The task is as follows
Write a program that would run another process in memory and leave it running in an infinite loop. When the program is restarted, it must remove the previously started process from memory (you can use kill).
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <stdlib.h>
#include <spawn.h>
#include <sys/wait.h>
int main(void){
int pid = getpid(); // we find out the PID of the current process and store it in a variable
FILE *file = fopen("example.txt", "r"); // getting information from a file about a child process
int filePid = 0;
fscanf(file, "%d", filePid);
fclose(file);
switch (filePid){
case -1:{ // if there is no child process, then run it and write the PID to a file
filePid = fork();
file = fopen("example.txt", "w");
fprintf(file, "%d", filePid);
fclose(file);
break;
}
case 0:{ // if this process is a child, then we go into an infinite loop
for(;;){
sleep(7); // waiting for seven seconds so that the system is not heavily loaded
}
break;
}
default:{ // if this program is started again with a child process, then we send a signal to the child process
kill(filePid, SIGKILL); // we send a signal to the child process so that it ends, and after that we write the information to the file
file = fopen("example.txt", "w"); // we write information to the file that the child process is missing
fprintf(file, "%d", -1);
fclose(file);
}
}
return EXIT_SUCCESS;
}
/Yes, I have to do it through the qnx operating system./
the errors are as follows..I'm a little confused with getpid, because I haven't used the pid variable anywhere.
and another mistake.
I will be grateful for your help.since I'm a little confused...
UPD:
I can't get the value 0
UPD: how could it execute both cases, i mean "if" and "else" blocks at the same time?
how could it execute both cases, i mean "if" and "else" blocks at the same time?
You have to have a clear understanding of how fork works, when you use fork two identical processes are created parent and child and they run simultaneously, when you say filePid = fork() and if the operation is successful then the parent process will hold the process id of the child process and child process will hold 0. So here in parent process filePid == child process ID and in child process filePid == 0.
See man fork
On success, the PID of the child process is returned in the parent,
and 0 is returned in the child. On failure, -1 is returned in the
parent, no child process is created, and errno is set appropriately.
So whatever you put in the if (filePid == 0) block will be executed by the child process and whatever you put in the else block will be executed by the parent process simultaneously.
This will be helpful if you want to know more about fork.
fork() system call creates a child process and return its pid so once child is created you have two process running same instruction after fork() one is checking pid is greater which means its parent which gets child pid and in child context OS will set the pid value to 0 so it will enter condition block intended for child
Related
[SOLVED]
So for this practice exercise I was assigned we are practicing process control. I'm simply wondering why it seems that my child process will never execute (when child_pid == 0). Any help on this issue would be greatly appreciated!
Also, safe_fork() is a function our instructor wrote in order to avoid students opening too many processes.
#include "safe-fork.h"
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
int main(void) {
pid_t child_pid;
int pipefd[2];
int pipe_result;
int count;
pipe_result = pipe(pipefd);
if (pipe_result == -1) {
perror("pipe failure");
exit(1);
}
child_pid = safe_fork();
/*Parent Process*/
if (child_pid > 0) {
dup2(pipefd[0], STDIN_FILENO);
scanf("%d", &count);
if (count > 200) {
printf("Long enough!\n");
exit(0);
}
else {
printf("Too short!\n");
exit(1);
}
}
/* Child Process */
else {
dup2(pipefd[1], STDOUT_FILENO);
execl("/usr/bin/wc", "-w");
}
return 0;
}
Here is a suggestion, check for child_pid == 0 explicitly cause it could also be the case that your safe_fork() function failed, also check for those errors i.e child_pid < 0.
Add debug printf statements to check control flow if you already haven't and finally check if the execl call failed.
Also please have a look at this tutorial for creating pipes in C and using them to communicate between a parent and a child process. As I feel there are some issues with how you are handling those file descriptors. A possible reason that you can't see any output is that wc might still waiting for some input.
And another thing you might need to wait for the child process to complete/do something useful or actually be run before checking for its output. Check the other issues first before you worry about this.
