Confusion with forked process? - c

Question

I am trying to do first child function, for example, printing out a string : hello!, and go back to parent process. Then, I forked anothrer n processes for the second child function, which will count the shared memory number. I suppose there only shows one "all done" in the end, but it shows two? thanks for your help!
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/shm.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <errno.h>
#include <sys/wait.h>
#include <fcntl.h>
#define NUM_LINES 5
char * shm;
void child_func(char *shm)
{
while (atoi(shm) != NUM_LINES)
{
// P(sem);
printf("now reading word:%d\n", atoi(shm) );
*shm+=1;
// V(sem);
}
exit(0);
}
void parent_func(pid_t pid)
{
int status;
pid_t pid_wait;
do
{
pid_wait = waitpid(pid, &status, WNOHANG);
}while (pid_wait != pid);
// printf("Process %d done\n", getppid());
}
int main(int argc, char const *argv[])
{
/* declarations */
pid_t pid[8];
int ret, shmid, status,corpse, i, ave, n;
char fn[20];
key_t key = 123;
char *string_back;
/* create share memory */
if ((shmid = shmget(key, SHMSIZE, IPC_CREAT|666)) <0)
{
perror("shmget");
exit(1);
}
/* attach shm */
if ((shm = shmat(shmid, NULL, 0)) == (char*)-1)
{
perror("shmat");
exit(1);
}
/* init shm value */
*shm = '0';
/* input */
printf("please enter n:\n");
scanf("%d", &n);
/* section 1 */
pid[0] = fork();
if (pid[0] == 0) // child processes
{
printf("hello !\n");
}
else if (pid[0] >0)
{
parent_func(pid[0]);
}
/* section 2 */
for (i=0;i<n;i++)
{
pid[i] = fork();
if (pid[i] == 0) // child processes
{
child_func(shm);
}
else if (pid[i] >0)
{
parent_func(pid[i]);
}
}
printf("all done\n");
/* detach shm */
shmdt(shm);
/* destroy shm */
int retval = shmctl(shmid, IPC_RMID, NULL);
if (retval < 0) perror("remove shm");
return 0;
}

Answer 1

After adding exit(0) in first section, I only get one "all done" in the last, which is the answer I want.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/shm.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <errno.h>
#include <sys/wait.h>
#include <fcntl.h>
#define NUM_LINES 5
char * shm;
void child_func(char *shm)
{
while (atoi(shm) != NUM_LINES)
{
// P(sem);
printf("now reading word:%d\n", atoi(shm) );
*shm+=1;
// V(sem);
}
exit(0);
}
void parent_func(pid_t pid)
{
int status;
pid_t pid_wait;
do
{
pid_wait = waitpid(pid, &status, WNOHANG);
}while (pid_wait != pid);
// printf("Process %d done\n", getppid());
}
int main(int argc, char const *argv[])
{
/* declarations */
pid_t pid[8];
int ret, shmid, status,corpse, i, ave, n;
char fn[20];
key_t key = 123;
char *string_back;
/* create share memory */
if ((shmid = shmget(key, SHMSIZE, IPC_CREAT|666)) <0)
{
perror("shmget");
exit(1);
}
/* attach shm */
if ((shm = shmat(shmid, NULL, 0)) == (char*)-1)
{
perror("shmat");
exit(1);
}
/* init shm value */
*shm = '0';
/* input */
printf("please enter n:\n");
scanf("%d", &n);
/* section 1 */
pid[0] = fork();
if (pid[0] == 0) // child processes
{
printf("hello !\n");
/* problem solved here */
exit(0);
}
else if (pid[0] >0)
{
parent_func(pid[0]);
}
/* section 2 */
for (i=0;i<n;i++)
{
pid[i] = fork();
if (pid[i] == 0) // child processes
{
child_func(shm);
}
else if (pid[i] >0)
{
parent_func(pid[i]);
}
}
printf("all done\n");
/* detach shm */
shmdt(shm);
/* destroy shm */
int retval = shmctl(shmid, IPC_RMID, NULL);
if (retval < 0) perror("remove shm");
return 0;
}