Fork/ execlp in #pragma omp parallel for stops arbitrarily - openmp

Question

I am programming a Kalman-Filter based realtime program in C using a large number of realizations. To generate the realization output I have to execute an external program (groundwater simulation software) about 100 times. I am therefore using OpenMP with fork and exceclp for parallelization of this block:
#pragma omp parallel for private(temp)
for(i=1;i<=n_real;i++){
/* Start ensemble model run */
int = fork();
int ffork = 0;
int status;
if (pid == 0) {
//Child process
log_info("Start Dadia for Ens %d on thread %d",i,omp_get_thread_num());
sprintf(temp,"ens_%d",i);
chdir(temp);
execlp("../SPRING/dadia","dadia","-return","-replace_kenn","73","ini.csv","CSV",NULL);
log_info("Could not execute function dadia - exit anyway");
ffork = 1;
_exit(1);
}
else{
//Parent process
wait(NULL);
if (ffork == 0){
log_info("DADIA run ens_%d successfully finished",i);
}
}
}
In general the code runs smoothly for small number of realizations (with 6 threads). However sometimes the code hangs in the last cycle of parallel iterations. The occurrence only happens if number iterations >> number threads. I tried scheduling the for loop with different options, but it didn't solve the problem. I know that fork is not the best solution to use with OpenMP. But I am wondring why it's sometimes hangs at arbitrary points.
Thank's a lot for any kind if feedback.
Different Ubuntu versions tried (including difffrent comiler versions)

Answer 1

This version finally seems to run stable. Thank's for your hints:
#pragma omp parallel for private(temp) schedule(dynamic)
for(i=1;i<=n_real;i++) {
/* Start ensemble model run */
/* Calling bash-commands inside OpenMP enviroment requires using fork().
Else the parallel process is terminated.
Note exec automatically ends the child process after termination.
If Sitra fails, the child process needs to be terminated manually */
pid_t pid = 0;
int status;
pid = fork();
if (pid == 0) {
log_info("Start SITRA for Ens %d on thread %d",i,omp_get_thread_num());
sprintf(temp,"ens_%d",i);
chdir(temp);
execl("../SPRING/sitra","sitra","-return",(char *) 0);
perror("In exec(): ");
}
if (pid > 0) {
pid = wait(&status);
log_info("End of sitra for ensemble %d",i);
if (WIFEXITED(status)) {
printf("Sitra ended with exit(%d).\n", WEXITSTATUS(status));
}
if (WIFSIGNALED(status)) {
printf("Sitra ended with kill -%d.\n", WTERMSIG(status));
}
}
if (pid < 0){
perror("In fork():");
}
}