Only one process must execute a code portion at a time - c

Question

I am sorry I am repeating a question https://stackoverflow.com/questions/5687837/monitor-implementation-in-c but not getting a solution as yet. I have probably asked the question incorrectly.
Say I have a code portion B. A parent process spawns a number of child processes to execute code B but I would like only one process to be inside code portion B at a time. How can I do it in C on Linux platform?
Thanks for your help
An edit. Not threads but process.

Answer 1

You want a mutex.
pthread_mutex_t mutexsum;
pthread_mutex_init(&mutexsum, NULL);
pthread_mutex_lock (&mutexsum);
// Critical code
pthread_mutex_unlock (&mutexsum);
If you are serious about it being multiple processes instead of multiple threads, the mutex needs to be stored in a shared memory segment.

Answer 2

So what you want is to have exactly one child running at any point of time, then why spawn all the children processes all at once?
When a child process ends, a SIGCHLD is issued, you can write your own handler for this signal and call spawn from the handler. Then you have one new child process created when one perishes -- only one child process running. Below is a hack (useless, just for demo) to achieve this:
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <signal.h>
#include <stdlib.h>
void spawn(void){
pid_t child_pid=fork();
if(child_pid > 0){
printf("new child created by %d !\n",getpid());
sleep(1);
}else if(child_pid == 0){
printf("child %d created !\n",getpid());
}else{
exit(EXIT_FAILURE);
}
}
void handler(int sigval){
spawn();
}
int main(void){
signal(SIGCHLD,handler);
spawn();
return 0;
}