feat: add SSH transport, rsync-style CLI, and --stdio server mode

- Replace send()/recv() with read()/write() + io_set_fds() for fd abstraction
- Add client_connect_ssh() via socketpair + fork/exec; reap child in client_disconnect()
- Add server --stdio flag for SSH invocation
- Replace handle_arg() with rsync-style positional args (fastsync <src> <dest>)
- Add SSH vs TCP auto-detection via is_remote_dest()
- Add TransportType and ssh_destination to Config, fix uninitialized fields in
  config_receive() (pre-existing multithreaded crash)
- Sender threads now wait for server STATUS_OK before disconnecting
- Add --help, error handling for sendfile+SSH combos
- Add pre-flight checks and Posix Args test case to test.py
- Add test_config_ssh_dest() unit test

src/client/client.c

@@ -99,12 +99,21 @@ int load_files_multithreaded(void *pipeline_context) {
 
 int send_chunks_multithreaded(void *pipeline_context) {
   PipelineContextSender *context = (PipelineContextSender *)pipeline_context;
-  Client *client = client_create();
-  const char *env_ip = getenv("FASTSYNC_SERVER_IP");
-  const char *ip = env_ip ? env_ip : "127.0.0.1";
-  const char *env_port = getenv("FASTSYNC_SERVER_PORT");
-  int port = env_port ? atoi(env_port) : 8080;
-  client_connect(client, (char *)ip, port);
+  Client *client;
+  if (context->config->transport == TRANSPORT_SSH) {
+    if (context->config->use_sendfile) {
+      fprintf(stderr, "Error: -f/--sendfile is not supported with SSH transport\n");
+      return 1;
+    }
+    client = client_connect_ssh(context->config->ssh_destination);
+  } else {
+    client = client_create();
+    const char *env_ip = getenv("FASTSYNC_SERVER_IP");
+    const char *ip = env_ip ? env_ip : "127.0.0.1";
+    const char *env_port = getenv("FASTSYNC_SERVER_PORT");
+    int port = env_port ? atoi(env_port) : 8080;
+    client_connect(client, (char *)ip, port);
+  }
   config_send(client->file_descriptor, context->config);
 
   while (true) {
@@ -114,6 +123,11 @@ int send_chunks_multithreaded(void *pipeline_context) {
         &context->condition_not_full_loader, &context->loader_done);
     if (current_chunk == NULL) {
       send_status(client->file_descriptor, STATUS_FINISHED);
+      if (receive_status(client->file_descriptor) != STATUS_OK) {
+        client_disconnect(client);
+        client_delete(client);
+        return 1;
+      }
       client_disconnect(client);
       client_delete(client);
       return thrd_success;
@@ -127,12 +141,21 @@ int send_chunks_multithreaded(void *pipeline_context) {
 }
 
 int send_files(Config *config) {
-  Client *client = client_create();
-  const char *env_ip = getenv("FASTSYNC_SERVER_IP");
-  const char *ip = env_ip ? env_ip : "127.0.0.1";
-  const char *env_port = getenv("FASTSYNC_SERVER_PORT");
-  int port = env_port ? atoi(env_port) : 8080;
-  client_connect(client, (char *)ip, port);
+  Client *client;
+  if (config->transport == TRANSPORT_SSH) {
+    if (config->use_sendfile) {
+      fprintf(stderr, "Error: -f/--sendfile is not supported with SSH transport\n");
+      return 1;
+    }
+    client = client_connect_ssh(config->ssh_destination);
+  } else {
+    client = client_create();
+    const char *env_ip = getenv("FASTSYNC_SERVER_IP");
+    const char *ip = env_ip ? env_ip : "127.0.0.1";
+    const char *env_port = getenv("FASTSYNC_SERVER_PORT");
+    int port = env_port ? atoi(env_port) : 8080;
+    client_connect(client, (char *)ip, port);
+  }
   config_send(client->file_descriptor, config);
   DirectoryScanner *scanner = directory_scanner_create(config->send_directory, config->use_metadata);
   Chunk *current_chunk;
@@ -176,37 +199,62 @@ int send_files_multithreaded(Config *config) {
   return 0;
 }
 
