C++ wrong usage of close and dup2? - c++

Question

I wrote the following code:
void execute() {
std::cout << "smash pid is " << getpid() << std::endl;
}
int main()
{
int pid=fork();
if (pid==0)
{
int fd=open("my_file.txt", O_WRONLY | O_CREAT, 0666); // 3=my_file
dup2(fd,1); // replace output stream
close(fd); //close duplicate access to my_file
execute();
close (1); // close last access to my file
}
else if (pid>0)
{
std::cout << "Hello!" << std::endl;
}
return 0;
}
my question is am I doing things correctly? and can the main process still have access to print to terminal as usual?
I tried to add notes of what I am doing, please let me know if something isn't clear.
Version 2:
int main()
{
int pid=fork();
if (pid==0)
{
close (1);
int fd=open("my_file.txt", O_WRONLY | O_CREAT, 0666); // 3=my_file
execute();
close (1); // close last access to my file
}
else if (pid>0)
{
std::cout << "Hello!" << std::endl;
}
return 0;
}

Answer 1

my question is am I doing things correctly?
Your child will have its output directed into the opened file and the parent will write "Hello!" to the stdout that was provided to the program at startup. It looks like that's what you want, so, yes.
I'd use pid_t instead of int for the process id though, but that may be different on different platforms.
can the main process still have access to print to terminal as usual?
Yes. The dup2 in the child process does not affect the parent process in any way.
One note. Use fileno(stdout) instead of 1:
dup2(fd, fileno(stdout));
// and
close(fileno(stdout)); // not needed really
The int fileno(FILE*) function returns the internal file descriptor from the standard FILE* that is stdout. It's just a way of making it clearer to readers of the code.
You could however redirect stdout to a file using the standard C++ function std::freopen instead.
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <cstdio> // std::freopen
#include <iostream>
void execute() {
std::cout << "smash pid is " << getpid() << std::endl;
system("echo the redirect is inherited by the grand children too");
}
int main() {
pid_t pid = fork();
if(pid < 0) { // check for errors
std::perror("fork failed");
return 1;
}
if(pid == 0) { // child
// using std::freopen
if(std::freopen("my_file.txt", "w", stdout) == nullptr) {
std::perror("freopen failed");
return 1;
}
execute();
return 0;
}
// parent
std::cout << "Parent says hello!\n";
// wait for child
int wstatus;
if(waitpid(pid, &wstatus, 0) == pid) {
std::cout << "child exited with status " << wstatus << '\n';
} else {
std::perror("waitpid failed");
}
}