mirror of
https://github.com/nyanmisaka/ffmpeg-rockchip.git
synced 2025-10-31 04:26:37 +08:00
71 lines
2.7 KiB
Plaintext
71 lines
2.7 KiB
Plaintext
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.
|