Programming

/*
* pclose --
*    Pclose returns -1 if stream is not associated with a `popened' command,
*    if already `pclosed', or waitpid returns an error.
*/
int
pclose(iop)
FILE *iop;
{
    register struct pid *cur, *last;
    int pstat;
    pid_t pid;

    (void)fclose(iop);

    /* Find the appropriate file pointer. */
    for (last = NULL, cur = pidlist; cur; last = cur, cur = cur->next)
    if (cur->fp == iop)
    break;
    if (cur == NULL)
    return (-1);

    do {
        pid = waitpid(cur->pid, &pstat, 0);
    } while (pid == -1 && errno == EINTR);

    /* Remove the entry from the linked list. */
    if (last == NULL)
    pidlist = cur->next;
    else
    last->next = cur->next;
    free(cur);
    
    return (pid == -1 ? -1 : pstat);
}

for a hundred of times
{
     cout << "foo";
     cout << "bar" << endl;
}

for a hundred of times
      cout << "foo";
cout << "bar" << endl;

for (last = NULL, cur = pidlist; cur; last = cur, cur = cur->next)

int
pclose(iop)
FILE *iop;
{
    register struct pid *cur, *last;
    int pstat;
    pid_t pid;
    // Close the file.
    (void)fclose(iop);
    // pidlist must be a global variable of type 'struct pid', whatever that is.
    // It must contain the PID to wait for, and the matching FILE *, since it
    // wasn't contained in the FILE pointer itself.
    /* Find the appropriate file pointer. */
    // Whenever there's no brackets, it acts like it only encloses the next
    // statement, like this:
    for (last = NULL, cur = pidlist; cur; last = cur, cur = cur->next)
    {
        if (cur->fp == iop)
        {
            break;
        }
    }

    if (cur == NULL)
    {
        return (-1);
    }

    // Wait for the process.  Keep trying until it either succeeds or fails.
    do {
        pid = waitpid(cur->pid, &pstat, 0);
    } while (pid == -1 && errno == EINTR);

    /* Remove the entry from the linked list. */
    if (last == NULL)
    pidlist = cur->next;
    else
    last->next = cur->next;
    free(cur);
    
    return (pid == -1 ? -1 : pstat);
}

struct node
{
        struct node *next;
        struct node *prev;
        int value;
};


int main(void)
{
    struct node a,b, *z;
    a.prev=NULL;
    a.next=&b;
    a.value=3;

    b.prev=&a;
    b.next=NULL;
    b.value=5;

    z=&a;
    while(z != NULL)
    {
        printf("node %p=%d\n", z, z->value);
        z=z->next;
    }
}

$ ./a.out
node 0x7fff3f48bce0=3
node 0x7fff3f48bcc0=5
$