How to create unix process binary trees?

Question

Can some one help me out here, not necessarily complete my homework.

I need to create a process tree using fork(); in Unix/C so far the best I can get to to the 4th

Answer 1

The waitpid() function requires arguments.

The initial childpid = fork(); is unexpected.

You can sensibly write:

for (int i = 0; i < 1; i++)

for the loops unless you insist on using C89 instead of C99 syntax.

Attempt 2

Example output

Start 46480
i am 46480, my parent is 30775
i am 46482, my parent is 46480
i am 46481, my parent is 46480
i am 46483, my parent is 46481
i am 46484, my parent is 46482
i am 46486, my parent is 46482
i am 46487, my parent is 46483
Exit 46487
i am 46488, my parent is 46484
Exit 46488
i am 46485, my parent is 46481
i am 46491, my parent is 46484
Exit 46491
i am 46489, my parent is 46483
Exit 46489
Exit 46483
i am 46492, my parent is 46486
Exit 46492
Exit 46484
i am 46490, my parent is 46486
Exit 46490
i am 46493, my parent is 46485
Exit 46493
Exit 46486
Exit 46482
i am 46494, my parent is 46485
Exit 46494
Exit 46485
Exit 46481
Exit 46480

Example source

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>

static pid_t fork_ok(void)
{
    pid_t pid;
    if ((pid = fork()) < 0)
    {
        fprintf(stderr, "Fork failure in pid %d\n", (int)getpid());
        exit(1);
    }
    return pid;
}

int main(void)
{
    fprintf(stderr, "Start %d\n", (int)getpid());

    for (int level = 0; level < 3; level++)
    {
        if (fork_ok() == 0 || fork_ok() == 0)
            continue;
        break;
    }
    fprintf(stderr, "i am %d, my parent is %d\n",
            (int)getpid(), (int)getppid());
    while (wait(0) > 0)
        ;
    fprintf(stderr, "Exit %d\n", (int)getpid());
    return(0);
}

This uses a loop, a somewhat unusual loop. At each level, the parent process forks two children; the children each 'continue' to the next level, while the parent is done and exits the loop. The clean up code is the same as before (see below).

Recursive variant

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>

static pid_t fork_ok(void)
{
    pid_t pid;
    if ((pid = fork()) < 0)
    {
        fprintf(stderr, "Fork failure in pid %d\n", (int)getpid());
        exit(1);
    }
    return pid;
}

static void new_level(int level)
{
    if (level > 3)
        return;
    if (fork_ok() == 0 || fork_ok() == 0)
        new_level(level+1);
    else
    {
        printf("i am %d, my parent is %d\n",
                (int)getpid(), (int)getppid());
        while (wait(0) > 0)
            ;
        printf("Exit %d\n", (int)getpid());
    }
}

int main(void)
{
    printf("Start %d\n", (int)getpid());
    fflush(0);
    new_level(0);
    return(0);
}

---

Attempt 1

I'm not convinced you need any loops. This seemed to do the trick for me:

Example output

Start 44397
i am 44397, my parent is 30775
i am 44400, my parent is 44397
i am 44401, my parent is 44397
Exit 44401
i am 44399, my parent is 44397
i am 44398, my parent is 44397
i am 44404, my parent is 44400
Exit 44404
i am 44403, my parent is 44399
i am 44402, my parent is 44398
i am 44405, my parent is 44398
i am 44407, my parent is 44398
Exit 44407
Exit 44400
i am 44406, my parent is 44399
Exit 44406
i am 44408, my parent is 44402
i am 44410, my parent is 44402
i am 44411, my parent is 44405
Exit 44410
Exit 44411
i am 44409, my parent is 44403
Exit 44409
Exit 44405
i am 44412, my parent is 44408
Exit 44412
Exit 44403
Exit 44399
Exit 44408
Exit 44402
Exit 44398
Exit 44397

Source Code

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>

int main(void)
{
    fprintf(stderr, "Start %d\n", (int)getpid());
    if (fork() < 0 || fork() < 0 || fork() < 0 || fork() < 0)
    {
        fprintf(stderr, "Fork failure in pid %d\n", (int)getpid());
        exit(1);
    }
    fprintf(stderr, "i am %d, my parent is %d\n",
            (int)getpid(), (int)getppid());
    while (wait(0) > 0)
        ;
    fprintf(stderr, "Exit %d\n", (int)getpid());
    return(0);
}