在子进程中编写c ++

Question

I am trying to change a variable value inside a child process and keep it for the rest of the program. So In the below example why my cout keeps printing 0, and how do I solve it?

int var = 0;
int pid = fork();

if (pid == 0){ //child process
   var = 1;
   exit(1);
}
else if (pid> 0){  //parent process
         if (-1 == wait (0))
              perror ("there was an error with wait");
}

cout << var;

Answer 1

This is how code execution is going to go.

Child Process

int var =0;

var = 1;
exit (1);

Parent Process

int var =0;
if (-1 == wait (0))
  perror ("there was an error with wait);

cout << var;

So as you can see, var will not be changed for the parent process.

When you fork , the child and parent have separate copies of the variables that were present before they split off into execution paths. Processes do not share the same block of memory. Therefore, altering var for the child will not affect the var in the parent.

If you want multiple execution paths on the same piece of memory, you should use threads .

Thank you for reading.

Answer 2

Looks like the fork() call has failed. Check the errno value

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.

ERRORS EAGAIN fork() cannot allocate sufficient memory to copy the parent's page tables and allocate a task structure for the child. EAGAIN It was not possible to create a new process because the caller's RLIMIT_NPROC resource limit was encountered. To exceed this limit, the process must have either the CAP_SYS_ADMIN or the CAP_SYS_RESOURCE capability. ENOMEM fork() failed to allocate the necessary kernel structures because memory is tight.

CONFORMING TO SVr4, 4.3BSD, POSIX.1-2001.