int proc_create(int n)
{
int pid;
n = n+1;
printf("The parent process id: %d\n", getpid());
while(1)
{
if(pid=fork() < 0){
perror("Fork Failed!");
exit(1);
}else{
printf("The child process ID is: %d\n", pid);
}
}
}
I have written the above function that will create n child processes and each child processes will print out it's own child id. Can someone tell me the flaws and how i can improve the above function.
n is a local variable, so you just do n + 1 which doesn't change anything.
It creates infinite child processes, because the fork is inside a while(1) loop
int *proc_create(int n) {
int *childs = malloc(sizeof *childs * n);
printf("The parent process id: %d\n", getpid());
for (int i = 0; i < n; i++) {
int pid = fork();
if (pid < 0) {
perror("Fork Failed!");
exit(1);
} else if (pid == 0) {
return NULL;
} else {
childs[i] = pid;
printf("The child process ID is: %d\n", pid);
}
}
return childs;
}
This process spawn N children, when they return from proc_create() they will return NULL. The parent will return an array with the pids of its N children.
Related
Let's say there is a card game which the parent is the host and Children are players.
There could be 3~6 players, and Parent needs communicate through Pipe with Children. If there are i children, we could let each child to sleep i*n seconds to set the order, but I don't think this is efficient.
How could let the parent ask something to Child 1, and after Child 1 comes back to the parent, the parent asks something to Child 2.. and so on?
for (i = 1; i <= numChild * 2; i++) { //creating 2n pipes for N children.
pipe(fd[i]);
if (pipe(fd[i]) < 0) {
printf("Pipe creating error\n");
exit(1);
}
}
int childPid[numChild+1];
int pid[6];
for(int i=0;i <numChild; i++){
pid[i] = fork();
if (pid[i] < 0){
printf("Error fork\n");
}
else if (pid[i] > 0){ //Parent Process
wait(NULL);
While(1){
// ** Some Process **
}
else if (pid[i] == 0){ //Child Process
While(1){
usleep(i*1000);
// ** Some Process **
}
exit(0);
}
}
I am currently learning about the fork() function in c. I was playing around with the child pid's and trying to store them within an array but keep coming across errors:
void store(int pid){
int arr[10];
int i = 0;
for(i = 0; i < 10; i++){
if(arr[i] == 0){
arr[i] = pid;
printArray(arr);
break;
}
}
}
int stuff(int a){
int status = fork();
if(status == 0){
printf("PID IS %d\n", getpid());
store(getpid());
}
else {
printf("PID IS %d\n", getpid());
store(getpid());
}
return a + 1;
}
int main(int argc, char * argv[]){
int a = stuff(10);
return 0;
}
Instead, this outputs the same array with the two different PIDS in the same array index. I am not too sure what exactly is happening here and would be grateful for any explanation.
Keep it in mind that fork function is called once but returns twice. The difference in the returns is that the return value in the child
is 0, whereas the return value in the parent is the process ID of the new child. The child process and the parent process run in separate memory spaces.
That's the reason why your program outputs the same array with the two different PIDS in the same array index.
int stuff(int a){
int status = fork();
// fork will return a value of pid_t type
pid_t status = fork();
if(status == 0){
// this is in the child process, so getpid() will return the pid of the child
printf("PID IS %d\n", getpid());
store(getpid());
}
else {
// this is in then parent process, so getpid() will return the pid of the parent
printf("PID IS %d\n", getpid());
store(getpid());
}
return a + 1;
}
UPDATE
I thought this code block was giving me an error printing a printf statement out twice, but I commented out everything in my code BESIDES this and it worked just fine! What seems to be the issue then is the work I am doing with process IDs.
Here is the entire code:
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
pid_t pid, pid1;
int n;
int temp;
int stop = 1;
if (argc == 1) {
fprintf(stderr,"Usage: ./a.out <starting value>\n");
return -1;
}
n = atoi(argv[1]);
pid = fork();
if (pid < 0) { /* error occurred */
fprintf(stderr, "Fork Failed");
return 1;
}
else if (pid == 0) { /* child process */
pid1 = getpid();
printf("child: pid = %d\n", pid);
printf("child: pid1 = %d\n", pid1);
}
else { /* parent process */
pid1 = getpid();
printf("parent: pid = %d\n", pid);
printf("parent: pid1 = %d\n", pid1);
wait(NULL);
}
while (n!=1) {
if (n%2 == 0) {
printf("%d, ", n);
temp = (n/2);
n = temp;
}else {
printf("%d, ", n);
temp = (3*n+1);
n = temp;
}
}
printf("1\n");
return 0;
}
The output I'm expecting is something like:
parent: pid = 1444
parent: pid1 = 1443
child: pid = 0
child: pid = 1444
8, 4, 2, 1
But instead I get this Output:
parent: pid = 1444
parent: pid1 = 1443
child: pid = 0
child: pid = 1444
8, 4, 2, 1
8, 4, 2, 1
Might a child a parent process be printing out the sequence a second time?
