I have written a code that generates a random number in the child process using rand()... I need to send this random value to the parent process using pipe(). when I use read() and write () the values received by the parent is 0 instead of the random number.
I am getting no error just the output is Value received is : 0.
#include<stdio.h>
#include<unistd.h>
#include<stdlib.h>
#include<pthread.h>
#include<fcntl.h>
int main ()
{
pid_t Cpid;
int fd[2];
int rVal;
//pid will store the value of fork()
Cpid = fork();
pipe(fd);
if( Cpid == 0)
{
//CHILD
rVal = rand()%100;
;
//this is in child process
close (fd[0]);
rVal = rand()%100;
write(fd[1], &rVal,sizeof (rVal));
close (fd[1]);
printf(" Child Pid : %d sending random value : %d to parent : %d\n",
getpid(), rVal, getppid());
}
else if(Cpid != 0 )
{
printf (" Parent PID : %d\n", getpid());
//this is the parent process
close(fd[1]);
read(fd[0], &rVal, sizeof(rVal));
printf(" value recieved is : %d\n", rVal);
close(fd[0]);
printf(" value recieved by parent : %d\n", rVal);
}
return 0;
}
The code should setup the pipe before forking the children. Otherwise, the parent and the children will use different pipes, and will not be able to talk to each other.
int main ()
{
pid_t Cpid;
int fd[2];
// Create the pipe BEFORE forking.
pipe(fd);
//pid will store the value of fork()
Cpid = fork();
if( Cpid == 0)
.. REST OF CODE ~~~
Related
So I'm currently learning the fork() system call in C but I just can't seem to wrap my head around what exactly is happening.
This is the code
int id=fork();
int child_id, status;
//if id == 0 we are in the child process
if(id == 0 )
{
int pid = getpid()
printf("this is the child process %d\n", pid);
int id1=fork()
if(id1==0)
{
//this will return the pid of the grandparent
int pid2 = getppid();
printf("this is child no 2 process %d\n", pid2);
}
else
{
child_id = wait(&status);
printf("this is parent(%d) of child no 2\n", pid)
}
}
else
{
child_id = wait(&status);
printf("this is the parent process and we will execute after child %d\n", child_id);
}
And this is how I am visualizing it.
I've created the below code:
int main(int argc, char *argv[])
{
int i, n, N;
pid_t pid;
int status;
N = atoi(argv[1]);
for(i = 0; i < N; i++) {
pid = fork();
if(pid==0) {
srand(getpid() * getppid());
n = rand() % 10 + 1;
printf("I'm child nº %d with childpid %d, parent pid = %d, n = %d\n", i, getpid(), getppid(), n);
exit(n);
}
}
while ((pid = waitpid(pid, &status, 0))) {
if (pid < 0)
;
else if (WEXITSTATUS(status) < 4)
printf("exit status %d lower than 4\n", WEXITSTATUS(status));
}
wait(NULL);
return 0;
}
The idea is a parent process forking N child process and each of them exiting with a random value. I want my parent process to monitor all the child processes and fork a new child if the exit status is, for instance <4. This will be going on until all process exit with a status >= 4.
Solved creating a function copying the code from parent and child (keeping the code in main() untouched). The parent calls the function, the function forks another process, and the parent in the same function calls the function recursively under the conditions we choose.
I'm writing a C program that creates a child process. After creating the child process, the parent process should ouput two messages: "I am the parent" then it should print "The parent is done". Same for child process "I am child" and "The child is done". However I want to make sure, the second message of the child is always done before the second message of the parent. How can I achieve to print "The child is done" and "The parent is done" rather than printing their pid?
#include <unistd.h>
#include <stdio.h>
main()
{
int pid, stat_loc;
printf("\nmy pid = %d\n", getpid());
pid = fork();
if (pid == -1)
perror("error in fork");
else if (pid ==0 )
{
printf("\nI am the child process, my pid = %d\n\n", getpid());
}
else
{
printf("\nI am the parent process, my pid = %d\n\n", getpid());
sleep(2);
}
printf("\nThe %d is done\n\n", getpid());
}
You could have a flag variable, that is set in the parent, but then the child clears it. Then simply check for that for the last output.
