how to communicate with child process using pipe -- WIN32 - winapi

Question

I wrote a simple test file:
#include "iostream"
using namespace std;
int main(int argc, char const *argv[]) {
char s[512];
while(cin >> s) {
for(int i=0; s[i]; ++i) {
s[i] ^= 32;
}
cout << s << endl;
}
return 0;
}
And in another program, I want to launch it as a child process and communicate with it using pipe.
I use CreatePipe to create two pipe, and use CreateProcess to launch it with the redirected stdio flags.
code for creating pipe:
HANDLE _stdin_rd = NULL;
HANDLE _stdin_wr = NULL;
HANDLE _stdout_rd = NULL;
HANDLE _stdout_wr = NULL;
BOOL br;
SECURITY_ATTRIBUTES sa;
PROCESS_INFORMATION gdb_info; // the process information
void _init() {
sa.nLength = sizeof(SECURITY_ATTRIBUTES);
sa.bInheritHandle = TRUE; // the handles are inheritable
sa.lpSecurityDescriptor = NULL;
// Create stdin pipe.
br = CreatePipe(&_stdin_rd, &_stdin_wr, &sa, NULL);
if(!br) {
cerr << "Can't create stdin pipe." << endl;
abort();
}
// ensure the write handle to the pipe for stdin is not inherited.
br = SetHandleInformation(_stdin_wr, HANDLE_FLAG_INHERIT, 0);
if(!br) {
cerr << "change stdin pipe attr error" << endl;
abort();
}
// Create stdout pipe.
br = CreatePipe(&_stdout_rd, &_stdout_wr, &sa, NULL);
if(!br) {
cerr << "Can't create stdout pipe." << endl;
abort();
}
// ensure the read handle to the pipe for stdout is not inherited.
br = SetHandleInformation(_stdout_rd, HANDLE_FLAG_INHERIT, 0);
if(!br) {
cerr << "change stdout pipe attr error" << endl;
abort();
}
}
void _launch() {
STARTUPINFO si;
ZeroMemory(&si, sizeof(STARTUPINFO));
ZeroMemory(&gdb_info, sizeof(PROCESS_INFORMATION));
si.cb = sizeof(STARTUPINFO);
si.dwFlags |= STARTF_USESTDHANDLES; // so as to enable text pipe
si.hStdInput = _stdin_rd; // redirection
si.hStdOutput = _stdout_wr;
si.hStdError = _stdout_wr;
br = CreateProcess(
NULL,
"test", // command line
NULL, // process security attributes
NULL, // primary thread security attributes
TRUE, // handles are inherited
NULL, // creation flags
NULL, // use parent's environment
NULL, // use parent's current directory
&si, // STARTUPINFO pointer
&gdb_info // receives PROCESS_INFORMATION
);
if(!br) {
cerr << "Can't start test file!" << endl;
abort();
} else {
cout << "test file invoked." << endl
<< "Process " << gdb_info.dwProcessId << endl
<< "Thread " << gdb_info.dwThreadId << endl;
}
}
void test_loop() {
HANDLE console_out = GetStdHandle(STD_OUTPUT_HANDLE);
char buf[512];
DWORD len;
WriteFile(gdb_stdin_wr, "Helloworld", strlen("Helloworld"), &len, NULL);
ReadFile(gdb_stdout_rd, buf, 512, &len, NULL);
WriteFile(console_out, buf, len, &len, NULL);
//
WriteFile(_stdin_wr, "PipeCommuni", strlen("PipeCommuni"), &len, NULL);
ReadFile(_stdout_rd, buf, 512, &len, NULL);
WriteFile(console_out, buf, len, &len, NULL);
//
WriteFile(gdb_stdin_wr, "HAHAhehe", strlen("HAHAhehe"), &len, NULL);
ReadFile(gdb_stdout_rd, buf, 512, &len, NULL);
WriteFile(console_out, buf, len, &len, NULL);
}
The problem is, after I start the program, it launches the test program successfully. But the program just stuck there, no error reported.
If I change the command line "test" into a program like "gdb", then I could receive output from gdb.
Can anyone help me with this problem?