waitpid() error problem - fork

Question

I wrote code to print pid in order parent->g3->c2->g1->g2->c1.
So I used wait(), and waitpid(). But I failed.
So I wrote "finish"code to know what is problem.
And I knew that c1 ignore the waitpid and print what->finish.
How can I solve this problem
#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
#include <stdlib.h>
void main()
{
pid_t c1,c2,g1,g2,g3;
printf("parent:%d\n", (int)getpid());
c1=fork();
int status;
if (c1>0) {
c2=fork();
if (c2==0) {
g3=fork();
if (g3==0) {
printf("g3:%d\n",(int)getpid());
} else if (g3>0) {
wait(&status);
printf("c2:%d\n",(int)getpid());
}
}
} else if (c1==0) {
waitpid(c2,&status,WUNTRACED);
printf("what\n");
g1=fork();
if (g1>0) {
g2=fork();
if (g2==0) {
printf("g2:%d\n",(int)getpid());
} else if (g2>0) {
waitpid(g1,&status,WUNTRACED);
printf("c1:%d\n", (int)getpid());
}
} else if (g1==0) {
waitpid(g2,&status,WUNTRACED);
printf("g1:%d\n",(int)getpid());
} else {
printf("failed\n");
exit(1);
}
} else {
printf("main failed\n");
exit(1);
} printf("finish\n");
}

Answer 1

pid_t c1,c2,g1,g2,g3;
printf("parent:%d\n", (int)getpid());
c1=fork();
int status;
if (c1>0) {
....
} else if (c1==0) {
waitpid(c2,&status,WUNTRACED);
the c2 variable was never set, so it may contain whatever garbage it happened to be on the stack. The compiler would warn you about it if you run it with the -Wall flag.
Also, you should always check the return value of waitpid; that would've also caught the bug.
And after a fork, do not assume that the child will be scheduled to run before the parent or vice-versa.
printf("what\n");
g1=fork();
if (g1>0) {
...
} else if (g1==0) {
waitpid(g2,&status,WUNTRACED);
Same thing as above, the g2 variable is used without being initialized.
Also, the return value of main should be int, not void.
And you should always compile with the -O2 -Wextra -Wall flags on, that will save you a lot of trouble. If you find some warnings superfluous, you can turn them off individually; eg. (-Wno-parentheses, -Wno-unused).

Answer 2

I verified this code with gcc 6.3.0 and it is working properly there.
Well this code prints them in the order in which you asked:
#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/types.h>
static int *glob_var;
int main()
{
int c1,c2,g1,g2,g3;
glob_var = mmap(NULL, sizeof *glob_var, PROT_READ | PROT_WRITE,
MAP_SHARED | MAP_ANONYMOUS, -1, 0);
printf("parent:%d\n", (int)getpid());
*glob_var = 0;
c1=fork();
int status;
if (c1>0) {
c2=fork();
if (c2==0) {
g3=fork();
if (g3==0) {
printf("g3:%d\n",(int)getpid());
} else if (g3>0) {
wait(&status);
printf("c2:%d\n",(int)getpid());
*glob_var = 1;
}
}
} else if (c1==0) {
while(*glob_var == 0);
printf("what\n");
g1=fork();
if (g1>0) {
waitpid(g1,&status,WUNTRACED);
g2=fork();
if (g2==0) {
printf("g2:%d\n",(int)getpid());
} else if (g2>0) {
waitpid(g2,&status,WUNTRACED);
printf("c1:%d\n", (int)getpid());
}
} else if (g1==0) {
printf("g1:%d\n",(int)getpid());
} else {
printf("failed\n");
exit(1);
}
} else {
printf("main failed\n");
exit(1);
}
}
I made few chagnes.
I just added waitpid(g2,&status,WUNTRACED); in the starting of
else if (g2>0) {
waitpid(g2,&status,WUNTRACED);
printf("c1:%d\n", (int)getpid());
}
conditional statement so g2 will be finished before c1.
And waitpid(g1,&status,WUNTRACED); added in the conditional statement if(g1>0) so g1 will be finished before g2.
And also I used a glob_var which is a global variable declared as static int *glob_var; and used mmap to share it between parent and child.
glob_var = mmap(NULL, sizeof *glob_var, PROT_READ | PROT_WRITE,
MAP_SHARED | MAP_ANONYMOUS, -1, 0);
And used it as a semaphore to execute c2 before c1 and it is working like when c2 will be finished it will then change the value of semaphore from 0 to 1 and after that onlywhile(*glob_var == 0) loop will break else if(c1==0) condition will continue.
And also there is no need of waitpid(c2,&status,WUNTRACED); in else if (c1==0) so I removed it.