0,0 → 1,70 |
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 |
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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"); |
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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); |
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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" |
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Makefile changes |
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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. |