IREP Reference

IREP - An Intermediate Representation for Structured Input

// C/C++
#include "ir_extern.h"
#include "wkt_TABLENAME.h"

int ir_read(lua_State *L, const char *tbl_elem);
int ir_exists(lua_State *L, const char *t);
int ir_rtlen(lua_State *L, const char *s);
int ir_nprm(int npnr);
int ir_nret(int npnr);
int ir_unread(lua_State *L, const char *ir_tbl)
char *ir_get_stringref(lua_State *L, int n, int *len);
! Fortran
use ir_extern


The IREP is a toolkit and associated C library, to read Lua tables into a compiled data store. The toolkit defines the data store as a set of structured variables in C/C++ and Fortran. The library reads Lua tables, comparing table keys with the corresponding variable in the data store, and puts the table values into the data store.


Step one is to design and define one or more well-known tables for the data store (see below). After creating the well-known tables, you can build the IREP library, and link it to your C/C++ and/or Fortran code - the host code. Your host code reads Lua tables into the data store:

int nerr = ir_read(L, "mytable");

You can access a variable from mytable using native language operations in the host code:

// C
#include "wkt_mytable.h"
int n = mytable.subtable.n;
// C++ (The IREP datastore is in namespace "irep".)
#include "wkt_mytable.h"
int n = irep::mytable.subtable.n;
! Fortran
use wkt_mytable
integer n
n = mytable%subtable%n

You can query the Lua table to discover whether a particular element actually was given in the input:

int e = ir_exists(L, "mytable.subtable.n");
if (e) {
    printf("User input explicitly defines N\n");
} else {
    printf("Using IREP default value for N\n");

If a table element is a vector, you can ask how long the actual input array was:

int nelem = ir_rtlen(L, "mytable.subtable.v1");

If a table element is a string, you can copy it from the data store into a local variable using a macro or function from IREP:

// C
char *ss = strndup(IR_STR(;        // Access scalar string.
char *vs = strndup(IR_STR(t.vs[0]));     // First element of vector.
// C++
std::string my_ss(IR_STR(;    // Access scalar string
std::string my_vs(IR_STR(irep::t.vs[0])); // First element of vector.
! Fortran
character(len=64) :: my_ss, my_vs
my_ss = trim(fstr(t%ss))                 ! Access scalar string.
my_vs = trim(fstr(t%vs(:,1)))                    ! First element of vector.

The remaining functions in the IREP API are briefly described:

int ir_nprm(int npnr); // Return NPRM, given an npnr value.

int ir_nret(int npnr); // Return NRET, given an npnr value.

These two functions are typically used by the host code during evaluation of Lua callback functions (see Lua Callback Functions).

int ir_unread(lua_State *L, const char *ir_tbl)

This function is experimental at present. It is effectively the reverse of ir_read: IREP can read one of the tables in the data store, and create a corresponding table in the lua_State.

char *ir_get_function_name(lua_State *L,void *p);

This function is aimed mainly at error reporting, during callback function evaluation. When the Lua input file is read, IREP stores the full name of each callback function using the address of its associated lua_cb_data structure as a key. The function name can thus be accessed later, typically to produce better error messages.

char *ir_get_stringref(lua_State *L, int n, int *len);

This function handles the case where a Lua string of arbitrary length has been stored using the IREP ir_reference macro. (See ir_get_stringref.) The client code typically calls ir_get_stringref as follows:

if (ir_exists(L, "")) {
   int nn;
   std::string foo = ir_get_stringref(L,,&nn);

The nn parameter returns the length of the string; it can be passed as (char *) NULL if you do not need this value.

Defining the Data Store

The IREP data store is a collection of C/C++ “structs” or their equivalent Fortran “derived type”. The code defining each variable in the data store is written once, using cpp(1) macros that expand into either C/C++ or Fortran source code, depending on the context:

Example Table in the IREP Data Store:

IREP Input           C/C++ Output    Fortran Output
Beg_struct(irt_t)    struct irt_t {  type, bind(c) :: irt_t
  ir_int(a,1)          int a;          integer(c_int) :: a=1
  Vir_dbl(b,2)         double b[2];    real(c_double) :: b(2)
  ir_str(c,8,"foo")    char c[8];      character(c_char) :: c(8)="foo"
  ir_log(d,true)       _Bool d;        logical(c_bool) :: d=.true.
End_struct(irt_t)    } irt_t;        end type irt_t

The rules for constructing the IREP data store are precisely the rules for C and Fortran, constrained by the Fortran ISO_C_BINDING. Structures can be nested in arbitrary fashion. Scalar and one-dimensional array variables (vectors) of type integer, double, boolean, or character (strings) can be defined. In addition, special types are available to define Lua callback0m functions and Lua references.

Summary of IREP Macros


Begin declaration for a table of type T.


End declaration for a table of type T.


Define a table named ID, of type T.

Vir_wkt(T,ID,FB,CB) Define a vector of tables named ID, of type T. The vector has Fortran bounds FB0m and C bounds CB. E.g., if FB=``0:5’’, and CB=``6’’, the vector will have six elements indexed from zero to five. Since Lua normally indexes from 1, the 0th element will presumably be unused. This is useful if you want a particular index value to have the same semantics in all three of C/C++, Fortran, and Lua. Alternatively, if you set FB=``1:6’’, then the first element of the array would have index 0 in C/C++, but index 1 in Fortran and Lua.


Declare a table named ID, with typename T.


Declare a vector of tables ID, typename T. The vector has Fortran bounds FB and C bounds CB. (See Vir_wkt above.)


Declare scalar variable of type double named ID, default value DV.


