system() VS popen()

popen.cpp

#include <iostream> // cout, endl;
#include <stdio.h>  // popen, pclose

int main(void) {
  std::cout << "Caller starts" << std::endl;
  
  FILE *fp = popen("./writeTest", "r");
  pclose(fp);

  std::cout << "Caller ends" << std::endl;
  return(0);
}

system.cpp

#include <iostream> // cout, endl;
#include <stdlib.h> // system

int main(void) {
  std::cout << "Caller starts" << std::endl;
  
  system("./writeTest");
  
  std::cout << "Caller ends" << std::endl;
  return(0);
}

writeTest.cpp

#include <iostream> // cout, endl
#include <stdio.h>  // fopen, fputs, fclose
#include <unistd.h> // sleep

int main(void) {
  std::cout << "Callee starts" << std::endl;

  sleep(5);

  FILE *fp = fopen("test.txt", "w");
  fputs("test\n", fp);
  fclose(fp);

  std::cout << "Callee ends" << std::endl;

  return(0);
}