Capture Output of Spawned Process to string

Question

Background:

---

I'm working on a program that needs to be able to capture the stdout, stderr and return values of a program. Ideally, I would like to capture these in a string that I store inside of my object that holds details of the process. I currently have some code that works by saving the output into a file using some (in my opinion) archaic C file handle magic. Any time I want to output the results, I open up that file and I print the contents.

Sometimes (when a process I spawn is left running) the next execution of my executable will break down because it cannot open the file for writing.

Problem Statement:

---

I'm looking for a way to save the output from stdout of a created process in windows to one string and the stderr to another in a safer, more modern fashion. That way I could print those contents any time I feel like outputting the result of each created process.

My ugly code:

---

main chunk-

    int stdoutold = _dup(_fileno(stdout)); //make a copy of stdout
    int stderrold = _dup(_fileno(stdout)); //make a copy of stderr
    FILE *f; 

    if(!fopen_s(&f, "name_of_my_file", "w")){ //make sure I can write to the file
        _dup2(_fileno(f), _fileno(stdout)); //make stdout point to f
        _dup2(_fileno(f), _fileno(stderr)); //make stderr point to f

        fork("command_I_want_to_run", &pi); //run my fake fork (see below)
    }
    else{
        ...//error handling
    }
    _close(_fileno(stdout)); //close tainted stdout
    _close(_fileno(stderr)); //close tainted stderr
    _close(_fileno(f)); //close f
    _dup2(stdoutold, _fileno(stdout)); //fix stdout
    _dup2(stderrold, _fileno(stderr)); //fix stderr

fork- (you can think of this as just CreateProcess, but just in case anyone needs to see what happens here)

int fork(std::string s, PROCESS_INFORMATION* pi){
char infoBuf[INFO_BUFFER_SIZE];
int bufCharCount = 
    ExpandEnvironmentStrings(s.c_str(), infoBuf, INFO_BUFFER_SIZE ); 
...
    STARTUPINFO si;
    ZeroMemory( &si, sizeof(si) );
    si.cb = sizeof(si);
    ZeroMemory( pi, sizeof(*pi) );
    LPSTR str = const_cast<char *>(infoBuf);
    if(!CreateProcess(NULL,
        str,
        NULL,
        NULL,
        TRUE,
        0,
        NULL,
        NULL,
        &si,
        pi)
    ){
        int err = GetLastError();
        printf("CreateProcess failed (%d).\n", err);
        CloseHandle((*pi).hProcess);
        CloseHandle((*pi).hThread);
        return err;
    }
return 0;
}

Notes:

---

• I'm using VS 2010
• I want to remain using multiple processes, not threads because I need what I run to have the freedom of its own process

Edit:

---

An extra note: I also try to wait for the process to finish right after calling the function that runs the code given, so the results of stdout and stderr are available to me at that time.

Answer 1

Eddy Luten's answer led me in a good direction, but the MSDN documentation (while elaborate) had some issues. Mainly, you need to ensure you close all handles you don't use. Also it just has code it expects the user to understand.

So instead, here's my wall of code I expect people to just understand :D

#include <string>
#include <iostream>
#include <windows.h> 
#include <stdio.h>
#pragma warning( disable : 4800 ) // stupid warning about bool
#define BUFSIZE 4096
HANDLE g_hChildStd_OUT_Rd = NULL;
HANDLE g_hChildStd_OUT_Wr = NULL;
HANDLE g_hChildStd_ERR_Rd = NULL;
HANDLE g_hChildStd_ERR_Wr = NULL;

PROCESS_INFORMATION CreateChildProcess(void); 
void ReadFromPipe(PROCESS_INFORMATION); 

int main(int argc, char *argv[]){ 
    SECURITY_ATTRIBUTES sa; 
    printf("\n->Start of parent execution.\n");
    // Set the bInheritHandle flag so pipe handles are inherited. 
    sa.nLength = sizeof(SECURITY_ATTRIBUTES); 
    sa.bInheritHandle = TRUE; 
    sa.lpSecurityDescriptor = NULL; 
    // Create a pipe for the child process's STDERR. 
    if ( ! CreatePipe(&g_hChildStd_ERR_Rd, &g_hChildStd_ERR_Wr, &sa, 0) ) {
        exit(1); 
    }
    // Ensure the read handle to the pipe for STDERR is not inherited.
    if ( ! SetHandleInformation(g_hChildStd_ERR_Rd, HANDLE_FLAG_INHERIT, 0) ){
        exit(1);
    }
    // Create a pipe for the child process's STDOUT. 
    if ( ! CreatePipe(&g_hChildStd_OUT_Rd, &g_hChildStd_OUT_Wr, &sa, 0) ) {
        exit(1);
    }
    // Ensure the read handle to the pipe for STDOUT is not inherited
    if ( ! SetHandleInformation(g_hChildStd_OUT_Rd, HANDLE_FLAG_INHERIT, 0) ){
        exit(1); 
    }
    // Create the child process. 
    PROCESS_INFORMATION piProcInfo = CreateChildProcess();

