NAME

eval - evaluate simple arithmetic expressions

SYNOPSIS

#include <mba/eval.h>

struct eval *eval_new(symlook_fn symlook, void *context);
 
void eval_del(void *eval);
 
int eval_expression(struct eval *eval, const tchar *expr, const tchar *elim, unsigned long *result);

DESCRIPTION

The eval(3m) module will evaluate simple arithmentic expressions consisting of integers, symbols for which the provided symlook_fn returns an integer, and any of the operators |&^+-*/().OperatorPrecedence

Operator precedence is roughly the same as the C language.


 ( )  higher
 * /
 + -
^ & | lower

Prefixing integer tokens with minus '-' to indicate a negative value is currently not supported.

new: The eval_new function creates and returns a new context object for evaluating expressions. The symlook parameter is defined as follows:


typedef int (*symlook_fn)(const tchar *name, unsigned long *val, void *context);

The symlook_fn function will be called to resolve any non-numeric symbols and should place the value identified by name into val and return 0. If the symbol is not found -1 should be returned.

The eval_new function can be used repeatedly to evaluate any number of expressions before being destroyed with the eval_del function. The context parameter is passed uninterpreted to the symlook_fn (e.g. a map perhaps).

del: The eval_del function deletes the context object eval freeing any memory allocated with eval_new or during the evaluation of expressions.

eval_expression: The eval_expression function evaluates an infix expression like '(5 + 3) * N', converts it into a stack of tokens in postfix orientation, and reduces it with a simple translation matrix to produce a single integer value. The eval parameter is a context object created with eval_new. The expression at expr is evaluated up to, but not including, the memory at elim, and writes the resulting value in the memory at result.

RETURNS

eval_expression: The eval_expression function returns 0 if the expression was successfully reduced or -1 if an error occured in which case errno will be set appropriately (e.g. ENOENT if the symlook_fn could not resolve a symbol).