docs: update existing compatibility status

README.md

@@ -102,6 +102,7 @@ partial, alternate, and planned behavior.
 | `--max-size <n>` | Skip files larger than n bytes |
 | `--min-size <n>` | Skip files smaller than n bytes |
 | `--incremental` | Skip files unchanged since last transfer (size + mtime). Auto-enables `--preserve`. Incompatible with `-s`. |
+| `--existing` | Skip files not already present at the destination; update existing files normally. |
 | `--bwlimit <KB/s>` | Bandwidth limit in kilobytes per second |
 | `--chunk-size <n>` | Chunk size in bytes (default: 10485760) |
 | `--timeout <sec>` | I/O timeout in seconds (default: 30) |
@@ -465,7 +466,7 @@ defaults to the current directory. |
 
 ## Protocol and Security
 
-FastSync protocol version `2.2.0` is shared by the client and server. The
+FastSync protocol version `2.3.0` is shared by the client and server. The
 current protocol is sender-driven and includes configuration negotiation,
 incremental checks, checksums, manifests, keep-alives, abort handling, and
 FastSync-native delta messages. Client and server versions must currently

RSYNC_COMPAT.md

@@ -6,11 +6,11 @@ This document maps rsync's full feature set to FastSync's current implementation
 
 | Status | Count | Description |
 |--------|-------|-------------|
-| ✅ Implemented | 34 | Feature works end-to-end |
+| ✅ Implemented | 35 | Feature works end-to-end |
 | 🔀 Alt Arg | 3 | Functionality exists but under different flag/semantics |
 | ⚠️ Partial | 3 | Flag parsed/stored but behavior incomplete |
-| ❌ Not Implemented | 110 | Flag not recognized or no behavior |
-| **Total** | **150** | |
+| ❌ Not Implemented | 97 | Flag not recognized or no behavior |
+| **Total** | **138** | |
 
 ---
 
@@ -30,7 +30,6 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `--exclude=PATTERN` | Exclude files matching pattern | ✅ Implemented | Glob matching in scanner |
 | `--include=PATTERN` | Include files matching pattern | ✅ Implemented | Glob matching in scanner |
 | `-C`, `--cvs-exclude` | Auto-ignore CVS files | ❌ Not Implemented | Removed because it had no effect |
-| `--no-OPTION` | Turn off an implied option | ❌ Not Implemented | Rsync supports this for options such as `--no-D`; FastSync does not |
 
 ## 2. Modifying Output
 
@@ -44,7 +43,7 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `--out-format=FORMAT` | Custom output format | ❌ Not Implemented | Removed because it had no effect |
 | `--log-file=FILE` | Log to file | ✅ Implemented | `log_file` config field |
 | `--log-file-format=FMT` | Log format | ❌ Not Implemented | |
-| `-8`, `--8-bit-output` | Leave high-bit chars unescaped | ❌ Not Implemented | |
+| `--8-bit-output` | Leave high-bit chars unescaped | ❌ Not Implemented | |
 | `--list-only` | List files instead of copying | ❌ Not Implemented | Removed because it had no effect |
 
 ## 3. File Selection
@@ -53,8 +52,7 @@ This document maps rsync's full feature set to FastSync's current implementation
 |------|-------------------|-----------------|-------|
 | `--exclude-from=FILE` | Read exclude patterns from file | ✅ Implemented | Reads patterns from file |
 | `--include-from=FILE` | Read include patterns from file | ✅ Implemented | Reads patterns from file |
-| `-f`, `--filter=RULE` | Add a file-filtering rule | ❌ Not Implemented | `-f` is FastSync's sendfile flag |
-| `-F` | Add the default `.rsync-filter` rules | ❌ Not Implemented | |
+| `--filter=RULE` | Add file-filtering rule | ❌ Not Implemented | Removed because it had no effect |
 | `--files-from=FILE` | Read source file list from file | ❌ Not Implemented | Removed because it had no effect |
 | `-0`, `--from0` | Delimit *-from files with NULs | ❌ Not Implemented | |
 | `--max-size=SIZE` | Skip files larger than SIZE | ✅ Implemented | `max_size` in scanner |
@@ -62,10 +60,9 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `-I`, `--ignore-times` | Don't skip files matching size+time | ❌ Not Implemented | |
 | `--size-only` | Skip based on size only | ❌ Not Implemented | |
 | `-@`, `--modify-window=NUM` | Mod-time comparison accuracy | ❌ Not Implemented | |
-| `--existing` | Skip creating new files on receiver | ❌ Not Implemented | |
+| `--existing` | Skip creating new files on receiver | ✅ Implemented | Existing destination files continue through normal update handling |
 | `--ignore-existing` | Skip updating existing files | ❌ Not Implemented | |
 | `--remove-source-files` | Sender removes synced files | ❌ Not Implemented | |
-| `-x`, `--one-file-system` | Do not cross filesystem boundaries | ❌ Not Implemented | |
 