Declare scalar variable of type integer named ID, default value DV.


Declare scalar variable of type boolean named ID, default value


Declare scalar string named ID, max len LEN, default value


Declare vector variable of type double named ID, with NELEM0m elements, default value DV.


Declare vector variable of type integer named ID, with NELEM elements, default value DV.


Declare vector variable of type boolean named ID, with NELEM elements, default value DV.


Declare vector string named ID, with NELEM elements, max len LEN. Note that string vectors cannot set a default value.


Declare a Lua callback function named ID, with NPRM parameters, returning NRET double precision values.


Declare an IREP variable as a Lua reference. In practice, Lua stores an (integer) reference to the matching element in the Lua table, for later use by the host code. This is effectively an escape mechanism to allow embedding of arbitrary Lua tables, functions, or strings into the well known table. The host code will need to be specialized to handle each ir_reference value. See also ir_get_stringref.


Declare an IREP variable as a C pointer. This will normally require some special treatment by the IREP reader. At the present time, the only use is internal, by the Callback macro.

Doc(( a comment ))

Add an inline comment. This macro must occur at the end of a line containing one of the other IR macros. It can be multi-line and can contain free-form RST.

Reading a Well Known Table

The host code finds and loads (compiles) Lua input using the normal luaL_loadfile, lua_pcall sequence. If the Lua input contains a well-known table named, say, table1, that table can be read into the IREP data store using the call:

int n = ir_read(L, "table1");

This recursively reads the entire table. If table1 itself contains a subtable table2, that table could be separately read as:

int n = ir_read(L, "table1.table2");

That is, ir_read can start at any point in a Lua table, and reads recursively from that element down. The syntax of the second argument is equivalent to the Lua syntax that would reference the same element. Reading just the subtable ignores elements in table1 that are outside of table2. If the environment variable irep_debug is set to a positive integer value, ir_read will produce a listing to stderr of each variable read from the Lua table.

Lua Callback Functions

Lua functions are stored in the IREP data store using a struct:

  ir_int(fref, -1) // -1 == LUA_REFNIL
  ir_int(npnr, -9) // packed nprm,nret

The fref component stores a Lua reference to the (Lua) function. The npnr component packs the NPRM and NRET values into a single integer, to save space. The packing algorithm effectively will allow values for NPRM in the range [-9,1014] and for NRET, the range [-9,2097142]. (But note that the enforced lower bound for NPRM and NRET is currently -1, not -9.) Values for the NPRM parameter have the following meaning:

nprm == -1 // The Lua function accepts a variable number of arguments.
nprm >=  0 // The Lua function accepts exactly NPRM arguments.

Values for the NRET parameter have the following meaning:

nret == -1 // The Lua function will return a table of any length.
nret >=  0 // The Lua function will return exactly NRET values.

This is important to IREP because, for convenience and efficiency, IREP allows an extended set of Lua types to be treated as functions. Some examples may be useful.

Callback(f1,2,1)  // A function with NPRM==2, NRET=1.

For f1, either of the following Lua inputs is legal:

f1 = function(x,y) return math.sin(3) end
f1 = math.sin(3)

The first input is a true Lua function, that happens to return a constant value. The second input is a number, obviously constant, which IREP will also allow as the definition of f1. In the second case, IREP uses the data component of the lua_cb_data structure associated with f1. It allocates space for one double precision number, and stores math.sin(3) at that location. It sets the fref component to LUA_NOREF as an indication that there is no true Lua function to call in this case. Later on, the host code may evaluate f1. If fref==LUA_NOREF, the evaluator can use the stored value. Here is a second example:

Callback(f2,3,3)  // A function with NPRM==3, NRET=3.

For f2, any of the following Lua inputs is legal:

f2 = function(x,y,z) return 3,4,5 end
f2 = { 3,4,5 }
f2 = 3

The first input is a true Lua function, returning 3 values, which also happen to be constant in this example. The second input is Lua table, containing the same values returned by the function. IREP treats the table like it did the number in the first example: It allocates space for the 3 values, and stores them in the data component for later use by the host code. The third input is also legal. The scalar number will be treated as if the input was

f2 = { 3,3,3 }.

That is, broadcast to fill the 3 return slots expected for this function. Finally, the case where NRET == -1:

Callback(f3,3,-1) // A function with NPRM==3, NRET=-1.

In this case, IREP expects the function to return a table (a rank-1 array of double precision numbers) of arbitrary length. A true Lua function can of course calculate a new table each time it is called:

f3 = function(x,y,z) return { x, x+y, x+y+z } end

Note that in this case, the function returns a table, distinct from the list returned by the function in example two above. Or the input can be a (constant) Lua table:

f3 = { 1,2,3,4,5 }

If it is a table, IREP allocates space and stores the table values in the data component, as before. In this case, it also modifies the npnr component to reflect the actual number of elements read from the Lua table.

Return Values

  • ir_read returns the number of errors encountered.

  • ir_exists returns 1 if the element is found in the Lua input, 0 if not.

  • ir_rtlen returns -1 if the given Lua value is not present, 0 if the

    value is a (scalar) number, or whatever lua_objlen returns otherwise. For a string, this is the length of the string. For an array, it is the same as the # operator in Lua.


Variables in the IREP data store are generally static. This is a consequence of current ISO_C_BINDING rules for sharing data between Fortran and This means that the maximum length of arrays (and therefore strings) in the IREP data store is fixed at compile time.


IREP was created by Lee Busby at Lawrence Livermore National Laboratory.

See also

See Overview of IREP for more discussion of some of the code in IREP.