GAWK: Effective AWK Programming

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290 GAWK: Effective AWK ProgrammingCaution: This function is new as of gawk 3.1.4.get_scalar_argument(t, i, opt)This is a convenience macro that calls get_actual_argument.Caution: This macro is new as of gawk 3.1.4.get_array_argument(t, i, opt)This is a convenience macro that calls get_actual_argument.Caution: This macro is new as of gawk 3.1.4.void set_value(NODE *tree)This function is called from within a C extension function to set the returnvalue from the extension function. This value is what the awk program sees asthe return value from the new awk function.void update_ERRNO(void)This function is called from within a C extension function to set the value ofgawk’s ERRNO variable, based on the current value of the C errno variable. Itis provided as a convenience.void update_ERRNO_saved(int errno_saved)This function is called from within a C extension function to set the valueof gawk’s ERRNO variable, based on the saved value of the C errno variableprovided as the argument. It is provided as a convenience.Caution: This function is new as of gawk 3.1.5.void register_deferred_variable(const char *name, NODE *(*load_func)(void))This function is called to register a function to be called when a reference to anundefined variable with the given name is encountered. The callback functionwill never be called if the variable exists already, so, unless the calling code isrunning at program startup, it should first check whether a variable of the givenname already exists. The argument function must return a pointer to a NODEcontaining the newly created variable. This function is used to implement thebuiltin ENVIRON and PROCINFO variables, so you can refer to them for examples.Caution: This function is new as of gawk 3.1.5.void register_open_hook(void *(*open_func)(IOBUF *))This function is called to register a function to be called whenever a new datafile is opened, leading to the creation of an IOBUF structure in iop_alloc. Aftercreating the new IOBUF, iop_alloc will call (in reverse order of registration,so the last function registered is called first) each open hook until one returnsnon-NULL. If any hook returns a non-NULL value, that value is assigned tothe IOBUF’s opaque field (which will presumably point to a structure containingadditional state associated with the input processing), and no further openhooks are called.The function called will most likely want to set the IOBUF get_record methodto indicate that future input records should be retrieved by calling that methodinstead of using the standard gawk input processing.
Appendix C: Implementation Notes 291And the function will also probably want to set the IOBUF close_func methodto be called when the file is closed to clean up any state associated with theinput.Finally, hook functions should be prepared to receive an IOBUF structure wherethe fd field is set to INVALID_HANDLE, meaning that gawk was not able to openthe file itself. In this case, the hook function must be able to successfully openthe file and place a valid file descriptor there.Currently, for example, the hook function facility is used to implement the XMLparser shared library extension. For more info, please look in ‘awk.h’ and in‘io.c’.Caution: This function is new as of gawk 3.1.5.An argument that is supposed to be an array needs to be handled with some extra code,in case the array being passed in is actually from a function parameter.In versions of gawk up to and including 3.1.2, the following boilerplate code shows howto do this:NODE *the_arg;the_arg = get_argument(tree, 2); /* assume need 3rd arg, 0-based *//* if a parameter, get it off the stack */if (the_arg->type == Node_param_list)the_arg = stack_ptr[the_arg->param_cnt];/* parameter referenced an array, get it */if (the_arg->type == Node_array_ref)the_arg = the_arg->orig_array;/* check type */if (the_arg->type != Node_var && the_arg->type != Node_var_array)fatal("newfunc: third argument is not an array");/* force it to be an array, if necessary, clear it */the_arg->type = Node_var_array;assoc_clear(the_arg);For versions 3.1.3 and later, the internals changed. In particular, the interface wasactually simplified drastically. The following boilerplate code now suffices:NODE *the_arg;the_arg = get_argument(tree, 2); /* assume need 3rd arg, 0-based *//* force it to be an array: */the_arg = get_array(the_arg);/* if necessary, clear it: */assoc_clear(the_arg);As of version 3.1.4, the internals improved again, and became even simpler:NODE *the_arg;the_arg = get_array_argument(tree, 2, FALSE); /* assume need 3rd arg, 0-based */Again, you should spend time studying the gawk internals; don’t just blindly copy thiscode.
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