Windows: system_np() - Creating a Child Process with Redirected Input and Output

system_np.c

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

// Reworked MSDN sample starting with https://stackoverflow.com/questions/14147138/capture-output-of-spawned-process-to-string
// https://msdn.microsoft.com/en-us/library/windows/desktop/ms682499(v=vs.85).aspx
// Implemented in ~C99 instead of C++
// Child Process window hidden (for use from WinMain() /SYSTEM:WINDOWS executable)
// Creates background thread for peeking the pipes to avoid deadlocks.
// Implements timeout in milliseconds and tames CPU utilization with WaitForMultipleObjects.

// Does not allocate memory (no malloc() free() calls)

#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 data_stdout[16 * 1024 * 1024];
static char data_stderr[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, data_stdout, sizeof(data_stdout), data_stderr, sizeof(data_stderr), &exit_code);
    if (r != 0) {
        fprintf(stderr, "system_np failed: %d 0x%08x %s", r, r, strerror(r));
        return r;
    } else {
        fwrite(data_stdout, strlen(data_stdout), 1, stdout);
        fwrite(data_stderr, strlen(data_stderr), 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