I am trying to create a small program that creates two sons ( proccess ), each son generate random number. the father wait for the sons and sum the results.
The Question: What is the modifications I need to do to make it work?
this is what I did so far.
#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>
#define SEGSIZE 100
void main()
{
key_t key;
int shmid;
char *segptr;
int status=0;
int r_pid=1;
int r_pid2=1;
key = ftok(".", 'T');
if((shmid = shmget(key, SEGSIZE,IPC_CREAT|IPC_EXCL|0666))== -1)
{
printf("Shared memory segment exists - opening as client\n");
if((shmid = shmget(key, SEGSIZE, 0)) == -1)
{
perror(" bad shmget");
exit(1);
}
}
else
{
printf("Creating new shared memory segment\n");
}
if((segptr = shmat(shmid, 0, 0)) == NULL)
{
perror("shmat");
exit(1);
}
r_pid=fork();
if(r_pid!=0)
r_pid2=fork();
if(r_pid<0 || r_pid2<0 )
{
printf("No child created");
exit(1);
}
if(r_pid==0 )
{
printf("The child process with PID number : %d"" (his parent PID is %d) writes a text to the shared"" memory\n",getpid(),getppid());
strcpy(segptr,"12");
}
else if(r_pid2==0)
{
segptr+=2;
printf("The child process with PID number : %d" " (his parent PID is %d) writes a text to the shared"" memory\n",getpid(),getppid());
strcat(segptr,"14");
}
else
{
r_pid = wait(&status);
r_pid2 = wait(&status);
printf(" The following text is received by the ""parent process with PID number pid: %d pid2 : %d text: %s\n",r_pid,r_pid2,segptr);
}
shmctl(shmid, IPC_RMID, 0);
}
Suggestions Are Welcomed! thanks.
EDIT
Here is some update:
If I initialize segptr with useless chars like: strpy(segptr,"a")
and then in the sons I do strcat(segptr,"test1") and in r_pid2 I do strcat(segptr,"test2")
the father will print test1test2 or vice versa.
There is no guarantee that the first child runs before the second. So, the
segptr+=2;
…
strcat(segptr,"14");
may be executed before
strcpy(segptr,"12");
causing the first digit of "14" to be overwritten by the terminating null character of "12".
What is the modifications I need to do to make it work?
Make sure that the writes do not overlap, e. g. by changing the
strcpy(segptr,"12");
to
strncpy(segptr, "12", 2);
Related
I write a program containing two processes: the first one contains a group of two semaphores and creates the child process that reads all data in the shared memory segment and prints them.
In the second one, the child process computes the data using a compute function that returns 0 when all data are computed. It transmits them to the parent through the shared memory segment.
To write data:
On the 1st semaphore the child makes P and the parent make V.
On the 2nd semaphore the child makes V and the parent make P.
But as I'm new in this topic and still getting hardness to understand, it seems like I'm doing something wrong because it's not working as it has to be.
Here is my code:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>
#include <semaphore.h>
#include <sys/shm.h>
#include <errno.h>
#include <sys/wait.h>
int sum =0;
int compute(int data){
sum += data;
return sum;
}
int main(){
int i;
int shm_id;
int data;
pid_t pid;
key_t shm_key;
sem_t *sem;
// unsigned int sem_value =2;
shm_key = ftok("/dev/null", 65);
shm_id = shmget(shm_id, sizeof(int), 0644 | IPC_CREAT);
if (shm_id < 0){
perror("shmgget");
exit(EXIT_FAILURE);
}
// data = shmat(shm_id, NULL, 0);
sem = sem_open("semaphore", O_CREAT | O_EXCL, 0644, 2);
for (i = 0; i < 2; i++){
pid = fork();
if (pid < 0)
{
perror("fork");
sem_unlink("semaphore");
sem_close(sem);
exit(EXIT_FAILURE);
}
else if (pid == 0)
{
break;
}
}
if (pid == 0)
{
puts("Enter the data:");
scanf("%d", &data);
//child process
sem_wait(sem);
printf("Child - %d is in critical section\n", i);
sleep(1);
puts("Enter the data:");
scanf("%d", &data);
// *shrd_value += data;
printf("Child - %d: new value of data = %d\n", i, data);
printf("Child - %d: sum of whole data by far = %d\n", i, compute(data));
sem_post(sem);
exit(EXIT_SUCCESS);
}
else if (pid > 0)
{
//parent process
while (pid = waitpid(-1, NULL, 0))
{
if (errno == ECHILD)
{
break;
}
}
puts("All children exited");
shmdt(&data);
shmctl(shm_id, IPC_RMID, 0);
sem_unlink("semaphore");
sem_close(sem);
exit(0);
}
}
Output:
Enter the data:
Enter the data:
2
Child - 0 is in critical section
1Enter the data:
Child - 1 is in critical section
Enter the data:
3
Child - 0: new value of data = 3
Child - 0: sum of whole data by far = 3
2
Child - 1: new value of data = 2
Child - 1: sum of whole data by far = 2
All children exited
I have also modified the way they write to shared memory: they write directly at the address given by shmat call that is missing in your code.