-void handle_arg(char *argument_given, char *argument_to_set, bool *result,
-                char *message) {
-  if (strcmp(argument_given, argument_to_set) == 0) {
-    *result = true;
-    log_message(LOG_LEVEL_INFO, message);
+static int is_remote_dest(const char *s) {
+  const char *colon = strchr(s, ':');
+  if (!colon) return 0;
+  if (colon == s) return 0;
+  for (const char *p = s; p < colon; p++) {
+    if (*p == '/') return 0;
   }
+  return 1;
 }
+
+static void print_usage(void) {
+  printf("Usage:\n");
+  printf("  fastsync [options] <source> <destination>\n");
+  printf("  fastsync [options] --source-dir <src> --dest-dir <dst>\n");
+  printf("\n");
+  printf("Destination formats:\n");
+  printf("  user@host:/path     SSH transport (rsync-style)\n");
+  printf("  host:/path          SSH transport (current user)\n");
+  printf("  /local/path         TCP transport (requires server on localhost:8080)\n");
+  printf("\n");
+  printf("Options:\n");
+  printf("  -c [level]          Enable compression (level 1-22, default 5)\n");
+  printf("  -m                  Enable multithreading\n");
+  printf("  -s                  Enable chunk serialization\n");
+  printf("  -f                  Enable sendfile (TCP only, not with -c or -s)\n");
+  printf("  -M, --preserve      Preserve file metadata\n");
+  printf("  --source-dir <path> Source directory\n");
+  printf("  --dest-dir <path>   Destination directory\n");
+  printf("  --save-to-disk      Write received files to disk\n");
+  printf("  --help              Show this help\n");
+}
+
 int main(int argc, char *argv[]) {
   const char *env_source = getenv("FASTSYNC_SOURCE_DIR");
   const char *env_dest = getenv("FASTSYNC_DEST_DIR");
   const char *env_save = getenv("FASTSYNC_SAVE_TO_DISK");
 
-  char *source_dir =
-      env_source ? str_dup((char *)env_source)
-                 : str_dup("/home/taptap/Nextcloud/Uni/moodle/B. Schnor： "
-                           "Konzepte Paralleler Programmierung, SoSe 2026");
-  char *dest_dir =
-      env_dest ? str_dup((char *)env_dest) : str_dup("./data_copied");
   bool save_to_disk = false;
   if (env_save &&
       (strcmp(env_save, "true") == 0 || strcmp(env_save, "1") == 0)) {
     save_to_disk = true;
   }
 
