Say I used fork twice to get two child processes. Then, I want to execute the 3 processes (including the parent) differently depending on the process is running. How will this be done in c language?
Will the following works:
Say PID1 is the process id for child 1 and similarly PID2 for child 2, then:
I first created the two child using:
pid1 = fork();
if(pid1 == 0){
PID1=getpid();
}
if(pid1 > 0){
pid2 = fork();
if(pid2 > 0){
printf("\nParent ProcessL %d\n",getpid());
}
else if(pid2 == 0){
PID2=getpid();
}
}
-----------------------------------------------------
further down in my code:
while(/*true for a certain time*/){
if (PID1==getpid()){
//execute the code for child 1
} else if (PID2==getpid()){
// execute code for child 2
} else {
// execute the code for the parent
}
by the way, do processes run randomly or they run in order (fixed time for each)?
It won't because the pid returned by fork() in the child is 0.
The simplest way to achieve your goal is to simply branch off right after each fork().
pit_t PID1, PID2;
if(0>(PID1=fork()) {/*handle error*/}
if(!PID1) _exit(code_for_child1());
if(0>(PID2=fork()) {/*handle error*/}
if(!PID2) _exit(code_for_child2());
/*continue the parent's work*/
Related
I have the following code in my main function
pid_t pid;
pid = fork(); //Two processes are made
if (pid > 0 && runBGflag==0) //Parent process. Waits for child termination and prints exit status
{
int status;
if (waitpid(pid, &status, 0) == pid && WIFEXITED(status))
{
printf("Exitstatus [");
for (int i = 0; i < noOfTokens; i++)
{
printf("%s ", commands[i]);
}
printf("\b] = %d\n", WEXITSTATUS(status));
}
}
else if (pid == 0) //Child process. Executes commands and prints error if something unexpected happened
{
if (runBGflag==1) insertElement(getpid(),ptr);
execvp(commands[0], commands);
printf ("exec: %s\n", strerror(errno));
exit(1);
}
In a nutshell, a child process is made and if the runBackGround flag is set, the parent process will not wait for the child process to exit, but rather continue running. If a background process is made, the PID of the background process is stored in a list. At a later point, this function is called
void delete_zombies(void)
{
pid_t kidpid;
int status;
char buffer[1337];
while ((kidpid = waitpid(-1, &status, WNOHANG)) > 0)
{
removeElement(kidpid,buffer,1337);
printf("Child %ld terminated\n", kidpid);
printf("its command was %s\n",buffer);
}
}
This function simply checks if any child processes have died and in that case deletes them. It will then search for the childs PID in the list, remove it and print it out.
The problem is, the delete_zombies function will find that a child has died and will then try to remove it from the list, but it only finds an empty list, as if the child process never inserted its PID into the list.
This is really strange, because delete_zombies only finds a dead child process, when there was one created with the background flag set, so we know insertElement must have been called, but strangely when the parent checks in the list nothing is there
Is the cause for that, that child process and parent process have seperate lists, or is the PID maybe wrong?
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 am having problems with C code about processes that are created with fork given by our professor to figure out the number of processes that it creates.
pid1 = fork();
if (pid1 != 0) {
pid2 = fork();
pid3 = fork();
}
else {
pid4 = fork();
}
Running the code is only somewhat helpful (and can't be done during the exam!)--better to draw a process tree. The rules are that every child process starts execution after the fork call that spawned it, parents have nonzero pid and children have a pid of zero.
main
/ | \
pid1 pid2 pid3
/ |
pid4 pid3
We can see main spawns three children, pid1, pid2 and pid3 (pid1 is trivial and the other two are spawned in the true if block). pid1 goes on to the else portion of the conditional and spawns pid4 before finishing. pid2 is the only trick: it spawns again at pid3 because that's where its execution starts.
Answer:
6 processes in total are created (main and 5 children)
I am trying to make a processor farm in C. I start with opening message queues, and afterwards try to make worker processes: (note that NROF_WORKERS is 5)
static void
makechildren (void) {
// Only the parent should fork. Any children created will become workers.
pid_t processID;
pid_t farmerPID = getpid(); // To identify who the farmer is
// Loop creating processes, indexed by NROF_WORKERS
int i = 0;
while (i < NROF_WORKERS){
if (getpid() == farmerPID){
i++;
printf ("Parent is creating a child!%d\n", getpid());
processID = fork();
}
}
if (processID < 0){
perror("fork() failed");
exit(1);
}
else {
// If parent, start farming
if (processID == farmerPID) {
printf("Parent reporting in!%d\n");
}
// If child, become a worker
if (processID == 0) {
printf("Child reporting in!%d\n", getpid());
join();
}
}
}
As you can see, I want the parent to report any time a child is created, and afterwards I want the parent and all children to report. However, this is all I get:
Parent is creating a child!11909
Parent is creating a child!11909
Parent is creating a child!11909
Parent is creating a child!11909
Parent is creating a child!11909
Child reporting in!11914
Now, I do notice the difference in 11909 and 11914 is 5. So my question: are the other processes created? If so, how come they don't report? And if not, what am I doing wrong? Also, the parent is not reporting at all, how is this caused?