Hope this Helps :)
Yes, But no. First you should carefully understand the meaning of fork().
fork(): creates a child process and it returns into TWO PROCESSES, with different return value in each. In parent, it always returns the number that OS has assigned for the newly created child's process id. At the same it returns in newly created child also, as child process will start executing the code copied from parent just after the fork() statement. In child fork always returns zero. So normally after a fork() system call, people check for returned value by fork to differentiate child process and parent process.
particularly in this case after a fork(), there will be two processes with same code and same program states(except both will have different value assigned in child_pid variable). In parent its a non zero value (which is corresponding to child's pid) and in child, its the value zero as the fork returns zero in child.
Now both processes will execute
if (child_pid > 0)
which is true only for parent so parent process will flow into 'if block' and is false for child so it will directly jump to else part and will execute the else block.
What if child_process id is zero?
OS will always assign a non zero value for a child so that fork() will work. And if you have somehow got child process id as zero then parent will fail to enter in 'if block' and will execute else block which is the same as child.
Can someone please explain to me what does fork() != 0 mean? From what I understand I think it means if fork is not false? Or if fork is true then.... I don't understand how Fork() can be true or false, seeing that it just creates a copy of a process into a parent and child. Also if a program where to say if (Fork() == 0) what would that mean?
#include "csapp.h"
int main(void)
{
int x = 3;
if (Fork() != 0)
printf("x=%d\n", ++x);
printf("x=%d\n", --x);
exit(0);
}
fork() returns -1 if it fails, and if it succeeds, it returns the forked child's pid in the parent, and 0 in the child. So if (fork() != 0) tests whether it's the parent process.
From man fork
Return Value
On success, the PID of the child process is returned in the parent,
and 0 is returned in the child. On failure, -1 is returned in the
parent, no child process is created, and errno is set appropriately.
Assuming success, fork returns twice: once in the parent, and once in the child.
OK, I did the OP a disservice: I don't know where csapp.h comes from, but if it's this one then it isn't doing you any favours. I guess it is a thin wrapper on POSIX (eg. around fork()), but maybe works on other platforms too?
Because you mentioned fork() before Fork(), I assumed the latter was a typo, whereas it's actually a library function.
If you had been using fork() directly, it would be reasonable to expect you to check the manpage.
Since you're using a Fork() function provided by some library, that library really ought to document it, and doesn't seem to.
Standard (non csapp) usage is:
pid_t child = fork();
if (child == -1) {
printf("fork failed - %d - %s\n", errno, strerror(errno));
exit(-1);
}
if (child) {
printf("I have a child with pid %d, so I must be the parent!\n", child);
} else {
printf("I don't have a child ... so I must be the child!\n")
}
exit(0);
Let's try explaining it differently... When the function starts there's 1 process, this process has a int x = 3
Once you hit this line of code:
if (fork() != 0)
Now, assuming the fork() worked, we have two processes. They both have the same execution space, they both are going to run the same code (to a point), but the child will get its own copy of x to play with.
fork() will return a 0 to the child process, so from the child processes' prospective, the rest of the function is this:
printf(("x=%d\n", --x);
exit(0);
The parent process on the other hand, will get a valid value back from the fork() command, thus it will execute:
printf("x=%d\n", ++x);
printf("x=%d\n", --x);
exit(0);
What the output at this point will be is anyone's guess... You can't tell if parent or child will run first
But if we assume the parent hits the ++x is the next operation then the output is:
x=4
x=3
x=2
As both parent and child will hit the --x. (the parent's x was 4 after the ++x, will be 3 at the end. The child's x was 3, will be 2 at the end)
From fork() manual:
Upon successful completion, fork() returns a value of 0 to the child process and returns the process ID of the child process to the parent
process. Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set to indi-
cate the error.
After the code execution you have two execution threads. Into the if statement you have the parent process' thread and in else statement you have the child process' thread.
if ( fork() ) {
printf("I am the parent!\n");
} else {
printf("I am the child\n");
}
EDIT
For clarification purpose: fork starts a process, which has a thread, memory and may have other resources. I tried (it seems like that without success) to emphasize the flows of execution by adding the "thread" word.
However, by no means, one can say that "parent" relates to "thread" in "parent process' thread".
Of course, my answer could be improved but I think there are already enough good answers here.
Fork returns 0 for the child process and the process id of the child to the parent process. Hence commonly code has if(fork){ }else code. Which implies that the code inside the if is going to be executed only in a parent.
