feat: add -f/--sendfile for zero-copy file transfer

- Add file_send_sendfile() using sendfile() syscall to send file
  content directly from fd to socket, bypassing userspace memory
- Add use_sendfile field to Config struct (default false)
- Add -f / --sendfile flag parsing in client.c
- sendfile path in send_chunk() used when -f is set without -c or -s
- send_files_multithreaded() falls back to single-threaded when
  sendfile is enabled (loader pipeline becomes unnecessary)
- Add Sendfile (-f) test case to test.py

README.md

@@ -49,6 +49,7 @@ The client-server communication uses the following status codes:
 | `-m` | Enable multithreading mode |
 | `-c [level]` | Enable compression with optional level (1-22, default: 5) |
 | `-s` | Enable chunk serialization (batch-transfer all files per chunk) |
+| `-f` | Enable sendfile (zero-copy file transfer, bypasses userspace memory) |
 | `--source-dir <path>` | Source directory to sync (overrides `FASTSYNC_SOURCE_DIR`) |
 | `--dest-dir <path>` | Server-side destination directory (overrides `FASTSYNC_DEST_DIR`) |
 | `--save-to-disk` | Persist received files to disk |
@@ -118,6 +119,9 @@ make
 
 # Multithreaded with compressed chunk serialization
 ./build/client -m -s -c 3
+
+# Sendfile (zero-copy, bypasses userspace for large files)
+./build/client -f
 ```
 
 ## Testing

src/client/client.c

@@ -29,6 +29,11 @@ int send_chunk(Client *client, Chunk *chunk, Config *config) {
     }
     send_data(client->file_descriptor, data->data, data->size);
     data_destroy(data);
+  } else if (config->use_sendfile && !config->use_compression) {
+    for (int i = 0; i < chunk->element_count; i++) {
+      send_status(client->file_descriptor, STATUS_NEXT);
+      file_send_sendfile(chunk->items[i], client->file_descriptor);
+    }
   } else {
     for (int i = 0; i < chunk->element_count; i++) {
       send_status(client->file_descriptor, STATUS_NEXT);
@@ -130,8 +135,10 @@ int send_files(Config *config) {
   DirectoryScanner *scanner = directory_scanner_create(config->send_directory);
   Chunk *current_chunk;
   while ((current_chunk = directory_scanner_next(scanner)) != NULL) {
+    if (!config->use_sendfile) {
       for (int i = 0; i < current_chunk->element_count; i++)
         file_load_data(current_chunk->items[i]);
+    }
     send_chunk(client, current_chunk, config);
     chunk_destroy(current_chunk);
   }
@@ -145,6 +152,11 @@ int send_files(Config *config) {
 }
 
 int send_files_multithreaded(Config *config) {
+  if (config->use_sendfile) {
+    log_message(LOG_LEVEL_INFO, "Sendfile enabled, falling back to single-threaded");
+    return send_files(config);
+  }
+
   PipelineContextSender *context =
       pipeline_context_sender_create(config, queue_create(100, chunk_destroy),
                                      queue_create(100, chunk_destroy));
@@ -215,6 +227,9 @@ int main(int argc, char *argv[]) {
       config->receive_root_directory = str_dup(argv[++i]);
     } else if (strcmp(argv[i], "--save-to-disk") == 0) {
       config->save_to_disk = true;
+    } else if (strcmp(argv[i], "-f") == 0 || strcmp(argv[i], "--sendfile") == 0) {
+      config->use_sendfile = true;
+      log_message(LOG_LEVEL_INFO, "Enabled sendfile");
     } else {
       handle_arg(argv[i], "-m", &config->use_multithreading,
                  "Enabled Multithreading");

src/shared/config.c

@@ -20,6 +20,7 @@ Config *config_create(char *version, char *send_directory,
   config->use_compression = use_compression;
   config->compression_level = compression_level;
   config->num_connections = num_connections;
+  config->use_sendfile = false;
   return config;
 }
 

src/shared/config.h

@@ -11,6 +11,7 @@ typedef struct Config {
   bool use_multithreading;
   bool use_chunk_serialization;
   bool use_compression;
+  bool use_sendfile;
   bool use_single_send_per_file;
   int compression_level;
   int num_connections;

src/shared/file.c

@@ -1,9 +1,12 @@
 #include <dirent.h>
+#include <fcntl.h>
 #include <libgen.h>
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/sendfile.h>
+#include <unistd.h>
 #include <zstd.h>
 
 #include "data.h"
@@ -68,6 +71,31 @@ void file_send_single_calls(File *file, int file_descriptor) {
   send_data(file_descriptor, file->data->data, file->data->size);
 }
 
+void file_send_sendfile(File *file, int file_descriptor) {
+  send_str(file_descriptor, file->path);
+
+  int fd = open(file->path, O_RDONLY);
+  if (fd == -1) {
+    perror("Could not open file for sendfile");
+    exit(EXIT_FAILURE);
+  }
+
+  unsigned long long file_size = file->stats.st_size;
+  send_n_data(file_descriptor, &file_size, sizeof(unsigned long long));
+
+  off_t offset = 0;
+  while (offset < file_size) {
+    ssize_t sent = sendfile(file_descriptor, fd, &offset, file_size - offset);
+    if (sent == -1) {
+      perror("sendfile failed");
+      close(fd);
+      exit(EXIT_FAILURE);
+    }
+  }
+
+  close(fd);
+}
+
 size_t file_content_to_buffer(File *file) {
   FILE *file_pointer = fopen(file->path, "rb");
   if (file_pointer == NULL) {

src/shared/file.h

@@ -20,6 +20,7 @@ void file_destroy(void *item);
 void file_load_data(File *file);
 void file_print(void *item);
 void file_send_single_calls(File *file, int file_descriptor);
+void file_send_sendfile(File *file, int file_descriptor);
 size_t file_content_to_buffer(File *file);
 
 FileReceive *file_receive_create(char *path, Data *data);

test.py

@@ -48,6 +48,7 @@ TEST_CASES = [
         "name": "Multithreading + Compression + Chunk Serialization (-m -c -s)",
         "flags": ["-m", "-c", "-s"],
     },
+    {"name": "Sendfile (-f)", "flags": ["-f"]},
 ]