Something like
int is_parent = 1; // Important to create and initialize before the fork
pid = fork();
if (pid == -1) { ... }
if (pid == 0)
{
printf("\nI am the child process, my pid = %d\n\n", getpid());
in_parent = 0; // We're not in the parent anymore
}
else { ... }
printf("\nThe %s is done\n\n", is_parent ? "parent" : child");
Call wait(2) in the parent process for the child to complete.
else
{
wait(0);
printf("\nI am the parent process, my pid = %d\n\n", getpid());
}
You should check if wait() succeeds and main()'s return type should be int.
In my code below, I'm running a parent process which forks off into two child processes. After child(getpid());, both children exit with a status.
However, when I run the parent process, it somehow always decides to run the parent section twice (sets two different pid values), and I just can't get it to run just once. Is there a way to make wait stop after getting one value?
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>
void child(int n) { //n: child pid
printf("\nPID of child: %i \n", n);
//random number rand
int randFile = open("/dev/random", O_RDONLY);
int r;
if(rand < 0)
printf("ERROR: %s\n", strerror(errno));
else {
unsigned int seed;
read(randFile, &seed, 4); //&rand is a pointer, 4 bytes
int randClose = close(randFile);
srand(seed); //seeds rand() with random from /dev/random
r = rand();
if(randClose < 0)
printf("ERROR: %s\n", strerror(errno));
//range between 5 and 20 seconds
r = r % 20;
if( r < 5)
r = 5;
}
// printf("\n%i\n", r);
sleep(r);
// sleep(1);
printf("\n child with pid %i FINISHED\n", n);
exit( r );
}
int main() {
printf("\nPREFORK\n");
int parentPID = getpid();
int child0 = fork();
if(child0 < 0)
printf("ERROR: %s\n", strerror(errno));
int child1 = fork();
if(child1 < 0)
printf("\nERROR: %s\n", strerror(errno));
if(getpid() == parentPID)
printf("\nPOSTFORK\n");
//if children
if(child1 == 0) //using child1 as child-testing value b/c when child1 is set, both children are already forked
child(getpid());
int status;
int pid = wait(&status);
//parent
if(getpid() != 0) {
if( pid < 0)
printf("\nERROR: %s\n", strerror(errno));
if ( pid > 0 && pid != parentPID) {
printf("\nPID of FINISHED CHILD: %i\n Asleep for %i seconds\n", pid, WEXITSTATUS(status));
printf("PARENT ENDED. PROGRAM TERMINATING");
}
}
return 0;
}
The parent is doing:
int child0 = fork(); // + test if fork failed
int child1 = fork(); // + test if fork failed
First you only have the parent.
After 1st fork you have the parent and the 1st child, both at the same execution point, so just before the next fork.
So just after that the parent re-creates a child, and the 1st child creates its own child (and will act like the parent).
You have to use if/else so that you are sure that the child don't fork. i.e.:
child0 = fork(); // add check for errors
if (child0 == 0) {
// the 1st child just have to call that
child(getpid());
exit(0);
}
// here we are the parent
child1 = fork();
if (child1 == 0) {
// the 2nd child just have to call that
child(getpid());
exit(0);
}
You can do that differently, of course, this is just an example. The main point is to not call fork() within the child.
I need to create a program that creates n number of processes and displays information. When each process ends, I am to print it's PID and the exit status. The way I am doing it, the parent program waits to create the next process until the current one ends. I need it so that it keeps creating the child processes and just displays the exit information when ever one process ends without blocking the parent from continuing. I can;t figure out where to put my wait to ensure this. Below is my code:
int main (int argc, char *argv[])
{
if (argc < 2)
{
printf("\n\nUsage: %s <enter a number (12 or less)>\n\n", argv[0]);
exit (-1);
}
else
{
int *processNum = (int *)malloc(sizeof(12));
int processNumTemp;
processNumTemp = atoi(argv[1]);
processNum = &processNumTemp;
if(*processNum > 12 || *processNum < 1)
{
printf("\n\nUsage: %s <enter a number (12 or lrss)>\n\n", argv[0]);
}
else
{
parentInfo(processNum);
createChildProcess(processNum);
}
}
return 0;
}
//Name: parentInfo
//Description: Displays information about the parent process
//Parameters: processNum - stores the number of child processes to create
// (entered at the command line).