    // Read from pipe that is the standard output for child process. 
    printf( "\n->Contents of child process STDOUT:\n\n", argv[1]);
    ReadFromPipe(piProcInfo); 

    printf("\n->End of parent execution.\n");

    // The remaining open handles are cleaned up when this process terminates. 
    // To avoid resource leaks in a larger application, 
    //   close handles explicitly.
    return 0; 
} 

// Create a child process that uses the previously created pipes
//  for STDERR and STDOUT.
PROCESS_INFORMATION CreateChildProcess(){
    // Set the text I want to run
    char szCmdline[]="test --log_level=all --report_level=detailed";
    PROCESS_INFORMATION piProcInfo; 
    STARTUPINFO siStartInfo;
    bool bSuccess = FALSE; 

    // Set up members of the PROCESS_INFORMATION structure. 
    ZeroMemory( &piProcInfo, sizeof(PROCESS_INFORMATION) );

    // Set up members of the STARTUPINFO structure. 
    // This structure specifies the STDERR and STDOUT handles for redirection.
    ZeroMemory( &siStartInfo, sizeof(STARTUPINFO) );
    siStartInfo.cb = sizeof(STARTUPINFO); 
    siStartInfo.hStdError = g_hChildStd_ERR_Wr;
    siStartInfo.hStdOutput = g_hChildStd_OUT_Wr;
    siStartInfo.dwFlags |= STARTF_USESTDHANDLES;

    // Create the child process. 
    bSuccess = CreateProcess(NULL, 
        szCmdline,     // command line 
        NULL,          // process security attributes 
        NULL,          // primary thread security attributes 
        TRUE,          // handles are inherited 
        0,             // creation flags 
        NULL,          // use parent's environment 
        NULL,          // use parent's current directory 
        &siStartInfo,  // STARTUPINFO pointer 
        &piProcInfo);  // receives PROCESS_INFORMATION
    CloseHandle(g_hChildStd_ERR_Wr);
    CloseHandle(g_hChildStd_OUT_Wr);
    // If an error occurs, exit the application. 
    if ( ! bSuccess ) {
        exit(1);
    }
    return piProcInfo;
}

// Read output from the child process's pipe for STDOUT
// and write to the parent process's pipe for STDOUT. 
// Stop when there is no more data. 
void ReadFromPipe(PROCESS_INFORMATION piProcInfo) {
    DWORD dwRead; 
    CHAR chBuf[BUFSIZE];
    bool bSuccess = FALSE;
    std::string out = "", err = "";
    for (;;) { 
        bSuccess=ReadFile( g_hChildStd_OUT_Rd, chBuf, BUFSIZE, &dwRead, NULL);
        if( ! bSuccess || dwRead == 0 ) break; 

        std::string s(chBuf, dwRead);
        out += s;
    } 
    dwRead = 0;
    for (;;) { 
        bSuccess=ReadFile( g_hChildStd_ERR_Rd, chBuf, BUFSIZE, &dwRead, NULL);
        if( ! bSuccess || dwRead == 0 ) break; 

        std::string s(chBuf, dwRead);
        err += s;

    } 
    std::cout << "stdout:" << out << std::endl;
    std::cout << "stderr:" << err << std::endl;
}

Answer 2

Shawn Blakesley code is good rework of Microsoft sample code but it has a bit of a problem when there is massive stdout and stderr interleaved streams that are out of order. And some handles are leaked (which is OK for the sample code). Having background thread and PeekNamedPipe() calls makes sure the the code behave more similar to POSIX system call:

#include <windows.h> 
#include <stdio.h>
#include <malloc.h>

#ifdef __cplusplus
#define BEGIN_C extern "C" {
#define END_C } // extern "C"
#define null nullptr
#else
#define BEGIN_C
#define END_C
#define null ((void*)0)
#endif

