爲什麼鏈接libsqlite3.dylib和libsqlcipher.a時不會出現重複符號錯誤？

Question

我正在根據本教程將sqlcipher整合到一個iOS項目中：https://www.zetetic.net/sqlcipher/ios-tutorial/ 有一個小貼士說：如果libsqlite3.dylib或其他SQLite框架列在Link Binary With Libraries列表中，請務必將其刪除，或者你會看到重複的符號錯誤。
但我試圖將libsqlite3.tbd（xcode 7中沒有dylib）和libsqlcipher.a添加到鏈接二進制文件庫，不會出現重複符號錯誤！爲什麼？
我注意到如果我點擊#import <sqlite3.h>行，xcode顯示xcode的lib文件夾中的sqlite3.h文件，而不是sqlcipher中的。 但是，sqlcipher確實有效，數據庫真的是加密的。

Answer 1

libsqlite3.tbd只是一個文本文件，它定義了庫，目標平臺，dylib安裝路徑和導出符號所支持的平臺體系結構。使用.tbd文件，可以減少捆綁的應用程序大小，因爲庫二進制文件已包含在平臺設備中，因此無需將其鏈接到應用程序映像中。

libsqlite3.tbd看起來是這樣的：

--- 
archs:   [ armv7, armv7s, arm64 ] 
platform:  ios 
install-name: /usr/lib/libsqlite3.dylib 
current-version: 216.4 
compatibility-version: 9.0 
exports:   
    - archs:   [ armv7, armv7s, arm64 ] 
    symbols:   [ __sqlite3_lockstate, __sqlite3_purgeEligiblePagerCacheMemory, 
         __sqlite3_system_busy_handler, __sqlite_auto_profile, 
         __sqlite_auto_profile_syslog, __sqlite_auto_trace, 
         __sqlite_auto_trace_syslog, _sqlite3OsShmHasMultipleLinks, 
         _sqlite3OsShmRenamedWhileOpen, _sqlite3OsShmWasTruncated, 
         _sqlite3OsShmWasUnlinkedWhileOpen, _sqlite3VersionNumber, 
         _sqlite3VersionString, _sqlite3_aggregate_context, 
         _sqlite3_aggregate_count, _sqlite3_auto_extension, 
         _sqlite3_backup_finish, _sqlite3_backup_init, _sqlite3_backup_pagecount, 
         _sqlite3_backup_remaining, _sqlite3_backup_step, 
         _sqlite3_bind_blob, _sqlite3_bind_blob64, _sqlite3_bind_double, 
         _sqlite3_bind_int, _sqlite3_bind_int64, _sqlite3_bind_null, 
         _sqlite3_bind_parameter_count, _sqlite3_bind_parameter_index, 
         _sqlite3_bind_parameter_name, _sqlite3_bind_text, 
         _sqlite3_bind_text16, _sqlite3_bind_text64, _sqlite3_bind_value, 
         _sqlite3_bind_zeroblob, _sqlite3_blob_bytes, _sqlite3_blob_close, 
         _sqlite3_blob_open, _sqlite3_blob_read, _sqlite3_blob_reopen, 
         _sqlite3_blob_write, _sqlite3_busy_handler, _sqlite3_busy_timeout, 
         _sqlite3_cancel_auto_extension, _sqlite3_changes, 
         _sqlite3_clear_bindings, _sqlite3_close, _sqlite3_close_v2, 
         _sqlite3_collation_needed, _sqlite3_collation_needed16, 
         _sqlite3_column_blob, _sqlite3_column_bytes, _sqlite3_column_bytes16, 
         _sqlite3_column_count, _sqlite3_column_decltype, 
         _sqlite3_column_decltype16, _sqlite3_column_double, 
         _sqlite3_column_int, _sqlite3_column_int64, _sqlite3_column_name, 
         _sqlite3_column_name16, _sqlite3_column_text, _sqlite3_column_text16, 
         _sqlite3_column_type, _sqlite3_column_value, _sqlite3_commit_hook, 
         _sqlite3_compileoption_get, _sqlite3_compileoption_used, 
         _sqlite3_complete, _sqlite3_complete16, _sqlite3_config, 
         _sqlite3_context_db_handle, _sqlite3_create_collation, 
         _sqlite3_create_collation16, _sqlite3_create_collation_v2, 
         _sqlite3_create_function, _sqlite3_create_function16, 
         _sqlite3_create_function_v2, _sqlite3_create_module, 
         _sqlite3_create_module_v2, _sqlite3_data_count, 
         _sqlite3_data_directory, _sqlite3_db_config, _sqlite3_db_filename, 
         _sqlite3_db_handle, _sqlite3_db_mutex, _sqlite3_db_readonly, 
         _sqlite3_db_release_memory, _sqlite3_db_status, 
         _sqlite3_declare_vtab, _sqlite3_enable_shared_cache, 
         _sqlite3_errcode, _sqlite3_errmsg, _sqlite3_errmsg16, 