The better way to deal with it is
pid = fork()
if(pid){
// I am parent. Let us do something that only the parent has to do
}else{
// I am child. Let us do something only the child has to do
}
// This code is common to both
The child pid may be useful to wait upon later or to detach from the parent.
I recommend replacing the if with a switch because there are 3 possible results:
#include <sys/types.h>
#include <unistd.h>
pid_t pid;
switch ((pid = fork ())) {
case -1: /* error creating child. */
break;
case 0: /* I am the child process. */
break;
default: /* I am the parent process. */
break;
}
Consider the code:
#include <stdio.h>
#include <errno.h>
#include <sys/types.h>
#include <unistd.h>
/* main --- do the work */
int main(int argc, char **argv)
{
pid_t child;
if ((child = fork()) < 0) {
fprintf(stderr, "%s: fork of child failed: %s\n",
argv[0], strerror(errno));
exit(1);
} else if (child == 0) {
// do something in child
}
} else {
// do something in parent
}
}
My question is from where does in the code the child process starts executing, i.e. which line is executed first??
If it executes the whole code, it will also create its own child process and thing will go on happening continuously which does not happen for sure!!!
If it starts after the fork() command, how does it goes in if statement at first??
It starts the execution of the child in the return of the fork function. Not in the start of the code. The fork returns the pid of the child in the parent process, and return 0 in the child process.
When you execute a fork() the thread is duplicated into memory.
So what effectively happens is that you will have two threads that executes the snippet you posted but their fork() return values will be different.
For the child thread fork() will return 0, so the other branch of the if won't be executed, same thing happens for the father thread.
When fork() is called the operating system assigns a new address space to the new thread that is going to spawn, then starts it, they will both share the same code segment but since the return value will be different they'll execute different parts of the code (if correctly split, like in your example)
The child starts by executing the next instruction (not line) after fork. So in your case it is the assignment of the fork's return value to the child variable.
Well, if i understand your question correctly, i can say to you that your code will run as a process already.When you run a code,it is already a process , so that this process goes if statement anyway. After fork(), you will have another process(child process).
In Unix, a process can create another process, that's why that happens.
Code execution in a child process starts from the next instruction following the fork() system call.
fork() system call just creates a seperate address space for the child process therefore it is a cloned copy of the parent process and the child process has all the memory elements of it's parent's process.
Thus, after spawning a child process through fork(), both processes (the parent process and the child process) resumes the execution right from the next instruction following the fork() system call.
I'm new to c language and Linux. I have a problem related to fork(),getpid()and exec()function.
I wrote a c program using fork() call the code of my program is following"
code:
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
void fun()
{
printf("\n this is trial for child process");
}
int main (int argc, char const *argv[])
{
int i,status,pid,t;
if(pid=fork()<0)
{
printf("\nfailed to create the process\n");
}
if(pid=fork()==0)
{
printf("\n the child process is created");
fun();
exit(1);
}
while(wait(&status)!=pid);
return 0;
}
The out put of this program is following:
the child process is created
this is trial for child process
the child process is created
this is trial for child process
Now my questions are as follows:
Why the output of program showing same thing twice? The output supposed to be "child process is created this is trial for child process"
Why the output is not according to code ?
Can we have a program which has 4 processes and all the processes perform different task for example one process print "my name". One process print "my age", the other process print "my address ?
How to make multiple process in main function ?
How to control the execution of multiple process ?
what does the exec() function do? Can anyone please explain me the working of exec(), fork(), getpid() with a source code?
Please help this novice fellow.
Your code calls fork() multiple times:
if(pid=fork()<0) /* calls fork() */
{
...
}
if(pid=fork()==0) /* also calls fork() */
{
...
}
Each successful fork() creates a new child process. To make matters worse, the second fork() is called by both the parent and the first child.
If you're trying to create a single child process, you should call fork() just once:
pid_t pid; /* note the correct return type of fork() */
...
pid = fork();
if (pid < 0)
{
...
}
else if (pid == 0)
{
...
}
If you want to create multiple child processes, you can have the parent process call fork() in a loop.
As to questions like "what does exec do?", my advice is to learn how to use man and then come back with specific questions if there's something in the manpages that remains unclear.