I have fixed some bugs and simplifed the code (removed the array - added detailed logging especially before and after entering the critial section):
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>
#include <semaphore.h>
#include <sys/shm.h>
#include <errno.h>
#include <sys/wait.h>
int main(){
int i;
int shm_id;
pid_t pid;
int *addr;
int data;
pid_t current_pid;
key_t shm_key;
sem_t *sem;
shm_key = ftok("/dev/null", 65);
shm_id = shmget(shm_key, sizeof(int), 0644 | IPC_CREAT);
if (shm_id < 0){
perror("shmget");
exit(EXIT_FAILURE);
}
sem_unlink("semaphore");
sem = sem_open("semaphore", O_CREAT, 0644, 1);
if (sem == SEM_FAILED) {
perror("sem_open");
exit(EXIT_FAILURE);
}
addr = (int *) shmat(shm_id, (void *) 0, 0);
if (addr == (void *) -1) {
perror("shmat");
exit(EXIT_FAILURE);
}
*addr = 0;
for (i = 0; i < 2; i++){
pid = fork();
if (pid < 0)
{
perror("fork");
sem_close(sem);
sem_unlink("semaphore");
exit(EXIT_FAILURE);
}
}
if (pid == 0)
{
current_pid = getpid();
printf("Child %d: waiting for critical section \n", current_pid);
sem_wait(sem);
printf("Child %d: enters in critical section \n", current_pid);
printf("child %d: Enter the data:\n", current_pid);
scanf("%d", &data);
printf("Child %d: new value of data = %d\n", current_pid, data);
printf("Child %d: sum of whole data so far = %d\n", current_pid, *addr += data);
sem_post(sem);
printf("Child %d exits from critical section\n", current_pid);
exit(EXIT_SUCCESS);
}
else if (pid > 0)
{
//parent process
while (pid = waitpid(-1, NULL, 0))
{
if (errno == ECHILD)
{
break;
}
}
puts("All children exited");
shmdt(addr);
shmctl(shm_id, IPC_RMID, 0);
sem_close(sem);
sem_unlink("semaphore");
exit(0);
}
exit(0);
}
Note that semaphore initial value must be 1 to have a true critical section for 2 processes.
I have also removed the sleep calls and we can see that one of the process is waiting:
Child 22514: waiting for critical section
Child 22514: enters in critical section
child 22514: Enter the data:
Child 22515: waiting for critical section
333
Child 22514: new value of data = 333
Child 22514: sum of whole data so far = 333
Child 22514 exits from critical section
Child 22515: enters in critical section
child 22515: Enter the data:
666
Child 22515: new value of data = 666
Child 22515: sum of whole data so far = 999
Child 22515 exits from critical section
All children exited
All children exited
Here's the code with producer and consumer process
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h> // O_CREAT, O_EXEC
#include <errno.h> // errno, ECHILD
#include <unistd.h>
#include <sys/shm.h> // shmat(), IPC_RMID
#include <sys/wait.h>
#include <semaphore.h> // sem_open(), sem_destroy(), sem_wait()...
