Edit File: tablegen.txt
Writing a table generator This documentation is preliminary. Parts of the API are not good and should be changed. Basic concepts A table generator consists of two files, *_tablegen.c and *_tablegen.h. The .h file will provide the variable declarations and initialization code for the tables, the .c calls the initialization code and then prints the tables as a header file using the tableprint.h helpers. Both of these files will be compiled for the host system, so to avoid breakage with cross-compilation neither of them may include, directly or indirectly, config.h or avconfig.h. This means that e.g. libavutil/mathematics.h is ok but libavutil/libm.h is not. Due to this, the .c file or Makefile may have to provide additional defines or stubs, though if possible this should be avoided. In particular, CONFIG_HARDCODED_TABLES should always be defined to 0. The .c file This file should include the *_tablegen.h and tableprint.h files and anything else it needs as long as it does not depend on config.h or avconfig.h. In addition to that it must contain a main() function which initializes all tables by calling the init functions from the .h file and then prints them. The printing code typically looks like this: write_fileheader(); printf("static const uint8_t my_array[100] = {\n"); write_uint8_t_array(my_array, 100); printf("};\n"); This is the more generic form, in case you need to do something special. Usually you should instead use the short form: write_fileheader(); WRITE_ARRAY("static const", uint8_t, my_array); write_fileheader() adds some minor things like a "this is a generated file" comment and some standard includes. tablegen.h defines some write functions for one- and two-dimensional arrays for standard types - they print only the "core" parts so they are easier to reuse for multi-dimensional arrays so the outermost {} must be printed separately. If there's no standard function for printing the type you need, the WRITE_1D_FUNC_ARGV macro is a very quick way to create one. See libavcodec/dv_tablegen.c for an example. The .h file This file should contain: - one or more initialization functions - the table variable declarations If CONFIG_HARDCODED_TABLES is set, the initialization functions should not do anything, and instead of the variable declarations the generated *_tables.h file should be included. Since that will be generated in the build directory, the path must be included, i.e. #include "libavcodec/example_tables.h" not #include "example_tables.h" Makefile changes To make the automatic table creation work, you must manually declare the new dependency. For this add a line similar to this: $(SUBDIR)example.o: $(SUBDIR)example_tables.h under the "ifdef CONFIG_HARDCODED_TABLES" section in the Makefile.