Redirecting output of an external application started with glib - glib

Question

I'm trying to use vala to start an external application using GLib with spawn_command_line_sync().
According to the documentation (http://valadoc.org/#!api=glib-2.0/GLib.Process.spawn_sync) you can pass a string to store the output of the external application.
While this works fine when starting a script which prints a couple of lines, I need to call a program which will print the content of a binary file.
(for example "cat /usr/bin/apt-get")
Is there any way how I can receive the output of the external program not in a string, but in a DataStream or something like that ?
I'm planning to write the ouput of the external program to a file, so just calling "cat /usr/bin/apt-get > outputfile" would be an alternative (not as nice), but it doesn't seem to work.
Anyway I would prefer it to get some kind of Output Stream.
I would appreciate any help.
Code im using:
using GLib;
static void main(string[] args) {
string execute = "cat /usr/bin/apt-get";
string output = "out";
try {
GLib.Process.spawn_command_line_sync(execute, out output);
} catch (SpawnError e) {
stderr.printf("spawn error!");
stderr.printf(e.message);
}
stdout.printf("Output: %s\n", output);
}

Answer 1

GLib.Process.spawn_async_with_pipes will let you do just that. It spawns the processes and returns a file descriptor for each of stdout, stderr, and stdin. There's a sample of code in the ValaDoc on how to set up IOChannels to monitor the output.

Answer 2

Thanks for that, I must have overread spawn_async_with_pipes() returning ints and not strings.
Is there anything wrong with doing it this way ?
(besides the buffer size of 1)
using GLib;
static void main(string[] args) {
string[] argv = {"cat", "/usr/bin/apt-get"};
string[] envv = Environ.get();
int child_pid;
int child_stdin_fd;
int child_stdout_fd;
int child_stderr_fd;
try {
Process.spawn_async_with_pipes(
".",
argv,
envv,
SpawnFlags.SEARCH_PATH,
null,
out child_pid,
out child_stdin_fd,
out child_stdout_fd,
out child_stderr_fd);
} catch (SpawnError e) {
stderr.printf("spawn error!");
stderr.printf(e.message);
return;
}
FileStream filestream1 = FileStream.fdopen(child_stdout_fd, "r");
FileStream filestream2 = FileStream.open("./stdout", "w");
uint8 buf[1];
size_t t;
while ((t = filestream1.read(buf, 1)) != 0) {
filestream2.write(buf, 1);
}
}