Extension Other Design Decisions (The GNU Awk User’s Guide)

From Get docs

C.5.3 Other Design Decisions

As an arbitrary design decision, extensions can read the values of predefined variables and arrays (such as ARGV and FS), but cannot change them, with the exception of PROCINFO.

The reason for this is to prevent an extension function from affecting the flow of an awk program outside its control. While a real awk function can do what it likes, that is at the discretion of the programmer. An extension function should provide a service or make a C API available for use within awk, and not mess with FS or ARGC and ARGV.

In addition, it becomes easy to start down a slippery slope. How much access to gawk facilities do extensions need? Do they need getline? What about calling gsub() or compiling regular expressions? What about calling into awk functions? (That would be messy.)

In order to avoid these issues, the gawk developers chose to start with the simplest, most basic features that are still truly useful.

Another decision is that although gawk provides nice things like MPFR, and arrays indexed internally by integers, these features are not being brought out to the API in order to keep things simple and close to traditional awk semantics. (In fact, arrays indexed internally by integers are so transparent that they aren’t even documented!)

Additionally, all functions in the API check that their pointer input parameters are not NULL. If they are, they return an error. (It is a good idea for extension code to verify that pointers received from gawk are not NULL. Such a thing should not happen, but the gawk developers are only human, and they have been known to occasionally make mistakes.)

With time, the API will undoubtedly evolve; the gawk developers expect this to be driven by user needs. For now, the current API seems to provide a minimal yet powerful set of features for creating extensions.