BEGIN_C

int system_np(const char* command, int timeout_milliseconds, 
              char* stdout_data, int stdout_data_size, 
              char* stderr_data, int stderr_data_size, int* exit_code);

typedef struct system_np_s {
    HANDLE child_stdout_read;
    HANDLE child_stderr_read;
    HANDLE reader;
    PROCESS_INFORMATION pi;
    const char* command;
    char* stdout_data;
    int   stdout_data_size;
    char* stderr_data;
    int   stderr_data_size;
    int*  exit_code;
    int   timeout; // timeout in milliseconds or -1 for INIFINTE
} system_np_t;

static char stdout_data[16 * 1024 * 1024];
static char stderr_data[16 * 1024 * 1024];

int main(int argc, char *argv[]) { 
    int bytes = 1;
    for (int i = 1; i < argc; i++) {
        bytes += (int)strlen(argv[i]) + 1;
    }
    char* command = (char*)alloca(bytes);
    command[0] = 0;
    char* p = command;
    for (int i = 1; i < argc; i++) {
        int n = (int)strlen(argv[i]);
        memcpy(p, argv[i], n); p += n;
        *p = (i == argc - 1) ? 0x00 : 0x20; 
        p++;
    }
    int exit_code = 0;
    if (command[0] == 0) {
        command = (char*)"cmd.exe /c \"dir /w /b\"";
    }
    int r = system_np(command, 100 * 1000, stdout_data, sizeof(stdout_data), stderr_data, sizeof(stderr_data), &exit_code);
    if (r != 0) {
        fprintf(stderr, "system_np failed: %d 0x%08x %s", r, r, strerror(r));
        return r;
    } else {
        fwrite(stdout_data, strlen(stdout_data), 1, stdout);
        fwrite(stderr_data, strlen(stderr_data), 1, stderr);
        return exit_code;
    }
}

static int peek_pipe(HANDLE pipe, char* data, int size) {         
    char buffer[4 * 1024];
    DWORD read = 0;  
    DWORD available = 0;
    bool b = PeekNamedPipe(pipe, null, sizeof(data), null, &available, null);
    if (!b) {
        return -1;
    } else if (available > 0) {
        int bytes = min(sizeof(buffer), available);
        b = ReadFile(pipe, buffer, bytes, &read, null);
        if (!b) {
            return -1;
        }
        if (data != null && size > 0) {
            int n = min(size - 1, (int)read);
            memcpy(data, buffer, n);
            data[n + 1] = 0; // always zero terminated
            return n;
        }
    } 
    return 0;
}

static DWORD WINAPI read_from_all_pipes_fully(void* p) {
    system_np_t* system = (system_np_t*)p;
    unsigned long long milliseconds = GetTickCount64(); // since boot time
    char* out = system->stdout_data != null && system->stdout_data_size > 0 ? system->stdout_data : null;
    char* err = system->stderr_data != null && system->stderr_data_size > 0 ? system->stderr_data : null;
    int out_bytes = system->stdout_data != null && system->stdout_data_size > 0 ? system->stdout_data_size - 1 : 0;
    int err_bytes = system->stderr_data != null && system->stderr_data_size > 0 ? system->stderr_data_size - 1 : 0;
    for (;;) {
        int read_stdout = peek_pipe(system->child_stdout_read, out, out_bytes);
        if (read_stdout > 0 && out != null) { out += read_stdout; out_bytes -= read_stdout; } 
        int read_stderr = peek_pipe(system->child_stderr_read, err, err_bytes);
        if (read_stderr > 0 && err != null) { err += read_stderr; err_bytes -= read_stderr; } 
        if (read_stdout < 0 && read_stderr < 0) { break; } // both pipes are closed
        unsigned long long time_spent_in_milliseconds = GetTickCount64() - milliseconds;
        if (system->timeout > 0 && time_spent_in_milliseconds > system->timeout) { break; }
        if (read_stdout == 0 && read_stderr == 0) { // nothing has been read from both pipes
            HANDLE handles[2] = {system->child_stdout_read, system->child_stderr_read};
            WaitForMultipleObjects(2, handles, false, 1); // wait for at least 1 millisecond (more likely 16)
        }
    }
    if (out != null) { *out = 0; }
    if (err != null) { *err = 0; }
    return 0;
}

