How can I save the data written to stdout to local buffer

Question

In MyProgram.c I am doing CreateProcess("GetData.exe" ....);. GetData when run on a command line prints about 100 lines of data to stdout. When I run my MyPragrame.exe I want that data not going to stdout instead I want it to be saved in the buffer DataBuffer[100][100] assuming there are 100 lines whose strlen is 100. Please help out. Thanks

MyProgram.c:

#include <Windows.h>
#include <stdio.h>

void
main()
{
    STARTUPINFO si;
    PROCESS_INFORMATION pi;
    char DataBuffer[100][100];

    ZeroMemory( &si, sizeof(si) );
    si.cb = sizeof(si);
    ZeroMemory( &pi, sizeof(pi) );

    if( !CreateProcess( NULL,  // No module name (use command line)
        "Getdata.exe",  // Command line
        NULL,           // Process handle not inheritable
        NULL,           // Thread handle not inheritable
        FALSE,          // Set handle inheritance to FALSE
        0,              // No creation flags
        NULL,           // Use parent's environment block
        NULL,           // Use parent's starting directory 
        &si,            // Pointer to STARTUPINFO structure
        &pi )           // Pointer to PROCESS_INFORMATION structure
    ) 
    {
        printf( "CreateProcess failed (%d).\n", GetLastError() );
        return;
    }
}

Answer 1

https://msdn.microsoft.com/en-us/library/windows/desktop/ms686331(v=vs.85).aspx Use with your STARTUPINFO parameter 'si', use STARTF_USESTDHANDLES in the dwFlags , and provide a HANDLE for hStdOutput .

Or, even easier, you could just redirect the output the file on the command line. Eg: GetData.exe > File.txt

https://msdn.microsoft.com/en-us/library/windows/desktop/ms682499(v=vs.85).aspx

"Creating a Child Process with Redirected Input and Output" sample, adapted to only read the stdout of the process, and fits your setup.

#include <comdef.h>

int main() {
STARTUPINFO si;
PROCESS_INFORMATION pi;
char DataBuffer[100][100];

ZeroMemory(&si, sizeof(si));
si.cb = sizeof(si);
ZeroMemory(&pi, sizeof(pi));

SECURITY_ATTRIBUTES saAttr;

// Set the bInheritHandle flag so pipe handles are inherited. 

saAttr.nLength = sizeof(SECURITY_ATTRIBUTES);
saAttr.bInheritHandle = TRUE;
saAttr.lpSecurityDescriptor = NULL;

HANDLE g_hChildStd_IN_Rd = NULL;
HANDLE g_hChildStd_OUT_Rd = NULL;
HANDLE g_hChildStd_OUT_Wr = NULL;

// Create a pipe for the child process's STDOUT. 

if (!CreatePipe(&g_hChildStd_OUT_Rd, &g_hChildStd_OUT_Wr, &saAttr, 0))
{
    printf("StdoutRd CreatePipe");
    return 0;
}

// Ensure the read handle to the pipe for STDOUT is not inherited.
if (!SetHandleInformation(g_hChildStd_OUT_Rd, HANDLE_FLAG_INHERIT, 0))
{
    printf("StdoutRd CreatePipe");
    return 0;
}

si.cb = sizeof(STARTUPINFO);
si.hStdError = g_hChildStd_OUT_Wr;
si.hStdOutput = g_hChildStd_OUT_Wr;
si.hStdInput = g_hChildStd_IN_Rd;
si.dwFlags |= STARTF_USESTDHANDLES;

if (!CreateProcess(NULL,  // No module name (use command line)
    "c:\\windows\\system32\\whoami.exe",  // Command line
    NULL,           // Process handle not inheritable
    NULL,           // Thread handle not inheritable
    TRUE,           // Set handle inheritance to TRUE
    0,              // No creation flags
    NULL,           // Use parent's environment block
    NULL,           // Use parent's starting directory 
    &si,            // Pointer to STARTUPINFO structure
    &pi)           // Pointer to PROCESS_INFORMATION structure
    )
{
    _com_error e(GetLastError());
    printf("CreateProcess failed (%d - %s).\n", GetLastError(), e.ErrorMessage());
    return 0;
}

DWORD bytesRead;
if (!ReadFile(g_hChildStd_OUT_Rd, DataBuffer, 100 * 100, &bytesRead, NULL))
{
    _com_error e(GetLastError());
    printf("ReadFile failed (%d - %s).\n", GetLastError(), e.ErrorMessage());
}
return 0;
}

Now, the output of whoami.exe is in DataBuffer. Note that this sample disregards a lot of safety issues, which you may want to address, especially if you are not providing GetData.exe yourself.