I think both are viable, sensible options; nmcli's man page even says:
Typical uses include:
- Scripts: Utilize NetworkManager via nmcli instead of managing network connections manually. nmcli supports a terse output format which is better suited for script processing.
So, being called externally is definitely a design goal, and with --terse you will get output that the maintainers put thought into keeping stable for usage by computer programs parsing it.
I think libnm mostly makes more sense if
- your C program already uses
GObject anyways, and
- you don't want to directly handle D-Bus calls yourself, but
- also don't want to work very abstracted from the D-Bus API of NetworkManager.
Since the operations you describe are "large" anyways, meaning that the things they cause will take multiple milliseconds, maybe even seconds, the overhead of a single popen (which internally creates pipes, forks, invokes a shell, and tells the shell to execute a command) is really insignificant.
So, I'd go for invoking nmcli --terse {subcommand} from your C program. Whether you do that via the simple
FILE* output = popen("nmcli --terse foobar", "r");
which involves spawning a shell, or, more advanced, but maybe advantageous if you want to do things like use nmcli connection monitor,
char* args[] = {"nmcli", "--terse", "foobar"};
char* nmcli_path = "/usr/bin/nmcli";
struct io_pipe {
int read_end;
int write_end;
};
struct io_pipe pipe_fd;
int success = pipe(&pipe_fd); // check success >= 0!
int child_pid = fork(); // check new_pid >= 0!
if(child_pid > 0) { // we're the parent!
close(pipe_fd.write_end); // we don't need the write end
/* Do whatever you would have done with the FILE* coming out of popen here,
* e.g.,
* fd_set watched_fds;
* FD_ZERO(&watched_fds);
* FD_SET(io_pipe.read_end, &watched_fds);
* struct timeval timeout = {.tv_sec = 5, .tv_usec = 0};
* char* buffer = NULL;
* while(select(1, // how many file descriptors?
&watched_fds, // which read descriptors
NULL, NULL, // which write, exception descriptors
timeout // timeval specifying timeout.
)) {
* if(!buffer) {
* buffer = malloc(4096);
* }
* int howmuch = read(pipe_fd.read_end, buffer, 4096);
* // Do clever stuff with the things you read!
* }
* free(buffer);
*/
int wstatus;
waitpid(child_pid, &wstatus, 0);
/* see `man 2 wait` for what you can do with wstatus */
} else { // we're the child!
close(pipe_fd.read_end); // we don't need the read end
// we connect our stdout to the write end:
dup2(STDOUT_FILENO, pipe_fd.write_end);
execv(nmcli_path, args); // we replace ourselves with nmcli
}
is fully up to you.
