linux - Process started from system command in C inherits parent fd's

Question

I have a sample application of a SIP server listening on both tcp and udp ports 5060. At some point in the code, I do a system("pppd file /etc/ppp/myoptions &");

After this if I do a netstat -apn, It shows me that ports 5060 are also opened for pppd! Is there any method to avoid this? Is this standard behaviour of the system function in Linux?

Thanks, Elison

Answer 1

Yes, by default whenever you fork a process (which system does), the child inherits all the parent's file descriptors. If the child doesn't need those descriptors, it SHOULD close them. The way to do this with system (or any other method that does a fork+exec) is to set the FD_CLOEXEC flag on all file descriptors that shouldn't be used by the children of you process. This will cause them to be closed automatically whenever any child execs some other program.

In general, ANY TIME your program opens ANY KIND of file descriptor that will live for an extended period of time (such as a listen socket in your example), and which should not be shared with children, you should do

fcntl(fd, F_SETFD, fcntl(fd, F_GETFD) | FD_CLOEXEC);

on the file descriptor.

---

As of the 2016? revision of POSIX.1, you can use the SOCK_CLOEXEC flag or'd into the type of the socket to get this behavior automatically when you create the socket:

listenfd = socket(AF_INET, SOCK_STREAM|SOCK_CLOEXEC, 0);
bind(listenfd, ...
listen(listemfd, ...

which guarentees it will be closed properly even if some other simultaneously running thread does a system or fork+exec call. Fortunately, this flag has been supported for awhile on Linux and BSD unixes (but not OSX, unfortunately).