Low-level API for Writing Extensions in GrADS v2.0

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The OpenGrADS User Defined Extensions in GrADS v1.9.0-rc1 was implemented in terms of the low-level GrADS data structures. While such approach provided maximum flexibility in so far as to provided access to all GrADS internals, it was a very fragile and tied to a particular version of GrADS. An internal change in data structure, even if it did not require changes in the extension source code, very often required re-compilation of the extensions. Therefore, this is not a recommended interface for developing new User Defined Extensions.

For each User Defined Function (UDF), the user provides a C function with a standard interface:

#include "grads.h"
int f_hello (struct gafunc *pfc, struct gastat *pst);

where grads.h is the main GrADS include file. The data structures gafunc and gastat are the same structures used to implement GrADS intrinsic functions. In fact, a UDF implemented this way can be easily implemented as a GrADS intrinsic function. In addition, for each UDF, the user provides an entry in a User Defined Extension" (UDX) table which tells GrADS where to look for the UDF.

#  Type  API     Name            Function                 Library
# ------ --- ---------------  ----------------  ---------------------------  
   udf    0    hello             f_hello         ^hello.gex

The Name (hello in this example) is the name of the function to be used in a GrADS expression; Function is the name of the C-function (f_hello in this case) inside the dynamically linked library (DLL) Library. The API version (0 in this example) is used to denote a particular version of the UDF API, allowing for future extensions.

The optional caret (^) in this example is useful to simplify installation: it tells GrADS that the DLL is found at the same directory where User Defined Extension table is located, freeing the user from having to set environment variables such as LD_LIBRARY_PATH.

Example: Hello World

Here is the full source code for the Hello World! extension:

#include <stdio.h>
#include "grads.h"

static counter = 0; /* Illustrates how to keep a state */
/* Sample Hello, World! user defined command */
int c_hello ( int argc, char **argv, struct gacmn *pcm) {
   int i;
   printf("[%d] Hello, GrADS v2.0 World! \n", counter);
   for (i=0; i<argc; i++)
   printf("    argv[%d] = <%s>\n", i, argv[i]);
   return 0;
/* Sample Hello, World! user defined function */
int f_hello (struct gafunc *pfc, struct gastat *pst) {
 int rc;
 printf("[%d] Hello, GrADS v2.0 World!\n", counter);
 rc = gaexpr("0",pst);                    /* return a zero grid  */
 return (rc);