 ## 4. Directory Options
 
@@ -75,7 +72,6 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `-R`, `--relative` | Use relative path names | ❌ Not Implemented | Removed because it had no effect |
 | `--no-implied-dirs` | Don't send implied dirs with -R | ❌ Not Implemented | |
 | `-d`, `--dirs` | Transfer dirs without recursing | ❌ Not Implemented | |
-| `--old-dirs`, `--old-d` | Transfer directories without recursing | ❌ Not Implemented | Compatibility aliases for `--dirs` |
 | `--mkpath` | Create missing path components | ❌ Not Implemented | |
 
 ## 5. Transfer Modifications
@@ -87,7 +83,7 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `--append` | Append data to shorter files | ❌ Not Implemented | Removed because it had no effect |
 | `--append-verify` | Append with old-data checksum | ❌ Not Implemented | Removed because it had no effect |
 | `-W`, `--whole-file` | Copy whole file (no delta) | ❌ Not Implemented | |
-| `-B`, `--block-size=SIZE` | Force checksum block-size | ⚠️ Partial | Parsed as `--delta-block`; controls delta transfer block size |
+| `--block-size=SIZE` | Force checksum block-size | ⚠️ Partial | Parsed as `--delta-block`; controls delta transfer block size |
 
 ## 6. Destination Handling
 
@@ -98,14 +94,12 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `--backup-dir=DIR` | Backup directory hierarchy | ✅ Implemented | `backup_dir` config field |
 | `--suffix=SUFFIX` | Backup suffix (default ~) | ✅ Implemented | `suffix` config field |
 | `--delay-updates` | Put updated files in place at end | ❌ Not Implemented | |
-| `-T`, `--temp-dir=DIR` | Create temporary files in DIR | ❌ Not Implemented | `-T` is FastSync's timeout alias |
 
 ## 7. Deletion
 
 | Flag | Rsync Description | FastSync Status | Notes |
 |------|-------------------|-----------------|-------|
 | `--delete` | Delete extraneous files from dest | ✅ Implemented | `use_delete` config field |
-| `--del` | Delete during transfer | ❌ Not Implemented | Alias for `--delete-during` |
 | `--delete-before` | Delete before transfer | ❌ Not Implemented | Removed because it had no effect |
 | `--delete-during` | Delete during transfer | ❌ Not Implemented | |
 | `--delete-delay` | Find deletions during, delete after | ❌ Not Implemented | |
@@ -114,7 +108,7 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `--max-delete=NUM` | Max files to delete | ❌ Not Implemented | Removed because it had no effect |
 | `--ignore-errors` | Delete even with I/O errors | ❌ Not Implemented | |
 | `--force` | Force deletion of non-empty dirs | ❌ Not Implemented | |
-| `-m`, `--prune-empty-dirs` | Prune empty dir chains | ❌ Not Implemented | `-m` enables FastSync multithreading instead |
+| `--prune-empty-dirs` | Prune empty dir chains | ❌ Not Implemented | Removed because it had no effect |
 
 ## 8. Metadata Preservation
 
@@ -141,12 +135,6 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `-J`, `--omit-link-times` | Omit symlinks from --times | ❌ Not Implemented | |
 | `--super` | Receiver attempts super-user activities | ❌ Not Implemented | |
 | `--fake-super` | Store/recover privileged attrs via xattrs | ❌ Not Implemented | |
-| `--open-noatime` | Avoid changing access time when opening files | ❌ Not Implemented | |
-| `--numeric-ids` | Do not map uid/gid by name | ❌ Not Implemented | |
-| `--usermap=STRING` | Map usernames | ❌ Not Implemented | |
-| `--groupmap=STRING` | Map group names | ❌ Not Implemented | |
-| `--chown=USER:GROUP` | Map owner and group | ❌ Not Implemented | |
-| `--copy-as=USER[:GROUP]` | Perform the copy as another user/group | ❌ Not Implemented | |
 