In this code you are creating Three process not including your main process.
pid=fork()
is itself a statement , which forks a new process even though it is inside an if statement condition. After the first fork() call the remaining codes will be executed twice. so next fork call will be called twice. You have already created a new process.
fork returns zero to itself and its
process id to its parent
That is consider a process A forks B (not from your code)
pid = fork();
printf("pid is : %d",pid);
printf statement executes twice(one for A and one for B). For A it prints(A is parent)
pid is : 512 //some integer value
process id
and B prints
pid is : 0
So in your question
if(pid=fork()==0)
{
printf("\n the child process is created");
fun();
exit(1);
}
this is the second fork which is already executing twice. So each of this execution creates a new child process. For both childs pid value is 0. So your print statement executes, which is what you see in the output. But for both parents a pid value will be there and your if condition fails, so it wont print. These two childs are your second and third processes..So in short you create 3 processes along with the main process
The output is generated twice because you are forking twice:
if(pid=fork()<0) // Fork #1
{
printf("\nfailed to create the process\n");
}
if(pid=fork()==0) // Fork #2
System information: I am running 64bit Ubuntu 10.10 on a 2 month old laptop.
Hi everyone, I've got a question about the fork() function in C. From the resources I'm using (Stevens/Rago, YoLinux, and Opengroup) it is my understanding that when you fork a process, both the parent and child continue execution from the next command. Since fork() returns 0 to the child, and the process id of the child to the parent, you can diverge their behavior with two if statements, one if(pid = 0) for the child and if(pid > 0), assuming you forked with pid = fork().
Now, I am having the weirdest thing occur. At the beginning of my main function, I am printing to stdout a couple of command line arguments that have been assigned to variables. This is this first non assignment statement in the entire program, yet, it would seem that sometimes when I call fork later in the program, this print statement is executed.
The goal of my program is to create a "process tree" with each process having two children, down to a depth of 3, thus creating 14 total children of the initial executable. Each process prints its parent's process ID and its process ID before and after the fork.
My code is as follows and is properly commented, command line arguments should be "ofile 3 2 -p" (i haven't gotten to implementing -p/-c flags yet":
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
int main (int argc, char *argv[])
{
if(argc != 5)//checks for correct amount of arguments
{
return 0;
}
FILE * ofile;//file to write to
pid_t pid = 1;//holds child process id
int depth = atoi(argv[2]);//depth of the process tree
int arity = atoi(argv[3]);//number of children each process should have
printf("%d%d", depth, arity);
ofile = fopen(argv[1], "w+");//opens specified file for writing
int a = 0;//counter for arity
int d = 0;//counter for depth
while(a < arity && d < depth)//makes sure depth and arity are within limits, if the children reach too high(low?) of a depth, loop fails to execute
//and if the process has forked arity times, then the loop fails to execute
{
fprintf(ofile, "before fork: parent's pid: %d, current pid: %d\n", getppid(), getpid());//prints parent and self id to buffer
pid = fork(); //forks program
if(pid == 0)//executes for child
{
fprintf(ofile, "after fork (child):parent's pid: %d, current pid: %d\n", getppid(), getpid());//prints parent's id and self id to buffer
a=-1;//resets arity to 0 (after current iteration of loop is finished), so new process makes correct number of children
d++;//increases depth counter for child and all of its children
}
if(pid > 0)//executes for parent process
{
waitpid(pid, NULL, 0);//waits on child to execute to print status
fprintf(ofile, "after fork (parent):parent's pid: %d, current pid: %d\n", getppid(), getpid());//prints parent's id and self id to buffer
}
a++;//increments arity counter
}
fclose(ofile);
}
When I run "gcc main.c -o ptree" then "ptree ofile 3 2 -p", the console is spammed with "32" a few times, and the file "ofile" is of seemingly proper format, but a bit too large for what I think my program should be doing, showing 34 child processes, when there should be 2^3+2^2+2^1=14. I think this is somehow related to the statement that is printing "32", as that would seem to possibly spawn more forks than intended.
Any help would be greatly appreciated.
When you call printf, the data is stored in a buffer internally. When you fork, that buffer is inherited by the child. At some point, (when you call printf again, or when you close the file), the buffer is flushed and data is written to the underlying file descriptor. To prevent the data in the buffer from being inherited by the child, you can flush the FILE * before you call fork, via fflush.