Program prints only lines from zombie process - c

I have the following source code:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
int main() {
int p, i;
p=fork();
if (p == -1) {perror("fork() error "); exit(EXIT_FAILURE);}
if (p == 0) {
for (i = 0; i < 10; i++)
printf("Child: i=%d pid=%d, ppid=%d\n", i, getpid(), getppid());
exit(0);
} else {
for (i = 0; i < 10; i++)
printf("Parent: i=%d pid=%d ppid=%d\n", i, getpid(), getppid());
wait(0);
}
}
If the parent process decides not to wait for child process ( delete the line which contains wait(0) statement), on the terminal will be printed only those lines within the child process. If I choose to redirect the output of the program into an arbitrary file ( that would be: ./source_code > some_file), some_file file will contain lines from parent process and child process, not only from child process like above. How is this possible?

Related

fork() in C. I need explanation on this code

So, i have this piece of C code
I can't grasp what the second 'for' segment is about. When does it get terminated abnormally?
Can someone enlighten me on that?
#include<unistd.h>
#include<stdio.h>
#include <sys/wait.h>
#define N 30
int main() {
pid_t pid[N];
int i;
int child_status;
for (i = 0; i < N; i++) {
pid[i] = fork();
if (pid[i] == 0) {
sleep(60 - 2 * i);
exit(100 + i);
}
}
for (i = 0; i < N; i++) {
pid_t wpid = waitpid(pid[i], & child_status, 0);
if (WIFEXITED(child_status)) {
printf("Child%d terminated with exit status %d\n", wpid, WEXITSTATUS(child_status));
} else {
printf("Child%d terminated abnormally\n", wpid);
}
}
return (0);
}
When child is terminate ,to be able to find with which value the child was terminated (either with exit or with return) i have to pash the second parametre in waitpid() with pointer to an integer.So in that integer on return from the call it will include 2 types of information
a) if child was terminated well with return or exit or stoped unexpectedly
b)the second type will be having the termination value.
If i want to know the information from (a) i need to use the macro WIFEXITED(), if this give me true the (b) emerged from macro WEXITSTATUS().This is a simple example
#include <stdio.h>
#include <stdlib.h> /* For exit() */
#include <unistd.h> /* For fork(), getpid() */
#include <sys/wait.h> /* For waitpid() */
void delay() { /* Just delay */
int i, sum=0;
for (i = 0; i < 10000000; i++)
sum += i;
printf("child (%d) exits...\n", getpid());
exit(5); /* Child exits with 5 */
}
int main() {
int pid, status;
pid = fork();
if (pid == 0) /* child */
delay();
printf("parent (%d) waits for child (%d)...\n", getpid(), pid);
waitpid(pid, &status, 0);
if (WIFEXITED(status)) /* Terminated OK? */
printf("child exited normally with value %d\n", WEXITSTATUS(status));
else
printf("child was terminated abnormaly.\n");
return 0;
}
SOS The macro WEXITSTATUS() return only the 8 least important bits of the value when the child is terminate.So if the child wants to "say" something to his parent through exit/waitpid it must be a number up to 255.

Set fork()'s child into foreground, and exit parent

I am trying to change the PID of a process in Linux.
Then the idea is to use fork() to spawn a child, and give the control of the terminal to this child, then exit from parent.
The result would be a new process with new process PID without interrupting the execution flow.
I've coded the following PoC, but it doesn't work. If I don't use wait() on the parent, the child process doesn't take control of the terminal. I mean, it continues outputting but the stdin is not attached to the child, is owned by bash.
#include <string.h>
#include <stdio.h>
#include <asm/ptrace.h>
#include <unistd.h>
#include <sys/syscall.h>
#include <linux/sched.h>
#include <stdlib.h>
#include <signal.h>
int main(void) {
int i = 0, ret = 0;
pid_t pid = 0;
printf("PID = %d\n", getpid());
pid = fork();
if (pid == -1) {
return -1;
}
if (pid != 0) {
printf("Parent PGID = %d\n", getpgid(0));
printf("Parent PGID for 0 = %d\n", tcgetpgrp(0));
signal(SIGTTOU, SIG_IGN);
for (i = 0; i < 3; i++) {
ret = tcsetpgrp(i, pid);
}
exit(0);
}
setpgid(0, 0);
printf("Child PGID = %d\n", getpgid(0));
printf("Child PGID for 0 = %d\n", tcgetpgrp(0));
for (i = 0; 1; i++) {
printf("iter = %d, PID = %d\n", i, getpid());
usleep(300000);
}
return 0;
}
Thanks in advance.

