Access to facilities within
gawk is achieved
by calling through function pointers passed into your extension.
API function pointers are provided for the following kinds of operations:
Registration functions. You may register:
All of these are discussed in detail later in this chapter.
ERRNO, or unsetting it.
Some points about using the API:
The following types, macros, and/or functions are referenced
gawkapi.h. For correct use, you must therefore include the
corresponding standard header file before including
The list of macros and related header files is shown in Table 17.1.
|C entity||Header file|
Due to portability concerns, especially to systems that are not
fully standards-compliant, it is your responsibility
to include the correct files in the correct way. This requirement
is necessary in order to keep
gawkapi.h clean, instead of becoming
a portability hodge-podge as can be seen in some parts of
gawk source code.
gawkand/or pass such values to it, you must include the
<mpfr.h>header before including
gawkapi.hfile may be included more than once without ill effect. Doing so, however, is poor coding practice.
inlinekeyword. If your compiler does not support this keyword, you should either place ‘
-Dinline=’ on your command line or use the GNU Autotools and include a
config.hfile in your extensions.
gawkpoint to memory managed by
gawkand should be treated by the extension as read-only. Memory for all strings passed into
gawkfrom the extension must come from calling one of
gawk_realloc(), and is managed by
gawkfrom then on.
The API defines several simple
structs that map values as seen
awk. A value can be a
double, a string, or an
array (as in multidimensional arrays, or when creating a new array).
String values maintain both pointer and length, because embedded NUL characters are allowed.
NOTE: By intent,
gawkmaintains strings using the current multibyte
encoding (as defined by
LC_xxxenvironment variables) and not using wide characters. This matches how
gawkstores strings internally and also how characters are likely to be input into
and output from files.
NOTE: String values passed to an extension by
NUL-terminated. Thus it is safe to pass such string values to standard library and system routines. However, because
gawkallows embedded NUL characters in string data, before using the data as a regular C string, you should check that the length for that string passed to the extension matches the return value of
When retrieving a value (such as a parameter or that of a global variable or array element), the extension requests a specific type (number, string, scalar, value cookie, array, or “undefined”). When the request is “undefined,” the returned value will have the real underlying type.
However, if the request and actual type don’t match, the access function returns “false” and fills in the type of the actual value that is there, so that the extension can, e.g., print an error message (such as “scalar passed where array expected”).
You may call the API functions by using the function pointers
directly, but the interface is not so pretty. To make extension code look
more like regular code, the
gawkapi.h header file defines several
macros that you should use in your code. This section presents
the macros as if they were functions.