static int create_child_process(system_np_t* system) {
    SECURITY_ATTRIBUTES sa = {0}; 
    sa.nLength = sizeof(SECURITY_ATTRIBUTES); 
    sa.bInheritHandle = true; 
    sa.lpSecurityDescriptor = null; 
    HANDLE child_stdout_write = INVALID_HANDLE_VALUE;
    HANDLE child_stderr_write = INVALID_HANDLE_VALUE;
    if (!CreatePipe(&system->child_stderr_read, &child_stderr_write, &sa, 0) ) {
        return GetLastError(); 
    }
    if (!SetHandleInformation(system->child_stderr_read, HANDLE_FLAG_INHERIT, 0) ){
        return GetLastError(); 
    }
    if (!CreatePipe(&system->child_stdout_read, &child_stdout_write, &sa, 0) ) {
        return GetLastError(); 
    }
    if (!SetHandleInformation(system->child_stdout_read, HANDLE_FLAG_INHERIT, 0) ){
        return GetLastError(); 
    }
    // Set the text I want to run
    STARTUPINFO siStartInfo = {0};
    siStartInfo.cb = sizeof(STARTUPINFO); 
    siStartInfo.hStdError = child_stderr_write;
    siStartInfo.hStdOutput = child_stdout_write;
    siStartInfo.dwFlags |= STARTF_USESTDHANDLES | STARTF_USESHOWWINDOW;
    siStartInfo.wShowWindow = SW_HIDE;
    bool b = CreateProcessA(null, 
        (char*)system->command,
        null,               // process security attributes 
        null,               // primary thread security attributes 
        true,               // handles are inherited 
        CREATE_NO_WINDOW,   // creation flags 
        null,               // use parent's environment 
        null,               // use parent's current directory 
        &siStartInfo,       // STARTUPINFO pointer 
        &system->pi);       // receives PROCESS_INFORMATION
    int err = GetLastError();
    CloseHandle(child_stderr_write);
    CloseHandle(child_stdout_write);
    if (!b) {
        CloseHandle(system->child_stdout_read); system->child_stdout_read = INVALID_HANDLE_VALUE;
        CloseHandle(system->child_stderr_read); system->child_stderr_read = INVALID_HANDLE_VALUE;
    }
    return b ? 0 : err;
}

int system_np(const char* command, int timeout_milliseconds, 
              char* stdout_data, int stdout_data_size, 
              char* stderr_data, int stderr_data_size, int* exit_code) {
    system_np_t system = {0};
    if (exit_code != null) { *exit_code = 0; }
    if (stdout_data != null && stdout_data_size > 0) { stdout_data[0] = 0; }
    if (stderr_data != null && stderr_data_size > 0) { stderr_data[0] = 0; }
    system.timeout = timeout_milliseconds > 0 ? timeout_milliseconds : -1; 
    system.command = command;
    system.stdout_data = stdout_data;
    system.stderr_data = stderr_data;
    system.stdout_data_size = stdout_data_size;
    system.stderr_data_size = stderr_data_size;
    int r = create_child_process(&system);
    if (r == 0) {
        system.reader = CreateThread(null, 0, read_from_all_pipes_fully, &system, 0, null);
        if (system.reader == null) { // in theory should rarely happen only when system super low on resources
            r = GetLastError();
            TerminateProcess(system.pi.hProcess, ECANCELED);
        } else {
            bool thread_done  = WaitForSingleObject(system.pi.hThread, timeout_milliseconds) == 0;
            bool process_done = WaitForSingleObject(system.pi.hProcess, timeout_milliseconds) == 0;
            if (!thread_done || !process_done) {
                TerminateProcess(system.pi.hProcess, ETIME);
            }
            if (exit_code != null) {
                GetExitCodeProcess(system.pi.hProcess, (DWORD*)exit_code);
            }        
            CloseHandle(system.pi.hThread);
            CloseHandle(system.pi.hProcess);
            CloseHandle(system.child_stdout_read); system.child_stdout_read = INVALID_HANDLE_VALUE;
            CloseHandle(system.child_stderr_read); system.child_stderr_read = INVALID_HANDLE_VALUE;
            WaitForSingleObject(system.reader, INFINITE); // join thread
            CloseHandle(system.reader);
        }
    }
    if (stdout_data != null && stdout_data_size > 0) { stdout_data[stdout_data_size - 1] = 0; }
    if (stderr_data != null && stderr_data_size > 0) { stderr_data[stderr_data_size - 1] = 0; }
    return r; 
} 

END_C

Answer 3

You'll have to use pipes to capture the contents of your process's stdout stream. There's an elaborate example on MSDN on how to accomplish this:

MSDN: Creating a Child Process with Redirected Input and Output