Synchronization of parent and child with SIGUSR signal in C. Make parent and child read one after the other

I have created a two way communication between parent and child processes using two pipes. Parent and child write data and I was able to make them read the data from each other. Parent writes numbers 1 to 5, and child writes numbers from 6 to 10. But I want parent to start reading data the first, and then reading continues in this order switching from parent to child until all the data are read: 6,1,7,2,8,3,9,4,10,5. I have tried to synchronize the reading with SIGUSR1 but when the parent is reading for the second time the program stops. I have searched a lot to find where the problem can be, and tried some tips and alike working examples, but nothing seems to help. Here is my code:
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/stat.h>
void paction(int dummy)
{
printf("P*************************************************\n");
}
void caction(int dummy)
{
printf("C*************************************************\n");
}
int main()
{
int pfd[2];
int pfd2[2];
pid_t cfork, pfork;
if (pipe(pfd) == -1 || pipe(pfd2) == -1) {
fprintf(stderr,"Pipe failed");
exit(1);
}
cfork = fork();
signal(SIGUSR1, paction);
if (cfork == -1) {
printf("Fork Failed\n");
exit(1);
}
else if (cfork > 0) { /*parent process*/
int numbers[] = {1, 2,3, 4, 5};
int numbers2[] = { 6, 7,8, 9, 10 };
close(pfd[0]); /*close read end, write and then close write end*/
/*write part*/
int limit = 5;
int i;
for (i = 0; i < limit; i++) {
printf("Parent sends: %d\n", numbers[i]);
write(pfd[1], &numbers[i], sizeof(numbers[i]));
printf("Child sends: %d\n", numbers2[i]);
write(pfd2[1], &numbers2[i], sizeof(numbers2[i]));
}
printf("***************************************************\n");
close(pfd[1]);
close(pfd2[1]);
/*read part/////////////////////////////////////////*/
int temp;
int reads = 5;
int j;
for (j = 0; j < reads; j++) {
sleep(1);
read(pfd2[0], &temp, sizeof(temp));
printf("Parent gets: %d\n", temp);
kill(cfork, SIGUSR1);
pause();
}
/*printf("***************************************************\n");*/
kill( cfork, SIGUSR1 );
close(pfd2[0]);
}
else { /*child process*/
signal(SIGUSR1, caction);
close(pfd[1]);
int temp;
int reads = 5;
int j;
pfork = getppid();
for (j = 0; j < reads; j++) {
sleep(1);
read(pfd[0], &temp, sizeof(temp));
printf("Child gets: %d\n", temp);
kill(getppid(), SIGUSR1);
pause();
}
/*printf("***************************************************\n");*/
close(pfd[0]);
close(pfd2[0]);
}
return 0;
}
My output looks like this:
> Parent sends:1
> Child sends:6
> Parent sends:2
> Child sends:7
> Parent sends:3
> Child sends:8
> Parent sends:4
> Child sends:9
> Parent sends:5
> Child sends:10
> **************************************************************
Parent gets:6
> C************************************************************
> Child gets:1
> P*************************************************************
> Parent gets:7
And here is when it stops.
If someone can help me I would really appreciate it because I really want to know where the problem is, and since I am a beginner in C programming and processes!
Thank you in advance
printf() is not an async-safe function. Calling printf() in both normal code and a signal handler will cause undefined behavior. In particular, printf() may need to take a lock on the output-stream, while taking locks in signal-handlers is very inadvisable (risk of self-deadlock).
Maybe it is a bad idea to use signals, but I had a task in which it was assigned to use SIGUSR1. I solved the issue by adding:
static struct sigaction pact, cact;
/* set SIGUSR1 action for parent */;
pact.sa_handler = p_action;
sigaction(SIGUSR1, &pact, NULL);
After the parent was assigned the first action, it worked fine.
Thank you:)

Calling every child process at once to kill?

