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.

Figure 17.1: Loading the extension


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.

Figure 17.2: Registering a new function


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.

Figure 17.3: Calling the new function


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’s do_xxx values, reflecting command-line options, like do_lint, do_profiling, and so on (see section API Variables). These are informational: an extension cannot affect their values inside gawk. 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.