-  Config *config = config_create(str_dup("1.0.0"), source_dir, dest_dir,
+  Config *config = config_create(str_dup("1.0.0"), NULL, NULL,
                                  save_to_disk, false, false, false, false, 5, 20, false);
+
+  int positional_args[2];
+  int positional_count = 0;
+
   for (int i = 1; i < argc; i++) {
-    if (strcmp(argv[i], "-c") == 0) {
+    if (strcmp(argv[i], "--help") == 0) {
+      print_usage();
+      return 0;
+    } else if (strcmp(argv[i], "-c") == 0) {
       config->use_compression = true;
       log_message(LOG_LEVEL_INFO, "Enabled Compression");
-
       if (i + 1 < argc) {
         char *end_ptr;
         int level = strtol(argv[i + 1], &end_ptr, 10);
@@ -214,6 +262,7 @@ int main(int argc, char *argv[]) {
           config->compression_level = level;
           log_message(LOG_LEVEL_INFO, "Set Compression level to %d",
                       config->compression_level);
+          i++;
         }
       }
     } else if (strcmp(argv[i], "--source-dir") == 0 && i + 1 < argc) {
@@ -230,11 +279,52 @@ int main(int argc, char *argv[]) {
     } else if (strcmp(argv[i], "-f") == 0 || strcmp(argv[i], "--sendfile") == 0) {
       config->use_sendfile = true;
       log_message(LOG_LEVEL_INFO, "Enabled sendfile");
+    } else if (strcmp(argv[i], "-m") == 0) {
+      config->use_multithreading = true;
+      log_message(LOG_LEVEL_INFO, "Enabled Multithreading");
+    } else if (strcmp(argv[i], "-s") == 0) {
+      config->use_chunk_serialization = true;
+      log_message(LOG_LEVEL_INFO, "Enabled Chunk Serialization");
+    } else if (argv[i][0] == '-') {
+      fprintf(stderr, "Unknown option: %s\n", argv[i]);
+      print_usage();
+      return 1;
     } else {
-      handle_arg(argv[i], "-m", &config->use_multithreading,
-                 "Enabled Multithreading");
-      handle_arg(argv[i], "-s", &config->use_chunk_serialization,
-                 "Enabled Chunk Serialization");
+      if (positional_count < 2)
+        positional_args[positional_count++] = i;
+      else {
+        fprintf(stderr, "Unexpected argument: %s\n", argv[i]);
+        print_usage();
+        return 1;
+      }
+    }
+  }
+
+  if (positional_count == 2) {
+    free(config->send_directory);
+    config->send_directory = str_dup(argv[positional_args[0]]);
+    free(config->receive_root_directory);
+    config->receive_root_directory = str_dup(argv[positional_args[1]]);
+    config->save_to_disk = true;
+
+    if (is_remote_dest(config->receive_root_directory)) {
+      config->transport = TRANSPORT_SSH;
+      config->ssh_destination = str_dup(config->receive_root_directory);
+    }
+  } else if (positional_count == 1) {
+    fprintf(stderr, "Error: missing destination argument\n");
+    print_usage();
+    return 1;
+  } else {
+    if (!config->send_directory) {
+      config->send_directory =
+          env_source ? str_dup((char *)env_source)
+                     : str_dup("/home/taptap/Nextcloud/Uni/moodle/B. Schnor： "
+                               "Konzepte Paralleler Programmierung, SoSe 2026");
+    }
+    if (!config->receive_root_directory) {
+      config->receive_root_directory =
+          env_dest ? str_dup((char *)env_dest) : str_dup("./data_copied");
     }
   }
 
@@ -243,6 +333,11 @@ int main(int argc, char *argv[]) {
     return 1;
   }
 
+  if (config->transport == TRANSPORT_SSH && config->use_sendfile) {
+    fprintf(stderr, "Error: -f/--sendfile is not supported with SSH transport\n");
+    return 1;
+  }
+
   if (config->use_multithreading)
     return send_files_multithreaded(config);
   return send_files(config);

src/server/server.c

@@ -10,6 +10,7 @@
 #include "utils.h"
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <threads.h>
 
 File *file_receive(Config *config, int file_descriptor) {
@@ -161,12 +162,18 @@ void handler(int file_descriptor) {
     thrd_join(receiver, NULL);
     thrd_join(writer, NULL);
     pipeline_context_receiver_destroy(context);
+    send_status(file_descriptor, STATUS_OK);
   } else
     receive_files(config, file_descriptor);
   close(file_descriptor);
 }
 
-int main() {
+int main(int argc, char *argv[]) {
+  if (argc > 1 && strcmp(argv[1], "--stdio") == 0) {
+    io_set_fds(STDIN_FILENO, STDOUT_FILENO);
+    handler(STDIN_FILENO);
+    return 0;
+  }
   Server *server = server_create(8080);
   server_listen(server, handler);
   server_delete(server);

src/shared/config.c

@@ -22,6 +22,8 @@ Config *config_create(char *version, char *send_directory,
   config->compression_level = compression_level;
   config->num_connections = num_connections;
   config->use_sendfile = use_sendfile;
+  config->transport = TRANSPORT_TCP;
+  config->ssh_destination = NULL;
   return config;
 }
 
@@ -29,6 +31,7 @@ void config_delete(Config *config) {
   free(config->version);
   free(config->send_directory);
   free(config->receive_root_directory);
+  free(config->ssh_destination);
   free(config);
 }
 
