Two way communication from the file that run with exec in child - c

Question

I am creating a child process with a fork in program y. In that child, I run another program with exec, in which I want the function in that program (let's call it program x) to return something to me. Is there a way to pass this returned value to the parent?
I provided some sort of a pseudo-code that demonstrates what I want to do below.
program.x:
int main(int argc, char** argv)
{
if(argc != 2)
{
printf("argument count does not match\n");
return -1;
}
printf("task1!\n");
...
char *value = "want this"; // how to pass this to the parent in the program y?
...
}
program y:
int main(int argc, char *argv[])
{
int fd[2];
pipe(fd);
pid_t p;
p = fork();
if(p==-1)
{
printf("There is an error while calling fork()");
}
if(p==0)
{
printf("We are in the child process\n");
printf("Calling hello.c from child process\n");
char *args[] = {"Hello", "C", "Programming", NULL};
execv("./hello", args);
close(fd[0]);
write(fd[1], ???, ??);
close(fd[0]);
}
else
{
printf("We are in the parent process");
wait(NULL);
close(fd[1]);
read(fd[0], ???,???);
close(fd[0]);
}
return 0;
}

Answer 1

The only thing you can pass directly is the exit code of the child (via wait()).
To pass a string between two processes, you need an IPC data structure like pipes. See the pipe() function in unistd.h.

Answer 2

For the simple case where there is one-way communication from a child to a parent), you can use popen. It's high level, simple to use, and has little overhead (if any) over fork/exec
int main(...)
{
...
FILE *fp = popen("./hello 'Hello', 'C', 'Programming'", "r") ;
char resp[200] ;
if ( fgets(resp, sizeof(resp, fp) ) {
// Do something
}
int result = pclose(fp) ;
}
Note that the way to pass command line arguments follows shell rules - arguments may need to be quoted (usually, single quote) to pass any special characters.
The 'pclose' result is the exit code of the executed program.