#include <sys/types.h> // key_t, sem_t, pid_t
#include <pthread.h>
#define BUFF 10
typedef struct data{
int buff[BUFF];
int size;
int index;
}DATA;
int main(int argc, char const *argv[])
{
sem_t *full, *empty, *access;
key_t shm_key;
int shm_id;
full = sem_open ("fullname", O_CREAT , 0644, 15);
empty = sem_open ("empty", O_CREAT , 0644, 0);
access = sem_open ("access", O_CREAT , 0644, 1);
if (argc!=2)
{
exit(1);
}
int value=atoi(argv[1]);
//initialize a shared variable in shared memory
shm_key = ftok("/dev/null", 60);
shm_id = shmget(shm_key, sizeof(DATA), 0);
// shared memory error check
if (shm_id < 0){
shm_id = shmget(shm_key, sizeof(DATA), 0644 | IPC_CREAT);
DATA *data = (DATA*) shmat (shm_id, NULL, 0);
data->size=0;
data->index=0; //index
}
printf("Shared memory id: %d\n",shm_id );
printf("Checking buffer...,\n");
//If in the buffer have free space then will wait for consumer to consume the data\n"
sem_wait(empty);
printf("\nLocking buffer to produce data\n");
sem_wait(access);
//initialize a shared variable in shared memory
shm_key = ftok("/dev/null", 60);
shm_id = shmget(shm_id, sizeof(DATA), 0644 | IPC_CREAT);
// shared memory error check
if (shm_id < 0){
perror("semaphore");
exit(EXIT_FAILURE);
}
//Shared variable
DATA *data = (DATA*) shmat (shm_id, NULL, 0);
int index=(data->size + data->index) % 15;
data->buff[index]=value;
data->size++;
printf("%d is located in %d on the buffer\n",value,index );
//consusming
// attach data to shared memory
index=data->index;
value=data->buff[index];
printf("%d now consumed\n",value );
data->size--;
data->index++;
sem_post(access);
sem_post(full);
return 0;
}
I have an assignment for class and I am confused on this part of the requirements. So we need to make a multi process word counter with n number of processes and n will be an input argument for the program. Each process needs to do their own mini word count of a select portion of the inputted file. So essentially the inputted file will be divided into 1/n parts and split between n processes.
I understand how to fork the processes through a for loop and how to use pipes to send the mini word count from the children processes to the parent process, but I unsure of how to tell a certain process to do a select part of the input file.
Would you use their PID values to check which process they are then assign them their task?
This is my code so far.
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#define MSGLEN 64
#define MESSES 3
int main(){
int fd[2];
pid_t pid;
int result;
//Creating a pipe
result = pipe (fd);
if (result < 0) {
//failure in creating a pipe
perror("pipe error\n");
exit (1);
}
//Creating a child process
for(int i = 0; i < MESSES; i++){
if ((pid = fork()) < 0) {
//failure in creating a child
perror ("fork error\n");
exit(2);
}
if(pid == 0)
break;
}
if (pid == 0) {
// ACTUALLY CHILD PROCESS
char message[MSGLEN];
//Clearing the message
memset (message, 0, sizeof(message));
printf ("Enter a message: ");
//scanf ("%s",message);
fgets (message, 1024, stdin);
close(fd[0]);
//Writing message to the pipe
write(fd[1], message, strlen(message));
close(fd[1]);
close(fd[0]);
exit (0);
}
else {
//Parent Process
char message[MSGLEN];
char *ptr;
long wc;
close(fd[1]);
while (1) {
//Clearing the message buffer
memset (message, 0, sizeof(message));
//Reading message from the pipe
if(read(fd[0], message, sizeof(message)) == 0)
exit(0);
printf("Message entered %s\n",message);
/*
Message entered needs to be in the format of number first space then string for it to work
*/
wc = 0;
wc = strtol(message, &ptr, 10);
printf("The number(unsigned long integer) is %ld\n", wc);
printf("String part is %s", ptr);
}
close(fd[0]);
wait(NULL);
// exit(0);
}
return 0;
}
The key thing to remember when using fork is that the parent and child share the same memory and a copy of everything the parent has is passed to the child. At which point the child has now forked the parents data.
