main() I will be killing child process and then respawn it again. Any ideas how to do that ?
I have something like that but when I'm using this function to create process I get everything twice. Its like after initiation of process() every statement is done twice and i dont want it. After adding sleep(100) after getpid() in child section seems working fine but I cannot kill it.
int process(int case){
if(case==1){
status=1;
childpid = fork();
if (childpid >= 0) /* fork succeeded */
{
if (childpid == 0) /* fork() returns 0 to the child process */
{
printf("CHILD PID: %d\n", getpid());
}
/* fork() returns new pid to the parent process *//* else
{
}*/
}
else
{
perror("fork");
exit(0);
}
}
else{
if(status!=0){
status=0;
//kill!!!!
system(a); //getting kill -9 PID ; but PID is equal 0 here...
printf("\nkilling child");
}
}
}
how to just spawn new child process and let it just exist, like some sort of worker in C#?
Assuming you are in Linux, here's an example that might clarify your view: parent process spawns a child, the child calls pause() which suspends it until a signal is delivered, and finally parent process kill's the child with SIGKILL.
#include <unistd.h>
#include <signal.h>
#include <stdio.h>
int main()
{
pid_t pid;
pid = fork();
if (pid < 0) { perror("fork"); exit(0); }
if (pid == 0) {
printf("Child process created and will now wait for signal...\n");
pause(); //waits for signal
}
else {
//do some other work in parent process here
printf("Killing child (%ld) from parent process!", (long) pid);
kill(pid, SIGKILL);
}
return 0;
}
Please note that fork() returns:
<0 on failure
0 in child process
the child's pid in parent process.
I'm trying to understand how to properly used fork() and execvp() to execute a command. So far I have the following code:
When I run ./test vim myFile.c it correctly opens myFile.c but I get strange behavior. It seems as though there are two processes running because whenever I enter anything it seems to happen twice. Why is this?
int main (int argc, char* argv[]) {
int fdin, pid, w, status;
fdin = 0;
if ((pid = fork()) < 0)
errorExit (EXIT_FAILURE);
execvp(argv[0],argv);
do {
w = waitpid(cpid, &status, WUNTRACED | WCONTINUED);
if (w == -1) {
perror("waitpid");
exit(EXIT_FAILURE);
}
if (WIFEXITED(status)) {
printf("exited, status=%d\n", WEXITSTATUS(status));
} else if (WIFSIGNALED(status)) {
printf("killed by signal %d\n", WTERMSIG(status));
} else if (WIFSTOPPED(status)) {
printf("stopped by signal %d\n", WSTOPSIG(status));
} else if (WIFCONTINUED(status)) {
printf("continued\n");
}
} while (!WIFEXITED(status) && !WIFSIGNALED(status))
}
When you call fork(), you create two almost completely identical processes, the parent process and the child process. The only difference between these two processes is the return value of fork(), which returns 0 to the child process and the pid of the child to the parent.
Hence, assuming fork succeeded, fork will return a nonnegative integer to both the parent and child process in line 4. Then, both the parent and child process will execute line 6, the execvp, and hence, you end up with two different processes running your vim myFile.c, causing all the problems you described.
The standard idiom is something like:
if ((pid = fork()) < 0) {
// Handle fork error
}
else if (pid == 0) {
// Child process
execvp(...);
}
else {
// Parent process
w = waitpid(pid, ...);
}
Since the return value for fork is 0 for the child, after fork succeeds, the test (pid == 0) will be true for the child, so execvp will be called.
For the parent, fork returns the pid of the child, so the check (pid == 0), which still get executed, is false, so the else condition is executed, causing the parent to wait for the child.
Both parent and child in your program get execvp():
if ((pid = fork()) < 0)
errorExit (EXIT_FAILURE);
execvp(argv[0],argv);
You should check, if you are in parent with pid != 0, and if you are in child otherwise.
You should look at the return value of fork, after a successful fork you will have two running processes at the same position in your program. The child process will get a return value of 0, the parent will get a return value which is the pid of the child. Most likely you want to do different things in the child process and the parent process.
You might also want to think again about how execvp is called. Do you really want to give "./test" as the first argument to execvp?
I'm hoping someone could shed some light on how to make the parent wait for ALL child processes to finish before continuing after the fork. I have cleanup code which I want to run but the child processes need to have returned before this can happen.
for (int id=0; id<n; id++) {
if (fork()==0) {
// Child
exit(0);
} else {
// Parent
...
}
...
}
pid_t child_pid, wpid;
int status = 0;
//Father code (before child processes start)
for (int id=0; id<n; id++) {
if ((child_pid = fork()) == 0) {
//child code
exit(0);
}
}
while ((wpid = wait(&status)) > 0); // this way, the father waits for all the child processes
//Father code (After all child processes end)
wait waits for a child process to terminate, and returns that child process's pid. On error (eg when there are no child processes), -1 is returned. So, basically, the code keeps waiting for child processes to finish, until the waiting errors out, and then you know they are all finished.
POSIX defines a function: wait(NULL)；. It's the shorthand for waitpid(-1, NULL, 0);, which will suspends the execution of the calling process until any one child process exits.
Here, 1st argument of waitpid indicates wait for any child process to end.
In your case, have the parent call it from within your else branch.
Use waitpid() like this:
pid_t childPid; // the child process that the execution will soon run inside of.
childPid = fork();
if(childPid == 0) // fork succeeded
{
// Do something
exit(0);
}
else if(childPid < 0) // fork failed
{
// log the error
}
else // Main (parent) process after fork succeeds
{
int returnStatus;
waitpid(childPid, &returnStatus, 0); // Parent process waits here for child to terminate.
if (returnStatus == 0) // Verify child process terminated without error.
{
printf("The child process terminated normally.");
}
if (returnStatus == 1)
{
printf("The child process terminated with an error!.");
}
}
Just use:
while(wait(NULL) > 0);
This ensures that you wait for ALL the child processes and only when all have returned, you move to the next instruction.