Yes, once the parent process has wait()ed on the child process, it continues down the code path and prints the sequence.
What you want is:
// ....
else if (pid == 0) { /* child process */
pid1 = getpid();
printf("child: pid = %d\n", pid);
printf("child: pid1 = %d\n", pid1);
while (n!=1) {
if (n%2 == 0) {
printf("%d, ", n);
temp = (n/2);
n = temp;
}else {
printf("%d, ", n);
temp = (3*n+1);
n = temp;
}
}
} else { /* parent process */
pid1 = getpid();
printf("parent: pid = %d\n", pid);
printf("parent: pid1 = %d\n", pid1);
wait(NULL);
}
After
wait(NULL);
You need an exit/return. The parent has done its job of bringing up the child and is done
I have the following piece of code:
int main() {
int n = 1;
if(fork() == 0) {
printf("child");
n = n + 1;
exit(0);
}
n = n + 2;
printf("%d: %d\n", getpid(), n);
wait(0);
return 0;
}
The problem is that I don't understand why the child process is not executing.
The child process is executing only if i set sleep(1) in the parent process
Thanks in advance.
It is getting executed and it should be outputting the text. No newlines should be necessary:
https://ideone.com/a1tznH
#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
int main() {
int n = 1;
if(fork() == 0) {
printf("child");
n = n + 1;
exit(0);
}
n = n + 2;
printf("%ld: %d\n", (long)getpid(), n); //this is how you should print pids
wait(0);
return 0;
}
Example output:
child23891: 3
Perhaps you didn't notice the child text was at the beginning of your next prompt:
18188: 3
child[21:17] pskocik#laptop: $
The child is executed but two processes are trying to write on the same FD - STDOUT (File Descriptor).
If you want to see the result, put "\n" in printf of the child.
int main() {
int n = 1;
if(fork() == 0)
{
printf("child\n");
n = n + 1;
exit(0);
}
n = n + 2;
printf("%d: %d\n", getpid(), n);
wait(0);
return 0;
}
Try
pid_t pid;
pid = fork();
if(pid < 0)
{
printf("fail to fork");
}
else if (pid == 0)
{
printf("running child");
exit(0);
}
else
{
print("running parent");
wait(0);
print("child done");
}
return 0;
This is the basic structure of a program I wrote recently which works. Not totally sure why yours didn't work though.
I'm trying to write a small program that forks processes from a single parent. Currently my code does this a few times but then the children create more child processes, which I want to eliminate.
int main() {
pid_t c;
for (int i = 0; i < 5; i++) {
c = fork();
if(c < 0) {
perror("fork");
exit(1);
}
else if( c > 0 ) {
printf("parentID = %d, childID = %d\n", getppid(i), getpid(i));
}
}
}
I'm not sure how to modify it so that fork is only forking from the parent though.
EDIT: thanks for the help, got the solution:
int main() {
pid_t c;
for (int i = 0; i < 5; i++) {
c = fork();
if(c < 0) {
perror("fork");
exit(1);
}
else if( c > 0 ) {
printf("parentID = %d, childID = %d\n", getppid(i), getpid(i));
}
else {
exit(0);
}
}
}
nothing in the posted code is recognizing the child (0 == pid)
so a child hits (and skips) the two 'if' statements.
hits the end of the loop,
branches back to the top of the loop, calls fork()....etc.
Suggest: adding
elseif( 0 == pid )
{ // then child ...
exit( EXIT_SUCCESS );
}
The child process does not enter any part of the if block, and just loops back to the top of the for loop creating more children. Also, the if (n > 0) block gets run for the parent, not the child, since fork returns 0 to the parent and the child's pid to the parent.
Change if (n > 0) to if (n == 0), and call exit() at the bottom of the block to prevent the child from continuing. Also, getpid() and getppid() don't take any arguments.
int main() {
pid_t c;
for (int i = 0; i < 5; i++) {
c = fork();
if(c < 0) {
perror("fork");
exit(1);
}
else if( c == 0 ) {
printf("parentID = %d, childID = %d\n", getppid(), getpid());
exit(0); // <-- here
}
}
}