Windows CreateProcess and output redirection - c

Question

I'm working in a C project, windows environment using WinAPI.
My function execs a command (e.g. : "dir C:\Users") it received as a param, and returns its output.
It is ran from a DLL so it must not spawn a cmd.exe windows, which means I can't use _popen. So I use CreateProcessA, and a pipe to catch stdout.
Here is the code :
char *runExec(char *command, int *contentLen) {
char *ret=NULL,*tmp=NULL;
DWORD readBytes;
int size=0;
char buffer[128],cmdBuf[4096];
HANDLE StdOutHandles[2];
CreatePipe(&StdOutHandles[0], &StdOutHandles[1], NULL, 4096);
STARTUPINFOA si;
memset(&si, 0, sizeof(si));
si.cb = sizeof(si);
si.dwFlags = STARTF_USESHOWWINDOW | STARTF_USESTDHANDLES;
si.wShowWindow = SW_HIDE;
si.hStdOutput = StdOutHandles[1];
si.hStdError = StdOutHandles[1];
PROCESS_INFORMATION pi;
snprintf(cmdBuf,4096,"cmd /C %s", command);
if (!CreateProcessA(NULL, cmdBuf, NULL, NULL, FALSE, CREATE_NO_WINDOW | DETACHED_PROCESS, NULL, NULL, &si, &pi)) {
printf("Error createProcess : %d\n",GetLastError());
return NULL;
}
CloseHandle(StdOutHandles[1]);
printf("Before read\n");
while (ReadFile(StdOutHandles[0], buffer, 127, &readBytes, NULL)){
printf("IN WHILE\n");
buffer[readBytes] = 0;
printf("read %d bytes\n", readBytes);
size += readBytes;
tmp = (char *)realloc(ret, size + 1);
if (tmp == NULL) {
free(ret);
return NULL;
}
ret = tmp;
strncpy(ret + (size - readBytes), buffer, readBytes);
ret[size] = 0;
}
printf("Readfile returned with %d, read %d\n", GetLastError(),readBytes);
CloseHandle(pi.hProcess);
CloseHandle(pi.hThread);
CloseHandle(StdOutHandles[0]);
printf("#%s#\n", ret);
return ret;
}
And the output is :
Before read
Readfile returned with 109, read 0
#(null)#
So from that I understand that :
- My loop isn't even executed once
- ReadFile reads nothing and returns Broken Pipe immediately
I have read various stackoverflow and MSDN pages, but I can't seem to make it work.

Answer 1

As pointed out in the comment, handles must be inheritable, which means replacing the CreatePipe like this :
SECURITY_ATTRIBUTES pipeAttrib;
memset(&pipeAttrib, 0, sizeof(pipeAttrib));
pipeAttrib.nLength = sizeof(SECURITY_ATTRIBUTES);
pipeAttrib.bInheritHandle = NULL;
pipeAttrib.bInheritHandle = TRUE;
if (!CreatePipe(&StdOutHandles[0], &StdOutHandles[1],&pipeAttrib, 4096)) {
printf("Create pipe error %d\n", GetLastError());
return NULL;
}
And setting CreateProcess 5th parameter (bInheritHandles) to TRUE instead of FALSE.