 ## 9. Symlink Handling
 
@@ -172,19 +160,19 @@ This document maps rsync's full feature set to FastSync's current implementation
 | Flag | Rsync Description | FastSync Status | Notes |
 |------|-------------------|-----------------|-------|
 | `--checksum` | Skip based on checksum | ✅ Implemented | With `--incremental`, compares xxHash64 content checksums; `-c` remains compression |
-| `--checksum-choice=STR` (`--cc`) | Choose checksum algorithm | ❌ Not Implemented | xxHash used internally |
+| `--checksum-choice=STR` | Choose checksum algorithm | ❌ Not Implemented | xxHash used internally |
 | `--compare-dest=DIR` | Compare dest files relative to DIR | ❌ Not Implemented | Removed because it had no effect |
 | `--copy-dest=DIR` | Include copies of unchanged files | ❌ Not Implemented | Removed because it had no effect |
 | `--link-dest=DIR` | Hardlink to files when unchanged | ❌ Not Implemented | Removed because it had no effect |
-| `-y`, `--fuzzy`, `--no-fuzzy` | Find similar file for basis | ❌ Not Implemented | `--no-fuzzy` has no short alias |
+| `--fuzzy`, `--no-fuzzy` | Find similar file for basis | ❌ Not Implemented | |
 
 ## 12. Compression
 
 | Flag | Rsync Description | FastSync Status | Notes |
 |------|-------------------|-----------------|-------|
 | `-z`, `--compress` | Compress file data | 🔀 Alt Arg | Always uses zstd (rsync supports multiple algorithms) |
-| `--compress-choice=STR` (`--zc`) | Choose compression algorithm | ❌ Not Implemented | Removed because it had no effect; FastSync always uses zstd |
-| `--compress-level=NUM` (`--zl`) | Set compression level | ✅ Implemented | 1-22, default 5 |
+| `--compress-choice=STR` | Choose compression algorithm | ❌ Not Implemented | Removed because it had no effect; FastSync always uses zstd |
+| `--compress-level=NUM` | Set compression level | ✅ Implemented | 1-22, default 5 |
 | `--compress-threads=NUM` | Set compression threads | ❌ Not Implemented | |
 | `--skip-compress=LIST` | Skip compress for suffixes | ❌ Not Implemented | Internal skip for hardcoded types; not user-configurable |
 
@@ -201,7 +189,6 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `--address=ADDRESS` | Bind address for outgoing socket | ❌ Not Implemented | Removed because it had no effect |
 | `-4`, `--ipv4` | Prefer IPv4 | ❌ Not Implemented | Removed because it had no effect |
 | `-6`, `--ipv6` | Prefer IPv6 | ❌ Not Implemented | Removed because it had no effect |
-| `--remote-option=OPT`, `-M` | Send an option only to the remote side | ❌ Not Implemented | `-M` is FastSync's metadata-preservation flag |
 
 ## 14. Daemon Mode
 
@@ -227,7 +214,6 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `--old-args` | Disable modern arg protection | ❌ Not Implemented | |
 | `--ignore-missing-args` | Ignore missing source args | ❌ Not Implemented | |
 | `--delete-missing-args` | Delete missing source args | ❌ Not Implemented | |
-| `--max-alloc=SIZE` | Limit a single memory allocation | ❌ Not Implemented | |
 
 ## 16. Batch Operations
 
@@ -248,6 +234,7 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `--iconv=CONVERT_SPEC` | Charset conversion | ❌ Not Implemented | |
 | `--checksum-seed=NUM` | Set checksum seed | ❌ Not Implemented | |
 | `-s`, `--secluded-args` | Use protocol to send args | ❌ Not Implemented | |
+| `--no-OPTION` | Turn off implied option | ❌ Not Implemented | |
 
 ---
 
@@ -260,7 +247,7 @@ Ranked by user demand, implementation complexity, and interoperability impact:
 | 1 | `--whole-file` / `-W` | Low | High — users expect opt-out of delta |
 | 2 | `--ignore-times` / `-I` | Low | Medium — useful for forcing re-transfer |
 | 3 | `--size-only` | Low | Medium — common migration scenario |
-| 4 | `--existing` / `--ignore-existing` | Low | Medium — common sync patterns |
+| 4 | `--ignore-existing` | Low | Medium — common sync patterns |
 | 5 | `--remove-source-files` | Low | High — common for moves/backup |
 | 6 | `--delete-during` | Medium | High — performance improvement |
 | 7 | `--delay-updates` | Medium | High — atomic updates |

src/client/client_cli.c

@@ -153,6 +153,7 @@ static const OptionEntry OPTION_TABLE[] = {
     {"--sparse", "-S", OPT_FLAG, offsetof(Config, preserve_sparse)},
     {"--inplace", NULL, OPT_FLAG, offsetof(Config, inplace)},
     {"--checksum", NULL, OPT_FLAG, offsetof(Config, checksum)},
+    {"--existing", NULL, OPT_FLAG, offsetof(Config, existing)},
 
