Win32 piped child example

child.c

// $ cc -nostartfiles -o child.exe child.c
// $ cl child.c /link /subsystem:console kernel32.lib
#define WIN32_LEAN_AND_MEAN
#include <windows.h>

int mainCRTStartup(void)
{
    HANDLE stdin  = GetStdHandle(STD_INPUT_HANDLE);
    HANDLE stdout = GetStdHandle(STD_OUTPUT_HANDLE);
    for (;;) {
        char buf[8];  // deliberately small to force multiple iterations
        DWORD len = sizeof(buf);
        if (!ReadFile(stdin, buf, len, &len, 0)) {
            break;
        }
        WriteFile(stdout, buf, len, &len, 0);
    }
    return 0;
}

parent.c

// $ cc -nostartfiles -o parent.exe parent.c
// $ cl parent.c /link /subsystem:console kernel32.lib
#define WIN32_LEAN_AND_MEAN
#include <windows.h>

int mainCRTStartup(void)
{
    HANDLE stdin[2];
    HANDLE stdout[2];
    CreatePipe(stdin+0,  stdin+1,  0, 0);
    CreatePipe(stdout+0, stdout+1, 0, 0);
    SetHandleInformation(stdin[0],  HANDLE_FLAG_INHERIT, HANDLE_FLAG_INHERIT);
    SetHandleInformation(stdout[1], HANDLE_FLAG_INHERIT, HANDLE_FLAG_INHERIT);

    STARTUPINFOW si = {0};
    si.cb = sizeof(si);
    si.hStdInput = stdin[0];
    si.hStdOutput = stdout[1];
    si.hStdError = GetStdHandle(STD_ERROR_HANDLE);
    si.dwFlags |= STARTF_USESTDHANDLES;

    PROCESS_INFORMATION pi;
    CreateProcessW(L"child.exe", L"child", 0, 0, TRUE, 0, 0, 0, &si, &pi);
    CloseHandle(pi.hThread);
    CloseHandle(stdin[0]);
    CloseHandle(stdout[1]);

    char msg[] = "hello world this is a test\n";
    DWORD len = sizeof(msg) - 1;
    WriteFile(stdin[1], msg, len, &len, 0);
    CloseHandle(stdin[1]);

    for (;;) {
        char buf[8];  // deliberately small to force multiple iterations
        DWORD len = sizeof(buf);
        if (!ReadFile(stdout[0], buf, len, &len, 0)) {
            break;
        }
        WriteFile(GetStdHandle(STD_OUTPUT_HANDLE), buf, len, &len, 0);
    }
    CloseHandle(stdin[0]);

    WaitForSingleObject(pi.hProcess, INFINITE);
    CloseHandle(pi.hProcess);
    return 0;
}