In the code below we're counting how many processes we've created. You could if you wanted use this as an argument in the child ie the nth child gets value n.
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#define PROCESS_COUNT 50
int main(void) {
pid_t pid;
size_t pid_count = 0;
//pid_t pid_array[PROCESS_COUNT];
for(int i = 0; i < PROCESS_COUNT; i++) {
if ((pid = fork()) < 0) {
perror ("fork error\n");
exit(2);
}
if (pid == 0) {//child
size_t n = 0;
size_t p = getpid();
while(n++ < 2) {
//Next line is illustration purposes only ie I'm taking liberties by
//printing a pid_t value
printf("child %zu has pid_count == %zu\n", p, pid_count);
sleep(1);
}
exit (0);
}
else {
//Count how many process we've created.
pid_count++;
int status;
waitpid( -1, &status, WNOHANG);
}
}
wait(NULL);
return 0;
}
If you want to get really fancy you can use IPC using pipes or shared memory. There are lots of ways to get data from one process to another, sometimes something as simple as temporary files is more than sufficient. For your problem I'd use mmap but it does not need to be that complicated
I'm trying to work with shared memory at the first time. I created one child process and I write to the shared memory from Parent and change it from Child, before program ends I print shared memory from Parent and shared memory hasn't change, here is my code:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <semaphore.h>
#include <fcntl.h>
#include <sys/shm.h>
#include <signal.h>
sem_t *semaphore;
int main(){
int i = 0, status;
pid_t pid=fork(), w;
int id;
if ((id = shmget(IPC_PRIVATE, sizeof(int), IPC_CREAT | 0666)) == -1){
perror("shmget");
exit(1);
}
int *sh;
if ((sh =(int *) shmat(id, NULL, 0)) == NULL){
perror("shmat");
exit(1);
}
*sh = 10;
if ((semaphore = sem_open("/semaphore", O_CREAT, 0666, 0)) == SEM_FAILED){
perror("semaphore");
exit(1);
}
if (pid==0) {
while(1){
sleep(1);
*sh = 50;
printf("child %d, %d\n", i, *sh);
i++;
if(i == 5){
sem_post(semaphore);
exit(0);
}
}
} else if (pid==-1) {
perror("process error\n");
} else {
printf("Parent, %d\n", *sh);
sem_wait(semaphore);
printf("child end => parent end\n");
printf("Parent, %d\n", *sh);
}
shmctl(id, IPC_RMID, NULL);
sem_close(semaphore);
sem_unlink("/semaphore");
return 0;
}
If I understand shared memory little bit than I can change it from everywhere if I have a pointer in my case is a "sh".
Output of program is:
Parent, 10
child 0, 50
child 1, 50
child 2, 50
child 3, 50
child 4, 50
child end => parent end
Parent, 10
Why is the number in shared memory different in Parent and in Child?
You fork() before you create the shared memory with the key IPC_PRIVATE, so both processes create their own shared memory and don't actually share it.
If you remove the =fork() from
pid_t pid=fork(), w;
and insert
pid = fork();
somewhere after the shmget call, it works the way you expect, since the child process will inherit the shared memory identifier from the parent and not create a different one.
I wrote the following code to help me understand how pipes work in C.
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
struct sum_ {
int a;
int b;
};
int main (void) {
int pipe1[2];
int pid;
struct sum_ sum;
if ( (pipe(pipe1) != 0)){
printf("pipe(): %d %s\n", errno, strerror(errno));
exit(1);
}
pid = fork();
if (pid == -1) {
printf("fork(): %d %s\n", errno, strerror(errno));
exit(1);
}
else if (pid == 0) { // Child
close(pipe1[0]);
sleep(5);
sum.a = read(pipe1[0], &sum.a, sizeof(sum.a));
printf("Your number was: %d", sum.a);
}
else { // Father
close(pipe1[1]);
printf("\nWrite a number: \n");
char a[4];
sum.a = atoi(fgets(a, 4, stdin));
write(pipe1[1], &sum.a, sizeof(sum.a));
}
return 0;
}
The code has a father and a son process. It is quite simple, the father uses a pipe to send a number to the son and the son displays the number for the user.