All of the children are created, but will loop forever in the while loop, as i is incremented only for the parent:
int i = 0;
while (i < NROF_WORKERS){
if (getpid() == farmerPID){
i++; // <---- This is happening for the parent process only.
printf ("Parent is creating a child!%d\n", getpid());
processID = fork();
}
}
The only child to terminate is the last one, for which the i is equal to NROF_WORKERS.
Also parent is "not reporting" since the processID you are checking to be equal to the parent PID is never equal to it, as it is equal to the latest fork result, i.e. the latest created child PID:
.........
processID = fork();
.........
.........
if (processID == farmerPID) {
printf("Parent reporting in!%d\n");
}
You always print the farmerPid! But as the message is printed 5 times, you effectively created 5 processes:
while (i < NROF_WORKERS){
if (getpid() == farmerPID){
i++;
printf ("Parent is creating a child!%d\n", getpid());
processID = fork();
}
}
If you want to print the children pids then your code must makes a difference in between parent and child, as in:
while (i < NROF_WORKERS){
if (getpid() == farmerPID){
i++;
printf ("Parent is creating a child!\n");
processID = fork();
if (processID==0) { // child
printf("I am the child %d\n",getpid());
} else { // parent
printf("Parent just created child %d\n",processID);
}
}
}
I'm starting to learn some C and while studying the fork, wait functions I got to a unexpected output. At least for me.
Is there any way to create only 2 child processes from the parent?
Here my code:
#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
int main ()
{
/* Create the pipe */
int fd [2];
pipe(fd);
pid_t pid;
pid_t pidb;
pid = fork ();
pidb = fork ();
if (pid < 0)
{
printf ("Fork Failed\n");
return -1;
}
else if (pid == 0)
{
//printf("I'm the child\n");
}
else
{
//printf("I'm the parent\n");
}
printf("I'm pid %d\n",getpid());
return 0;
}
And Here is my output:
I'm pid 6763
I'm pid 6765
I'm pid 6764
I'm pid 6766
Please, ignore the pipe part, I haven't gotten that far yet. I'm just trying to create only 2 child processes so I expect 3 "I'm pid ..." outputs only 1 for the parent which I will make wait and 2 child processes that will communicate through a pipe.
Let me know if you see where my error is.
pid = fork (); #1
pidb = fork (); #2
Let us assume the parent process id is 100, the first fork creates another process 101. Now both 100 & 101 continue execution after #1, so they execute second fork. pid 100 reaches #2 creating another process 102. pid 101 reaches #2 creating another process 103. So we end up with 4 processes.
What you should do is something like this.
if(fork()) # parent
if(fork()) #parent
else # child2
else #child1
After you create process , you should check the return value. if you don't , the seconde fork() will be executed by both the parent process and the child process, so you have four processes.
if you want to create 2 child processes , just :
if (pid = fork()) {
if (pid = fork()) {
;
}
}
You can create n child processes like this:
for (i = 0; i < n; ++i) {
pid = fork();
if (pid > 0) { /* I am the parent, create more children */
continue;
} else if (pid == 0) { /* I am a child, get to work */
break;
} else {
printf("fork error\n");
exit(1);
}
}
When a fork statement is executed by the parent, a child process is created as you'd expect. You could say that the child process also executes the fork statement but returns a 0, the parent, however, returns the pid.
All code after the fork statement is executed by both, the parent and the child.
In your case what was happening was that the first fork statement created a child process. So presently there's one parent, P1, and one child, C1.
Now both P1 and C1 encounter the second fork statement. The parent creates another child (c2) as you'd expect, but even the child, c1 creates a child process (c3). So in effect you have P1, C1, C2 and C3, which is why you got 4 print statement outputs.
A good way to think about this is using trees, with each node representing a process, and the root node is the topmost parent.
you can check the value as
if ( pid < 0 )
process creation unsuccessful
this tells if the child process creation was unsuccessful..
fork returns the process id of the child process if getpid() is used from parent process..
You can create a child process within a child process. This way you can have 2 copies of the original parent process.
int main (void) {
pid_t pid, pid2;
int status;
pid = fork();
if (pid == 0) { //child process
pid2 = fork();
int status2;
if (pid2 == 0) { //child of child process
printf("friends!\n");
}
else {
printf("my ");
fflush(stdout);
wait(&status2);
}
}
else { //parent process
printf("Hello ");
fflush(stdout);
wait(&status);
}
return 0;
}
This prints the following:
Hello my friends!