     {"--source-dir", NULL, OPT_STRING, offsetof(Config, send_directory)},
     {"--dest-dir", NULL, OPT_STRING, offsetof(Config, receive_root_directory)},

src/client/usage.c

@@ -28,6 +28,7 @@ void print_usage(void) {
   printf("  --max-size <n>      Skip files larger than n bytes\n");
   printf("  --min-size <n>      Skip files smaller than n bytes\n");
   printf("  --incremental       Skip files unchanged since last transfer\n");
+  printf("  --existing          Skip files not already present at destination\n");
   printf("  --delta             Delta transfer for changed files (requires --incremental)\n");
   printf("  --delta-block <n>   Delta block size in bytes (default: %d)\n",
          DELTA_BLOCK_SIZE_DEFAULT);

src/shared/config.c

@@ -68,6 +68,7 @@ static void config_set_defaults(Config* config) {
   config->debug_level = 0;
   config->list_only = false;
   config->human_readable = false;
+  config->existing = false;
   config->update = false;
   config->inplace = false;
   config->append = false;
@@ -123,12 +124,13 @@ static bool validate_received_config(const Config* config) {
          valid_wire_bool(config->copy_unsafe_links) &&
          valid_wire_bool(config->preserve_hard_links) && valid_wire_bool(config->preserve_acls) &&
          valid_wire_bool(config->preserve_xattrs) && valid_wire_bool(config->preserve_devices) &&
-         valid_wire_bool(config->preserve_sparse) && valid_wire_bool(config->update) &&
-         valid_wire_bool(config->inplace) && valid_wire_bool(config->append) &&
-         valid_wire_bool(config->append_verify) && valid_wire_bool(config->delete_excluded) &&
-         valid_wire_bool(config->delete_after) && valid_wire_bool(config->relative) &&
-         valid_wire_bool(config->prune_empty_dirs) && valid_wire_bool(config->partial) &&
-         valid_wire_bool(config->delete_before) && valid_wire_bool(config->checksum) &&
+         valid_wire_bool(config->preserve_sparse) && valid_wire_bool(config->existing) &&
+         valid_wire_bool(config->update) && valid_wire_bool(config->inplace) &&
+         valid_wire_bool(config->append) && valid_wire_bool(config->append_verify) &&
+         valid_wire_bool(config->delete_excluded) && valid_wire_bool(config->delete_after) &&
+         valid_wire_bool(config->relative) && valid_wire_bool(config->prune_empty_dirs) &&
+         valid_wire_bool(config->partial) && valid_wire_bool(config->delete_before) &&
+         valid_wire_bool(config->checksum) &&
          (!config->use_compression ||
           (config->compression_level >= 1 && config->compression_level <= 22)) &&
          config->chunk_size > 0 && config->chunk_size <= MAX_CHUNK_SIZE &&
@@ -243,10 +245,11 @@ static bool send_file_options(int fd, const Config* c) {
 }
 
 static bool send_selection_options(int fd, const Config* c) {
-  return send_int(fd, c->update) && send_int(fd, c->inplace) && send_int(fd, c->append) &&
-         send_int(fd, c->append_verify) && send_int(fd, c->delete_excluded) &&
-         send_int(fd, c->delete_after) && send_n_data(fd, &c->max_delete, sizeof(c->max_delete)) &&
-         send_int(fd, c->relative) && send_int(fd, c->prune_empty_dirs);
+  return send_int(fd, c->existing) && send_int(fd, c->update) && send_int(fd, c->inplace) &&
+         send_int(fd, c->append) && send_int(fd, c->append_verify) &&
+         send_int(fd, c->delete_excluded) && send_int(fd, c->delete_after) &&
+         send_n_data(fd, &c->max_delete, sizeof(c->max_delete)) && send_int(fd, c->relative) &&
+         send_int(fd, c->prune_empty_dirs);
 }
 
 static bool send_resume_options(int fd, const Config* c) {
@@ -304,7 +307,7 @@ static bool receive_file_options(int fd, Config* c) {
 }
 
 static bool receive_selection_options(int fd, Config* c) {
-  bool* flags[] = {&c->update,        &c->inplace,         &c->append,
+  bool* flags[] = {&c->existing,      &c->update,          &c->inplace,     &c->append,
                    &c->append_verify, &c->delete_excluded, &c->delete_after};
   for (size_t i = 0; i < sizeof(flags) / sizeof(flags[0]); i++) {
     if (!receive_wire_bool(fd, flags[i]))

src/shared/config.h

@@ -76,6 +76,7 @@ typedef struct Config {
   bool human_readable;
 