I always get -1 as result. What have I done wrong?
close(pipe1[0]);
sleep(5);
sum.a = read(pipe1[0], &sum.a, sizeof(sum.a));
You close the file descriptor pipe1[0], then read from it (and so get -1 returned). You make the equivalent error in the father, too. I think you mean to close pipep1[0] here and pipe1[1] in the father
Also, when you fix that, lthough you're reading into sum.a by passing the address, you're also setting it from the return value, which will overwrite what you read.
I know I'm going to need to use fork(), but this just creates a single child process. Do i simply call fork again from within the child process? Also, I need them to communicate through a signal or pipe, which is easier to implement and what do i need to know for doing that (functions, etc..)
To create a second process, call fork() again - either within the parent or the child (but not both!). Which you choose depends on whether you want this process to be a child of the original parent or a child of the first child process (it is usual for it to be a child of the original parent).
Communicating through a pipe is much simpler and more reliable than using signals. pipe(), close(), read(), write() and select() are the key functions here.
For example, to have the parent create two child processes, you would do something like:
pid_t child_a, child_b;
child_a = fork();
if (child_a == 0) {
/* Child A code */
} else {
child_b = fork();
if (child_b == 0) {
/* Child B code */
} else {
/* Parent Code */
}
}
Another fancy code using && operator:
pid_t c1_pid, c2_pid;
(c1_pid = fork()) && (c2_pid = fork()); // Creates two children
if (c1_pid == 0) {
/* Child 1 code goes here */
} else if (c2_pid == 0) {
/* Child 2 code goes here */
} else {
/* Parent code goes here */
}
#include <stdio.h>
#include <unistd.h>
void main(){
int pi_d ;
int pid ;
pi_d = fork();
if(pi_d == 0){
printf("Child Process B:\npid :%d\nppid:%d\n",getpid(),getppid());
}
if(pi_d > 0){
pid = fork();
if(pid > 0){
printf("\nParent Process:\npid:%d\nppid :%d\n",getpid(),getppid());
}
else if(pid == 0){
printf("Child Process A:\npid :%d\nppid:%d\n",getpid(),getppid());
}
}
}
output :
Parent Process:
pid:3648
ppid :2379
Child Process B:
pid :3649
ppid:3648
Child Process A:
pid :3650
ppid:3648
You can put the fork in a loop and generate as many child processes as you need.
I did that on a project recently.
for(nSon=0; nSon < nSonsAsked; nSon++) {
Log_Print("Setup son #%.2u ", nSon+1);
if((pid = fork()) == 0) {
/* Do child stuff init, like connect the pipes, close shared handles */
return iTMInChild(...); /* A specific function of the child work */
/* The life of the child should not go beyond that point, i.e. the loop is over
or else the child will spawn even more processes. */
}
else if(pid > 0) {
/* Father process stuff. Here I initialise an array with the pid of the forked */
/* processes, this way I can index with the number of processes.*/
pid[nSon] = pid;
}
else
return Err_Print(ERR_FORK_FAILED, "fork failed. errno=%d \"%s\"\n", errno, strerror(errno));
}
Log_Print() and Err_Print() are internal functions but quite obvious so I let them like they are.
There is one aspect with the variables that has to be explained. nSon and nSonAsked should be declared as globals not as stack variables. This way, their value persists in the forked process. This means that the nSon variable will have a different value in each of the children. This allows it to have a simpler numbering scheme than the ownpid() number.
To get it completely right, there are a lot of details to get right. You will have to set signal handlers in the father process to detect the death of a child, likewise the other way round (only possible on Linux, other Unix (at least Solaris) do not support parent death signals).
You have to be aware that open file descriptors in the father process will be also open in the child after fork and it will be the same one. This opens a lot of concurrency problems if you're not aware of it (the solution is using dup() and close() in the right places).
