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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.

Loading the extension

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.

Registering a new Function

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.

Calling the new function

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 Example: Some File Functions) and also in the testext.c code for testing the APIs.

Some other bits and pieces:

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