@@ -63,6 +66,8 @@ Config *config_receive(int file_descriptor) {
   config->compression_level = receive_int(file_descriptor);
   config->num_connections = receive_int(file_descriptor);
   config->use_sendfile = receive_int(file_descriptor);
+  config->transport = TRANSPORT_TCP;
+  config->ssh_destination = NULL;
   send_status(file_descriptor, STATUS_OK);
   return config;
 }

src/shared/config.h

@@ -3,6 +3,11 @@
 
 #include <stdbool.h>
 
+typedef enum {
+  TRANSPORT_TCP,
+  TRANSPORT_SSH
+} TransportType;
+
 typedef struct Config {
   char *version;
   char *send_directory;
@@ -16,6 +21,8 @@ typedef struct Config {
   bool use_metadata;
   int compression_level;
   int num_connections;
+  TransportType transport;
+  char *ssh_destination;
 } Config;
 
 Config *config_create(char *version, char *send_directory,

src/shared/socket.c

@@ -1,12 +1,134 @@
 #include "socket.h"
 #include "log.h"
 #include <arpa/inet.h>
+#include <fcntl.h>
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/socket.h>
+#include <sys/wait.h>
 #include <unistd.h>
 
+static __thread int io_read_fd = -1;
+static __thread int io_write_fd = -1;
+
+void io_set_fds(int read_fd, int write_fd) {
+  io_read_fd = read_fd;
+  io_write_fd = write_fd;
+}
+
+static int io_fd(int dir_fd, int file_descriptor) {
+  return (dir_fd != -1) ? dir_fd : file_descriptor;
+}
+
+typedef struct {
+  char user[256];
+  char host[256];
+  char remote_path[4096];
+} RemoteDest;
+
+static int parse_remote_dest(const char *dest, RemoteDest *r) {
+  const char *colon = strchr(dest, ':');
+  if (!colon) return -1;
+
+  size_t remote_path_len = strlen(colon + 1);
+  if (remote_path_len >= sizeof(r->remote_path)) return -1;
+  memcpy(r->remote_path, colon + 1, remote_path_len + 1);
+
+  const char *at = memchr(dest, '@', colon - dest);
+  if (at) {
+    size_t user_len = at - dest;
+    if (user_len >= sizeof(r->user)) return -1;
+    memcpy(r->user, dest, user_len);
+    r->user[user_len] = '\0';
+
+    size_t host_len = colon - at - 1;
+    if (host_len >= sizeof(r->host)) return -1;
+    memcpy(r->host, at + 1, host_len);
+    r->host[host_len] = '\0';
+  } else {
+    r->user[0] = '\0';
+    size_t host_len = colon - dest;
+    if (host_len >= sizeof(r->host)) return -1;
+    memcpy(r->host, dest, host_len);
+    r->host[host_len] = '\0';
+  }
+  return 0;
+}
+
+Client *client_connect_ssh(char *destination) {
+  RemoteDest r;
+  if (parse_remote_dest(destination, &r) != 0) {
+    fprintf(stderr, "Invalid remote destination: %s\n", destination);
+    exit(EXIT_FAILURE);
+  }
+
+  int sv[2];
+  if (socketpair(AF_UNIX, SOCK_STREAM, 0, sv) < 0) {
+    perror("socketpair failed");
+    exit(EXIT_FAILURE);
+  }
+
+  int exec_pipe[2];
+  if (pipe(exec_pipe) < 0) {
+    perror("pipe failed");
+    exit(EXIT_FAILURE);
+  }
+
+  pid_t pid = fork();
+  if (pid < 0) {
+    perror("fork failed");
+    exit(EXIT_FAILURE);
+  }
+
+  if (pid == 0) {
+    close(sv[0]);
+    close(exec_pipe[0]);
+    fcntl(exec_pipe[1], F_SETFD, FD_CLOEXEC);
+
+    if (sv[1] != STDIN_FILENO)
+      dup2(sv[1], STDIN_FILENO);
+    if (sv[1] != STDOUT_FILENO)
+      dup2(sv[1], STDOUT_FILENO);
+    if (sv[1] > 1) close(sv[1]);
+
+    char ssh_user[512];
+    if (r.user[0] != '\0')
+      snprintf(ssh_user, sizeof(ssh_user), "%s@%s", r.user, r.host);
+    else
+      snprintf(ssh_user, sizeof(ssh_user), "%s", r.host);
+
+    execlp("ssh", "ssh", "-o", "Compression=no", "-o",
+           "ControlMaster=no", ssh_user, "fastsync-server", "--stdio",
+           (char *)NULL);
+    perror("exec of ssh failed");
+    (void)write(exec_pipe[1], "x", 1);
+    _exit(1);
+  }
+
+  close(sv[1]);
+  close(exec_pipe[1]);
+
+  char exec_status;
+  ssize_t n = read(exec_pipe[0], &exec_status, 1);
+  close(exec_pipe[0]);
+
+  if (n > 0) {
+    close(sv[0]);
+    waitpid(pid, NULL, 0);
+    fprintf(stderr, "Error: could not launch 'fastsync-server --stdio' on remote\n");
+    exit(EXIT_FAILURE);
+  }
+
+  Client *client = malloc(sizeof(Client));
+  client->file_descriptor = sv[0];
+  client->address.sin_family = AF_UNIX;
+  client->address_length = 0;
+  client->ssh_child_pid = pid;
+  return client;
+}
+
 Server *server_create(int port) {
   Server *server = (Server *)malloc(sizeof(Server));
   if (server == NULL) {
@@ -82,6 +204,7 @@ Client *client_create() {
   client->file_descriptor = file_descriptor;
   client->address.sin_family = AF_INET;
   client->address_length = sizeof(client->address);
+  client->ssh_child_pid = -1;
   return client;
 }
 