I have to write an program which will generate a random amount of processes, and then will kill them one after one, after they all were created.
My problem is that I can't stop the child processes after being created.
Also, I try to call the termination-output to stdout from a child process, but don't really know how to solve it (because pid = 0 is for every child process).
#define _POSIX_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <time.h>
#include <signal.h>
#include <sys/wait.h>
int main(int argc, char const *argv[])
{
//int status;
srand(time(NULL));
int amount = (rand())%9+1;
pid_t fatherid = getpid();
printf("Hello I am a parent process, my PID is %d and I will now create %d children.\n",fatherid,amount);
pid_t pid = 1;
pid_t pidarr[amount];
for(int i = 0;i<amount;i++){
if(pid != 0){
pid = fork();
pidarr[i] = pid;
if(pid ==0){
printf("Hello I am a child process, my PID is %d and my parent has the PID %d.\n",getpid(),fatherid);
}
sleep(1);
}
}
if(pid != 0){
wait(NULL);
}
for(int i = (amount-1);i >= 0;i--){
if(pidarr[(i-1)] != 0){
printf("Hello I am a child process %d, I will terminate now.\n",getpid());
}
sleep(rand()%4);
if(pid != 0){
kill(pidarr[i],SIGKILL);
printf("Child Process %d was terminated.\n",pidarr[i]);
}
}
if(pid != 0){
printf("All child processes were terminated. I will terminate myself now.\n");
}
return EXIT_SUCCESS;
}
the following code shows how to handle fork and child processes.
the code compiles cleanly, is tested and works
#define _POSIX_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <time.h>
#include <signal.h>
#include <sys/wait.h>
int main( void )
{
//int status;
srand(time(NULL));
int amount = (rand())%9+1;
pid_t fatherid = getpid();
printf("Hello I am a parent process, my PID is %d and I will now create %d children.\n",fatherid,amount);
pid_t pid;
pid_t pidarr[amount];
for(int i = 0;i<amount;i++)
{
pid = fork();
if( -1 == pid )
{ //then, fork() error
perror( "fork() failed" );
exit(1);
}
// implied else, fork() successful
//pidarr[i] = pid;
if(!pid )
{ // then child process
printf("Hello I am a child process, my PID is %d and my parent has the PID %d.\n",getpid(),fatherid);
exit(0); // exit child process
}
// implied else, parent process
pidarr[i] = pid;
sleep(1);
} // end for
for(int i = (amount-1); i >= 0; i--)
{
kill(pidarr[i],SIGKILL);
printf("Child Process %d was terminated.\n",pidarr[i]);
}
printf("All child processes were terminated. I will terminate myself now.\n");
return(0);
} // end function: main
I am not sure about other parts of your logic (e.g. the if clause inside the fork loop), but
if(pid != 0){
wait(NULL);
}
looks suspiciously as of the parent process waits for a child to exit so that it doesn't get to the code which would kill the children at all (unless they exit on their own, but then the killing seems pointless).
Some issues in your code:
1) As #Peter Schneider points out,
parent process waits for a child to exit so that it doesn't get to the code which would kill the children
So first of all, you have to get rid of:
if(pid != 0){
wait(NULL);
}
2) The for loop that kills the children has to be executed only by the parent process, so the if clause embraces the for:
if(pid != 0){
for(int i = (amount-1);i >= 0;i--){
kill(pidarr[i],SIGKILL);
printf("Child Process %d was terminated.\n",pidarr[i]);
}
}
3) The child processes have to wait doing something until parent kills them, so append the following else clause to the above if:
else{
while(1){
printf("I am a child process %d. Will sleep for 2 senconds\n",getpid());
sleep(2);
}
}
4) the following code makes no sense, because when children are killed they simply stop working.
if(pidarr[(i-1)] != 0){
printf("Hello I am a child process %d, I will terminate now.\n",getpid());
}
If you want children to do something when the signal from kill() gets to them, you will have to use signals.

creating a second process in C

Im new in C programming and i have to do this:
Write a program that creates a second process, and then in both processes outputs the process ID and the owners user ID.
I don't know if thats right and how to continue from here. Here is what i have:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
int main(void) {
int ChildPID;
printf("This is the parent process number %d\n",getpid());
if ((ChildPID = fork()) == -1) {
perror("Could not fork");
exit(EXIT_FAILURE);
}
if (ChildPID == 0) {
//----In the child process
printf("This is the child process, number %d parent number %d\n", getpid(), getppid());
}
return(EXIT_SUCCESS);
}
The piece of code given below gives your solution. Here you can clearly identify parent code and child process code. Both are printing their corresponding pids.
void ExecuteChild(void);
void ExecuteParent(void);
int main(void)
{
pid_t pid;
pid = fork();
if (pid == 0)
ExecuteChild();
else
ExecuteParent();
}
void ExecuteChild(void)
{
int i;
for (i = 1; i <= 200; i++)
printf("CHILD[%d]: UserID[%d] printing - %d\n", getpid(),getuid(),i);
printf(" ------------- Child Exiting -------------\n");
}
void ExecuteParent(void)
{
int i;
for (i = 1; i <= 200; i++)
printf("PARENT[%d]: UserID[%d] printing - %d\n", getpid(),getuid(),i);
printf(" ------------- Parent Exiting -------------\n");
}

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