   // Issue #127: Transfer modes
+  bool existing;
   bool update;
   bool inplace;
   bool append;
@@ -128,7 +129,7 @@ typedef struct Config {
   char* compress_choice;
 } Config;
 
-#define PROTOCOL_VERSION "2.2.0"
+#define PROTOCOL_VERSION "2.3.0"
 #define DEFAULT_CHUNK_SIZE (10 * 1024 * 1024)
 
 Config* config_create(void);

src/shared/file_receive.c

@@ -29,6 +29,7 @@ bool file_save_to_disk(const char* root_directory, const File* file, const Confi
   const char* backup_dir = (config && config->backup_dir) ? config->backup_dir : NULL;
   const char* partial_dir = (config && config->partial_dir) ? config->partial_dir : NULL;
   char *confined_backup = NULL, *confined_partial = NULL, *disk_path = NULL;
+  char* destination_path = NULL;
   char *backup_path = NULL, *parent_copy = NULL;
 
   if (!file || !file->path || !file->data || (file->data->size != 0 && !file->data->data) ||
@@ -54,13 +55,27 @@ bool file_save_to_disk(const char* root_directory, const File* file, const Confi
 
   const char* actual_root =
       (partial_dir && config && config->partial) ? confined_partial : root_directory;
+  destination_path = path_cat(root_directory, file->path);
   disk_path = path_cat(actual_root, file->path);
-  if (disk_path == NULL) {
+  if (destination_path == NULL || disk_path == NULL) {
     free(confined_backup);
     free(confined_partial);
+    free(destination_path);
+    free(disk_path);
     return false;
   }
 
+  /* --existing checks the final destination, not a temporary partial path. */
+  if (config && config->existing && !file_path_exists_secure(destination_path)) {
+    free(confined_backup);
+    free(confined_partial);
+    free(destination_path);
+    free(disk_path);
+    return true;
+  }
+  free(destination_path);
+  destination_path = NULL;
+
   /* --update is receiver-side policy: never replace a newer destination. */
   if (config && config->update) {
     struct stat destination_stat;
@@ -68,6 +83,7 @@ bool file_save_to_disk(const char* root_directory, const File* file, const Confi
         destination_stat.st_mtime > file->metadata->mtime_sec) {
       free(confined_backup);
       free(confined_partial);
+      free(destination_path);
       free(disk_path);
       return true;
     }
@@ -109,6 +125,7 @@ bool file_save_to_disk(const char* root_directory, const File* file, const Confi
   free(backup_path);
   free(confined_backup);
   free(confined_partial);
+  free(destination_path);
   free(disk_path);
   return ok;
 
@@ -117,6 +134,7 @@ fail:
   free(backup_path);
   free(confined_backup);
   free(confined_partial);
+  free(destination_path);
   free(disk_path);
   return false;
 }

tests/integration/test_features.py

@@ -214,6 +214,34 @@ class TestIncremental:
             assert f.read() == b"hello world\n"
 
 
+class TestExisting:
+    def test_existing_updates_existing_and_skips_new(self, shared_server):
+        clean_dir(DEST_DIR)
+        result, _ = run_client(SOURCE_DIR, DEST_DIR, flags=["-M"], port=shared_server.port)
+        assert result.returncode == 0, f"Initial sync failed: {(result.stderr or result.stdout)[:200]}"
+
+        received = get_dest_received_dir(DEST_DIR, SOURCE_DIR)
+        source_file = os.path.join(SOURCE_DIR, "small.txt")
+        new_source_file = os.path.join(SOURCE_DIR, "new-existing-test.txt")
+        with open(source_file, "wb") as f:
+            f.write(b"updated existing content\n")
+        with open(new_source_file, "wb") as f:
+            f.write(b"this file must not be created\n")
+
+        try:
+            result, _ = run_client(SOURCE_DIR, DEST_DIR,
+                                   flags=["-M", "--existing"], port=shared_server.port)
+            assert result.returncode == 0, f"--existing sync failed: {(result.stderr or result.stdout)[:200]}"
+
+            with open(os.path.join(received, "small.txt"), "rb") as f:
+                assert f.read() == b"updated existing content\n"
+            assert not os.path.exists(os.path.join(received, "new-existing-test.txt"))
+        finally:
+            os.unlink(new_source_file)
+            with open(source_file, "wb") as f:
+                f.write(b"hello world\n")
+
+
 class TestDelete:
     def test_delete_removes_extra_files(self, shared_server):
         clean_dir(DEST_DIR)