@@ -100,7 +223,14 @@ void client_connect(Client *client, char *host, int port) {
   }
 }
 
-void client_disconnect(Client *client) { close(client->file_descriptor); }
+void client_disconnect(Client *client) {
+  close(client->file_descriptor);
+  if (client->ssh_child_pid > 0) {
+    int status;
+    waitpid(client->ssh_child_pid, &status, 0);
+    client->ssh_child_pid = -1;
+  }
+}
 
 void client_delete(Client *client) {
   if (client == NULL)
@@ -110,12 +240,11 @@ void client_delete(Client *client) {
 
 void send_n_data(int file_descriptor, void *data, size_t data_size) {
   log_message(LOG_LEVEL_DEBUG, "    Sending n Data: %zu", data_size);
+  int fd = io_fd(io_write_fd, file_descriptor);
   ssize_t total_bytes_send = 0;
   while (total_bytes_send < data_size) {
-    printf("Trying: %zu\n", data_size - total_bytes_send);
-    ssize_t bytes_send = send(file_descriptor, (char *)data + total_bytes_send,
-                              data_size - total_bytes_send, 0);
-    printf("Bytes send: %zd\n", bytes_send);
+    ssize_t bytes_send = write(fd, (char *)data + total_bytes_send,
+                               data_size - total_bytes_send);
     if (bytes_send <= 0) {
       perror("Could not send data!");
       exit(EXIT_FAILURE);
@@ -127,11 +256,12 @@ void send_n_data(int file_descriptor, void *data, size_t data_size) {
 
 void receive_n_data(int file_descriptor, void *data, size_t data_size) {
   log_message(LOG_LEVEL_DEBUG, "    Receiving n Data: %zu", data_size);
+  int fd = io_fd(io_read_fd, file_descriptor);
   size_t total_bytes_received = 0;
   while (total_bytes_received < data_size) {
-    long long bytes_received =
-        recv(file_descriptor, data + total_bytes_received,
-             data_size - total_bytes_received, 0);
+    ssize_t bytes_received =
+        read(fd, (char *)data + total_bytes_received,
+             data_size - total_bytes_received);
     if (bytes_received == -1 || bytes_received == 0) {
       perror("Could not receive bytes!");
       exit(EXIT_FAILURE);

src/shared/socket.h

@@ -21,13 +21,16 @@ typedef struct Client {
   struct sockaddr_in address;
   unsigned int address_length;
   int file_descriptor;
+  pid_t ssh_child_pid;
 } Client;
 
 Client *client_create();
 void client_disconnect(Client *client);
 void client_delete(Client *client);
 void client_connect(Client *client, char *host, int port);
+Client *client_connect_ssh(char *destination);
 
+void io_set_fds(int read_fd, int write_fd);
 void send_n_data(int file_descriptor, void *data, size_t data_size);
 void receive_n_data(int file_descriptor, void *data, size_t data_size);
 void send_str(int file_descriptor, char *data);