How do I call a bash built-in command?
"echo" for example.

I need the equivalent of C's system("echo hello");

You can use the syscall 'execve' to execute another program from within yours (it takes over your program).
The 'system' word in C does (as far as i can remember) also fork a new process to allow the calling program to continue, so the procedure is as follows:
##
Fork
And then from Child: execve ("/bin/echo",0,0)
##
See: man 2 execve, man 2 fork for usage.

Sorry, don`t have time to find/write the exact code for you at this momemt.