//Return: none
void parentInfo(int *processNum)
{
printf("Parent process ID: %d\n", getppid());
printf("Number of processes to create: %d\n", *processNum);
}
//Name: createChildProcess
//Description: Creates n number of child processes.
// For each child process, it says its a child process and it
// displays its PID.
// After each child process closes, the parent displays info.
//Parameters: processNum - stores the number of child processes to create
// (entered at the command line).
//Return: none
void createChildProcess(int *processNum)
{
int i;
int childStatus;
pid_t childpid;
/*The for loop will create n number of processes based on the value of processNum.*/
for(i = 1; i <= *processNum; i++)
childpid = fork();
//Executes if fork didn't work
if(childpid < 0)
{
perror("fork");
exit(1);
}
//Executes if the fork worked
else if( childpid == 0)
{
int pid = getpid();
//Prints a message and the child processe's PID
printf("\nHello I am a child process.\n");
printf("My PID is %d. \n", getpid());
for(int x = 1; x <= pid; x ++);
exit(15);
}
}
//Executes after the child process has ended
//Checks the child process's exit status
waitpid(childpid, &childStatus, WUNTRACED);
printf("\nPID of the child process that was just created: %d.\n", childpid);
if(WIFEXITED(childStatus))
{
printf("PID %d exited normally. Exit number: %d\n", childpid, WEXITSTATUS(childStatus));
}
else if(WIFSTOPPED(childStatus))
{
printf("PID %d was stopped by %d\n", childpid, WSTOPSIG(childStatus));
}
else if(WIFSIGNALED(childStatus))
{
printf("PID %d exited due to signal %d\n.", childpid, WTERMSIG(childStatus));
}
else
{
perror("waitpid");
}
}
Before fork code
signal(SIGCHLD, childHandler);
In childHandler put your waitpid code.
void childHandler(int signum)
{
pid_t childpid;
int childstatus;
while ((childpid = waitpid( -1, &childstatus, WNOHANG)) > 0)
{
if (WIFEXITED(childStatus))
{
printf("PID %d exited normally. Exit number: %d\n", childpid, WEXITSTATUS(childStatus));
}
else
if (WIFSTOPPED(childStatus))
{
printf("PID %d was stopped by %d\n", childpid, WSTOPSIG(childStatus));
}
else
if (WIFSIGNALED(childStatus))
{
printf("PID %d exited due to signal %d\n.", childpid, WTERMSIG(childStatus));
}
else
{
perror("waitpid");
}
}
}
}
You should not use async-unsafe calls like printf inside a signal handler so alter your code to save the status in a global or heap allocated array - you know the size to create from processNum - and print the status info outside the handler.
Also, as currently structured, your parent could end before reaping all the children. Add a counter for the children so that you wait on all of them before the parent exits.
Look into signal SIGCHLD. If you have it blocked, you must unblock it or might instead explicitly check for it.
The purpose of wait is to, well, wait, so the way to solve your problem is to first create all the children, then start waiting for them to terminate.
Here is a program which does that:
// fork
#include <unistd.h>
// wait
#include <sys/types.h>
#include <sys/wait.h>
// exit
#include <stdlib.h>
//printf
#include <stdio.h>
void child( int id)
{
int seed= id;
int x= rand_r( &seed) % 10;
sleep( x);
exit( x);
}
int main( int argc, char *argv[])
{
const int n= 5;
int i;
printf( "creating %d children.\n", n);
for ( i= 0; i < n; ++i) {
pid_t pid= fork();
if ( !pid)
child( i); // does not return
else
printf( "child [0x%x] created.\n", pid);
}
// all the children are created now
// now we wait for them to terminate
printf( "waiting for children to terminate.\n", n);
for ( i= 0; i < n; ++i) {
int result;
pid_t pid= wait( &result);
printf( "child [0x%x] terminated with result [%u].\n", pid, WEXITSTATUS( result));
}
puts( "all children terminated.");
}