Basically you will need to create an inter-process communication system on a Win32 platform.
You can do it in a few different ways: pipes, shared memory, IPC, WinSock, DDE...
All these features compete to offer you the crappiest possible API, with truckloads of inconsistent and useless parameters, unstandardized return codes and incoherent function names. And the bloody awkward circa 1995 semi-unicode handling, on top of that.
Here is an example with a named pipe.
#include <stdio.h>
#include <stdlib.h>
#include <windows.h>
// name of our glorious pipe
#define PIPE_NAME L"\\\\.\\pipe\\whatever" // bloody unicode string
// exit on fatal error
void panic(const char * msg)
{
fprintf(stderr, "***PANIC*** %s\n", msg);
exit(-1);
}
// father process
void father(const char * own_name) // name of our own executable to launch a copy of ourselve
{
printf("Father process starting\n");
// create a monodirectional father->child named pipe
HANDLE pipe = CreateNamedPipe (
PIPE_NAME, // name of the pipe
PIPE_ACCESS_OUTBOUND, // send only
PIPE_TYPE_BYTE, // send data as a byte stream
1, // only one instance
0, 0, 0, NULL); // default junk
if (pipe == INVALID_HANDLE_VALUE) panic("could not create pipe");
// spawn child process
{
STARTUPINFOA si;
PROCESS_INFORMATION pi;
ZeroMemory(&si, sizeof(si));
si.cb = sizeof(si);
ZeroMemory(&pi, sizeof(pi));
if (!CreateProcessA( // using ASCII variant to be compatible with argv
own_name, // executable name (ourself)
"child", // command line. This will be seen as argv[0]
NULL, NULL, FALSE, // default junk
CREATE_NEW_CONSOLE, // launch in another console window
NULL, NULL, // more junk
&si, &pi)) // final useless junk
panic("could not create child process");
}
// connect to child process
BOOL result = ConnectNamedPipe(pipe, NULL);
if (!result) panic("could not connect to child process");
// talk to child
for (;;)
{
// read an input line
char line[100];
printf("Say something >");
if (fgets(line, sizeof(line), stdin) == NULL)
panic("could not read from standard input");
// exit on an empty line
if (!strcmp(line, "\n")) break;
// send the line to the child
DWORD written = 0;
if (!WriteFile(
pipe,
line, // sent data
strlen(line), // data length
&written, // bytes actually written
NULL))
panic("could not write to pipe");
}
// close the pipe
CloseHandle(pipe);
}
void child(void)
{
printf("Child process starting\n");
// retrieve communication pipe
HANDLE pipe = CreateFile(
PIPE_NAME, // name of the pipe
GENERIC_READ, // read ONLY access (or else the call will fail)
0, NULL, // junk
OPEN_EXISTING, // opens existing pipe
0, NULL); // more junk
if (pipe == INVALID_HANDLE_VALUE) panic("could not connect to the pipe");
// read father's input
for (;;)
{
char buffer[80];
DWORD read = 0;
if (!ReadFile(
pipe,
buffer, // read data
sizeof(buffer)-1, // max length (leave room for terminator)
&read, // bytes actually read
NULL))
break; // exit if the pipe has closed
// display what our father said
buffer[read] = '\0'; // make sure what we just read will be displayable as a string
printf("Father said: %s", buffer);
}
// close pipe
CloseHandle(pipe);
}
int main(int argc, char *argv[])
{
// wait for a <return> keypress on exit
atexit(getchar);
// decide whether we are the father or the child
if (!strcmp (argv[0], "child")) child();
else father(argv[0]);
printf("Done\n");
return 0;
}
Sorry, I could not find a graceful way of using Ctrl-D as an exit signal. The only way I could think of achieving this would have required yet another couple of extremely tedious system calls, and I was daunted by the prospect.
So the father will terminate when the user enters an empty line.
By closing the pipe, the father will trigger a read error that will put the child out of its misery too (i.e. the child breaks out of the reading loop and dies when it gets a read error on the pipe). You can easily have the child react to whatever message instead, if that can please your teacher better.
I have added an extra wait for another keypress in case you're running this from an IDE, to avoid closing the windows too abruptly. Just remove the atexit at the start of main() if you don't want that.
