Extension Mechanism Outline (The GNU Awk User’s Guide)
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17.3 How It Works at a High Level
Communication between gawk
and an extension is two-way. First, when an extension is loaded, gawk
passes it a pointer to a struct
whose fields are function pointers. This is shown in Figure 17.1.
The extension can call functions inside gawk
through these function pointers, at runtime, without needing (link-time) access to gawk
’s symbols. One of these function pointers is to a function for “registering” new functions. This is shown in Figure 17.2.
In the other direction, the extension registers its new functions with gawk
by passing function pointers to the functions that provide the new feature (do_chdir()
, for example). gawk
associates the function pointer with a name and can then call it, using a defined calling convention. This is shown in Figure 17.3.
The do_xxx()
function, in turn, then uses the function pointers in the API struct
to do its work, such as updating variables or arrays, printing messages, setting ERRNO
, and so on.
Convenience macros make calling through the function pointers look like regular function calls so that extension code is quite readable and understandable.
Although all of this sounds somewhat complicated, the result is that extension code is quite straightforward to write and to read. You can see this in the sample extension filefuncs.c
(see section Example: Some File Functions) and also in the testext.c
code for testing the APIs.
Some other bits and pieces:
- The API provides access to
gawk
’sdo_xxx
values, reflecting command-line options, likedo_lint
,do_profiling
, and so on (see section API Variables). These are informational: an extension cannot affect their values insidegawk
. In addition, attempting to assign to them produces a compile-time error. - The API also provides major and minor version numbers, so that an extension can check if the
gawk
it is loaded with supports the facilities it was compiled with. (Version mismatches “shouldn’t” happen, but we all know how that goes.) See section API Version Constants and Variables for details.
Next: Extension API Description, Previous: Plugin License, Up: Dynamic Extensions [Contents][Index]