blob: 4c4f036e6a76eab3f059c5880e6d6ea9522f3de8
1 | Writing a table generator |
2 | |
3 | This documentation is preliminary. |
4 | Parts of the API are not good and should be changed. |
5 | |
6 | Basic concepts |
7 | |
8 | A table generator consists of two files, *_tablegen.c and *_tablegen.h. |
9 | The .h file will provide the variable declarations and initialization |
10 | code for the tables, the .c calls the initialization code and then prints |
11 | the tables as a header file using the tableprint.h helpers. |
12 | Both of these files will be compiled for the host system, so to avoid |
13 | breakage with cross-compilation neither of them may include, directly |
14 | or indirectly, config.h or avconfig.h. |
15 | This means that e.g. libavutil/mathematics.h is ok but libavutil/libm.h is not. |
16 | Due to this, the .c file or Makefile may have to provide additional defines |
17 | or stubs, though if possible this should be avoided. |
18 | In particular, CONFIG_HARDCODED_TABLES should always be defined to 0. |
19 | |
20 | The .c file |
21 | |
22 | This file should include the *_tablegen.h and tableprint.h files and |
23 | anything else it needs as long as it does not depend on config.h or |
24 | avconfig.h. |
25 | In addition to that it must contain a main() function which initializes |
26 | all tables by calling the init functions from the .h file and then prints |
27 | them. |
28 | The printing code typically looks like this: |
29 | write_fileheader(); |
30 | printf("static const uint8_t my_array[100] = {\n"); |
31 | write_uint8_t_array(my_array, 100); |
32 | printf("};\n"); |
33 | |
34 | This is the more generic form, in case you need to do something special. |
35 | Usually you should instead use the short form: |
36 | write_fileheader(); |
37 | WRITE_ARRAY("static const", uint8_t, my_array); |
38 | |
39 | write_fileheader() adds some minor things like a "this is a generated file" |
40 | comment and some standard includes. |
41 | tablegen.h defines some write functions for one- and two-dimensional arrays |
42 | for standard types - they print only the "core" parts so they are easier |
43 | to reuse for multi-dimensional arrays so the outermost {} must be printed |
44 | separately. |
45 | If there's no standard function for printing the type you need, the |
46 | WRITE_1D_FUNC_ARGV macro is a very quick way to create one. |
47 | See libavcodec/dv_tablegen.c for an example. |
48 | |
49 | |
50 | The .h file |
51 | |
52 | This file should contain: |
53 | - one or more initialization functions |
54 | - the table variable declarations |
55 | If CONFIG_HARDCODED_TABLES is set, the initialization functions should |
56 | not do anything, and instead of the variable declarations the |
57 | generated *_tables.h file should be included. |
58 | Since that will be generated in the build directory, the path must be |
59 | included, i.e. |
60 | #include "libavcodec/example_tables.h" |
61 | not |
62 | #include "example_tables.h" |
63 | |
64 | Makefile changes |
65 | |
66 | To make the automatic table creation work, you must manually declare the |
67 | new dependency. |
68 | For this add a line similar to this: |
69 | $(SUBDIR)example.o: $(SUBDIR)example_tables.h |
70 | under the "ifdef CONFIG_HARDCODED_TABLES" section in the Makefile. |
71 |