58 lines
2.1 KiB
Plaintext
58 lines
2.1 KiB
Plaintext
Writing a table generator
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This documentation is preliminary.
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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.
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The .h file will provide the variable declarations and initialization
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code for the tables, the .c describes the tables so they can be printed
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as a header file.
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Both of these files will be compiled for the host system, so to avoid
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breakage with cross-compilation neither of them may include, directly
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or indirectly, config.h or avconfig.h.
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Due to this, the .c file or Makefile may have to provide additional defines
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or stubs, though if possible this should be avoided.
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The .c file
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This file should include the *_tablegen.h and tableprint.h files and
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anything else it needs as long as it does not depend on config.h or
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avconfig.h.
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In addition to that it must contain a void tableinit(void) function
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which initializes all tables by calling the init functions from the .h
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file.
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It must also contain a "const struct tabledef tables[]" array describing
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the tables to be generated.
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Its entries consist of (in order):
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- a string suitable for declaring the table, up to but not including the =
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NULL terminates the table
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- a function to print the table - tableprint.h defines some defaults,
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e.g. write_uint8_array to print a uint8_t array.
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- a pointer to the table
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- the size of the first dimension of the array
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- if applicable, the size of the second dimension of the array
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The .h file
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This file should contain:
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- one or more initialization functions
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- the table variable declarations
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If CONFIG_HARDCODED_TABLES is set, the initialization functions should
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not do anything, and instead of the variable declarations the
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generated *_tables.h file should be included.
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Since that will be generated in the build directory, the path must be
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included, i.e.
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#include "libavcodec/example_tables.h"
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not
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#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
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new dependency.
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For this add a line similar to this:
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$(SUBDIR)example.o: $(SUBDIR)example_tables.h
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under the "ifdef CONFIG_HARDCODED_TABLES" section in the Makefile.
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