| Writing a table generator |
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| This documentation is preliminary. |
| Parts of the API are not good and should be changed. |
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| Basic concepts |
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| 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. |
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| 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. |
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| The .h file |
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| 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. |