Issue
I am trying to test how to create and manage processes with a program what should create a process and execute a command 2 times (each time executing a different command).
What I am trying to do exactly:
- Create a new PID
- Print the PID
- Execute a command
- Exit
Weirdly, when I execute 1 command, it works fine but when I execute the routine twice, the 2nd command is being executed twice also. Meaning that 3 commands get executed.
Here is my code:
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
void launch_command(char command[]) {
pid_t pid;
pid = fork();
if(pid==0)
{
printf("Creating process %d from %d\n", getpid(),getppid());
system(command);
exit(0);
}
else {
if ((pid = fork()) == -1)
perror("Error while creating a new process");
}
}
int main () {
printf("Root process: %d\n",getpid());
launch_command("/bin/ls -l");
while(wait(NULL) >= 0);
launch_command("/bin/date");
while(wait(NULL) >= 0);
return 0;
}
The output:
$ ./a.out
Root process: 8924
Creating process 8925 from 8924
Creating process 8928 from 8926
Sun 13 Nov 2022 23:54:51 EST
total 128
-rwxr-xr-x 1 gg staff 49784 13 Nov 23:54 a.out
-rw-r--r-- 1 gg staff 3499 11 Nov 10:59 create_process.c
Creating process 8931 from 8924
Sun 13 Nov 2022 23:54:51 EST
Could someone explain me this behaviour?
The desired result should be:
$ ./a.out
Root process: 8924
Creating process 8925 from 8924
total 128
-rwxr-xr-x 1 gg staff 49784 13 Nov 23:54 a.out
-rw-r--r-- 1 gg staff 3499 11 Nov 10:59 create_process.c
Creating process 8931 from 8924
Sun 13 Nov 2022 23:54:51 EST
Solution
As pointed out in the comments, the second call to fork
is creating too many processes.
After the first call to launch_command
returns, there will be both a parent process and a freshly made child process. They will both call the second launch_command
function, spawning two more processes that call system
, and then an additional process each, that again return from the function as well.
There will be four total processes that execute the second while
loop.
Remove the second fork
call, and use the same value of pid
again to check for failure.
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
void launch_command(const char *command)
{
pid_t pid = fork();
if (0 == pid) {
printf("Creating process %d from %d\n", getpid(), getppid());
system(command);
exit(EXIT_SUCCESS);
} else if (-1 == pid) {
perror("Error while creating a new process");
}
while (wait(NULL) >= 0);
}
int main(void)
{
printf("Root process: %d\n", getpid());
launch_command("/bin/ls -l");
launch_command("/bin/date");
}
Answered By - Oka Answer Checked By - Timothy Miller (WPSolving Admin)