tests/test_client_cli.c

@@ -339,6 +339,17 @@ static void test_parse_args_archive() {
   config_delete(cfg);
 }
 
+static void test_parse_args_existing() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--existing", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  EXPECT_EQ_INT(parse_args(cfg, 4, argv, positional_args, &positional_count), 0);
+  EXPECT_TRUE(cfg->existing);
+  config_delete(cfg);
+}
+
 void test_client_cli() {
   test_validate_config_required_paths();
   test_validate_config_incompatible_options();
@@ -360,4 +371,5 @@ void test_client_cli() {
   test_parse_args_unknown_option();
   test_parse_args_rejects_unimplemented_options();
   test_parse_args_archive();
+  test_parse_args_existing();
 }

tests/test_config.c

@@ -126,6 +126,7 @@ static void test_config_send_receive() {
   send_cfg->use_metadata = true;
   send_cfg->compression_level = 5;
   send_cfg->chunk_size = 1024;
+  send_cfg->existing = true;
 
   /* Use socketpair for bidirectional communication */
   int p[2];
@@ -160,6 +161,8 @@ static void test_config_send_receive() {
         ok = false;
       if (recv_cfg->chunk_size != 1024)
         ok = false;
+      if (!recv_cfg->existing)
+        ok = false;
     }
     config_delete(recv_cfg);
     close(p[0]);

tests/test_file.c

@@ -88,6 +88,50 @@ static void test_file_save_to_disk() {
   rmdir("test_save_tmp");
 }
 
+static void test_file_save_to_disk_existing() {
+  const char* root = "test_existing_tmp";
+  const char* existing_path = "test_existing_tmp/existing.txt";
+  const char* missing_path = "test_existing_tmp/missing.txt";
+  EXPECT_TRUE(file_write_to_disk(existing_path, "old", 3, false, false));
+
+  Config* cfg = config_create();
+  EXPECT_NOT_NULL(cfg);
+  cfg->existing = true;
+
+  File* existing = file_create("existing.txt");
+  EXPECT_NOT_NULL(existing);
+  existing->data->data = malloc(3);
+  EXPECT_NOT_NULL(existing->data->data);
+  memcpy(existing->data->data, "new", 3);
+  existing->data->size = 3;
+  EXPECT_TRUE(file_save_to_disk(root, existing, cfg));
+  file_destroy(existing);
+
+  File* missing = file_create("missing.txt");
+  EXPECT_NOT_NULL(missing);
+  missing->data->data = malloc(7);
+  EXPECT_NOT_NULL(missing->data->data);
+  memcpy(missing->data->data, "skipped", 7);
+  missing->data->size = 7;
+  EXPECT_TRUE(file_save_to_disk(root, missing, cfg));
+  file_destroy(missing);
+
+  FILE* fp = fopen(existing_path, "rb");
+  char content[4] = {0};
+  EXPECT_NOT_NULL(fp);
+  // cppcheck-suppress knownConditionTrueFalse
+  if (fp) {
+    EXPECT_EQ_INT((int)fread(content, 1, 3, fp), 3);
+    fclose(fp);
+  }
+  EXPECT_EQ_STR(content, "new");
+  EXPECT_EQ_INT(access(missing_path, F_OK), -1);
+
+  config_delete(cfg);
+  unlink(existing_path);
+  rmdir(root);
+}
+
 static void test_file_write_to_disk_basic() {
   const char* content = "Basic file_write_to_disk test";
   EXPECT_TRUE(file_write_to_disk("test_file_write_to_disk_basic.txt", content, strlen(content),
@@ -458,6 +502,7 @@ void test_file() {
   test_file_load_data();
   test_file_load_data_missing_file();
   test_file_save_to_disk();
+  test_file_save_to_disk_existing();
   test_file_write_to_disk_basic();
   test_file_write_to_disk_creates_dirs();
   test_file_write_to_disk_does_not_follow_symlink();