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
int main()
{
system ("clear");
int i ;
pid_t childa,childb,childa1,childa2,childb1,childb2;
printf("\n \t \t I am the parent process with ID %d \n",getpid());
childa=fork();
if (childa == 0 )
{
printf("\nI am a child A with PID %d and my parent ID is %d\n",getpid(),getppid());
}
else
{
childb = fork();
if (childb == 0)
{
printf("\nI am Child B with ID %d and my parent ID is %d\n",getpid(),getppid());
}
else
{
sleep(1);
}
}
}
In this example they are just sleeping for a few random sec. It also has all the pid, so we can send SIGNAL to communicate... Most of the #includes are commented cause they were useless where I compiled.
#include <stdlib.h> // exit() ...
#include <stdio.h> // printf() ...
// Compile with -lrt -> cc file_name.c -lrt
//#include <fcntl.h>
//#include <sys/stat.h>
//#include <sys/types.h>
//#include <sys/wait.h> // may need this for wait()
//#include <time.h>
//#include <unistd.h> // and this one for fork()
// In the start function you can do whatever you want.
void start (const int azon) {
// For children processes
srand( time(NULL) );
unsigned t = rand()%5; // printf("%d\n", t);
sleep(t);
printf("%d. process reached the end.\n", azon);
exit(0);
}
int main() {
const int N = 5;
pid_t pids[N];
int i;
// The 'for' loop make 'N' process with 'fork()'.
// The children processes will call the start function.
// Since after fork() you will get 2 process. One Parent, and One Child
// The returning value from fork() is saved in "pids" which is an
// integer AND it is (<0) IF something went wrong.
// it is (>0) IF 'we are' in the Parent process,
// because this number is the Child process' ID (pid).
// and Last it is (==0) IF 'we are' in the Child process.
for (i = 0; i < N; i++) {
pids[i] = fork();
sleep(1);
if (pids[i] == 0) start(i+1); // ... OR you can make a switch(..)
}
// This 'for' loop in the wait(NULL) statement ONLY move on when a
// process ended, so it waits until 'N' proc ends.
for (i = 0; i < N; i++)
wait(NULL);
printf("Partent process reached the end\n");
return 0;
}
Just a little contribution, if you want to create 2 childs from the same parent you could use this code below. In which one father create 2 child processes (lazy and active).
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
int main (){
pid_t lazy_child;
lazy_child = fork();
if(lazy_child == 0){ // This is the lazy child process.
printf("LAZY CHILD:%d\n", getpid());
}
else if(lazy_child > 0){ // This is the father process.
pid_t active_child = fork();
if(active_child == 0){ // This is the active child process.
printf("ACTIVE CHILD:%d\n", getpid());
}
else if(active_child > 0){ // This is the father process.
printf("FATHER:%d\n", getpid());
}
else{ // Fork doesnt work.
printf("fork error\n");
exit(1);
}
}
else{ // Fork doesnt work.
printf("fork error\n");
exit(1);
}
return 0;
}
If you run this code, you should get a similar output:
$ ./a.out
FATHER:14501
ACTIVE CHILD:14503
LAZY CHILD:14502
#include <sys/wait.h>
#include <stdio.h>
#include <unistd.h>
int main()
{
pid_t AliceID, BobID;
double n=0;
int i1 =0;
/* fork a child process */
AliceID = fork();
if (AliceID < 0) { /* error occurred */
fprintf(stderr, "Fork Failed");
return 1;
}
else if (AliceID == 0) { /* child Alice code */
for(int i=1; i<11; i++)
{n = n+i;
i1++; }
double avg1 = n/i1;
printf("From Alice: the average of 1,2, …, 10 is the-average-she-calculated");
printf(" sum = %.2f and avg = %.2f \n",n, avg1);
}
else {
BobID = fork();
if (BobID == 0) { /* Child Bob code */
printf("From Bob: I am born to print this and then die.\n");
} else { /* Parent Code */
/* parent will wait for the child to complete */
wait(NULL);
printf("From parent: AliceID is %d \n", AliceID);
printf("From parent: Bob is %d \n", BobID);
printf("Parent ID %d \n", getpid());
}
}
return 0;
}