Sounds like a trivial use-case for the system() function:
system("asterisk -rx reload");
If you need very fine-grained control of the child process there are better ways, but this is simple to get going.
This call starts a shell (such as bash) to run the command, which is why I removed the quotes around reload; they’re pointless for a single word and will be removed by the shell and never seen by the started program, anyway.