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 &\");

Answer 1

As others have stated, this is standard behavior that programs depend on.

When it comes to preventing it you have a few options. Firstly is closing all file descriptors after the fork(), as Dave suggests. Second, there is the POSIX support for using fcntl with FD_CLOEXEC to set a 'close on exec' bit on a per-fd basis.

Finally, though, since you mention you are running on Linux, there are a set of changes designed to let you set the bit right at the point of opening things. Naturally, this is platform dependent. An overview can be found at http://udrepper.livejournal.com/20407.html

What this means is that you can use a bitwise or with the 'type' in your socket creation call to set the SOCK_CLOEXEC flag. Provided you're running kernel 2.6.27 or later, that is.