         _sqlite3_errstr, _sqlite3_exec, _sqlite3_expired, 
         _sqlite3_extended_errcode, _sqlite3_extended_result_codes, 
         _sqlite3_file_control, _sqlite3_finalize, _sqlite3_free, 
         _sqlite3_free_table, _sqlite3_get_autocommit, _sqlite3_get_auxdata, 
         _sqlite3_get_table, _sqlite3_global_recover, _sqlite3_initialize, 
         _sqlite3_intarray_bind, _sqlite3_intarray_create, 
         _sqlite3_interrupt, _sqlite3_last_insert_rowid, 
         _sqlite3_libversion, _sqlite3_libversion_number, 
         _sqlite3_limit, _sqlite3_log, _sqlite3_malloc, _sqlite3_malloc64, 
         _sqlite3_memory_alarm, _sqlite3_memory_highwater, 
         _sqlite3_memory_used, _sqlite3_mprintf, _sqlite3_msize, 
         _sqlite3_mutex_alloc, _sqlite3_mutex_enter, _sqlite3_mutex_free, 
         _sqlite3_mutex_leave, _sqlite3_mutex_try, _sqlite3_next_stmt, 
         _sqlite3_open, _sqlite3_open16, _sqlite3_open_v2, 
         _sqlite3_os_end, _sqlite3_os_init, _sqlite3_overload_function, 
         _sqlite3_prepare, _sqlite3_prepare16, _sqlite3_prepare16_v2, 
         _sqlite3_prepare_v2, _sqlite3_profile, _sqlite3_progress_handler, 
         _sqlite3_randomness, _sqlite3_realloc, _sqlite3_realloc64, 
         _sqlite3_release_memory, _sqlite3_reset, _sqlite3_reset_auto_extension, 
         _sqlite3_result_blob, _sqlite3_result_blob64, _sqlite3_result_double, 
         _sqlite3_result_error, _sqlite3_result_error16, 
         _sqlite3_result_error_code, _sqlite3_result_error_nomem, 
         _sqlite3_result_error_toobig, _sqlite3_result_int, 
         _sqlite3_result_int64, _sqlite3_result_null, _sqlite3_result_text, 
         _sqlite3_result_text16, _sqlite3_result_text16be, 
         _sqlite3_result_text16le, _sqlite3_result_text64, 
         _sqlite3_result_value, _sqlite3_result_zeroblob, 
         _sqlite3_rollback_hook, _sqlite3_rtree_geometry_callback, 
         _sqlite3_rtree_query_callback, _sqlite3_set_authorizer, 
         _sqlite3_set_auxdata, _sqlite3_shutdown, _sqlite3_sleep, 
         _sqlite3_snprintf, _sqlite3_soft_heap_limit, _sqlite3_soft_heap_limit64, 
         _sqlite3_sourceid, _sqlite3_sql, _sqlite3_status, 
         _sqlite3_status64, _sqlite3_step, _sqlite3_stmt_busy, 
         _sqlite3_stmt_readonly, _sqlite3_stmt_status, _sqlite3_strglob, 
         _sqlite3_stricmp, _sqlite3_strnicmp, _sqlite3_table_column_metadata, 
         _sqlite3_temp_directory, _sqlite3_test_control, 
         _sqlite3_thread_cleanup, _sqlite3_threadsafe, _sqlite3_total_changes, 
         _sqlite3_trace, _sqlite3_transfer_bindings, _sqlite3_update_hook, 
         _sqlite3_uri_boolean, _sqlite3_uri_int64, _sqlite3_uri_parameter, 
         _sqlite3_user_data, _sqlite3_value_blob, _sqlite3_value_bytes, 
         _sqlite3_value_bytes16, _sqlite3_value_double, _sqlite3_value_int, 
         _sqlite3_value_int64, _sqlite3_value_numeric_type, 
         _sqlite3_value_text, _sqlite3_value_text16, _sqlite3_value_text16be, 
         _sqlite3_value_text16le, _sqlite3_value_type, _sqlite3_version, 
         _sqlite3_vfs_find, _sqlite3_vfs_register, _sqlite3_vfs_unregister, 
         _sqlite3_vmprintf, _sqlite3_vsnprintf, _sqlite3_vtab_config, 
         _sqlite3_vtab_on_conflict, _sqlite3_wal_autocheckpoint, 
         _sqlite3_wal_checkpoint, _sqlite3_wal_checkpoint_v2, 
         _sqlite3_wal_hook ] 
... 

您不會收到重複的符號鏈接錯誤，因爲你只包括一個單一的二進制庫，libsqlcipher.a。如果您希望在iOS應用程序中使用SQLCipher，則需要刪除libsqlite3.tbd參考，幷包含libsqlcipher.a。