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Manual Reference Pages  -  ICISTMT (1)

NAME

icistmt - ICI statements syntax and semantics

CONTENTS

Description

DESCRIPTION

    The if statement

The if statement has two forms:

        if ( expression ) statement
        if ( expression ) statement else statement

The parser converts both to an internal form. Upon execution, the expression is evaluated. If the expression evaluates to anything other than 0 (integer zero) or NULL, the following statement is executed; otherwise it is not. In the first form this is all that happens, in the second form, if the expression evaluated to 0 or NULL the statement following the else is executed; otherwise it is not.

The interpretation of both 0 and NULL as false, and anything else as true, is common to all logical operations in ici. There is no special boolean type.

The ambiguity introduced by multiple if statements with a lesser number of else clauses is resolved by binding else clauses with their closest possible if. Thus:

if (a) if (b) do_x(); else do_y();

is equivalent to:

if (a)
{
    if (b)
        do_x();
    else
        do_y();
}

The while statement

The while statement has the form:

while ( expression ) statement

The parser converts it to an internal form. Upon execution a loop is established. Within the loop the expression is evaluated, and if it is false (0 or NULL) the loop is terminated and flow of control continues after the while statement. But if the expression evaluates to true (not 0 and not NULL) the statement is executed and then flow of control moves back to the start of the loop where the test is performed again (although other statements, as seen below, can be used to modify this natural flow of control).

The do-while statement

The do-while statement has the following form:

do statement while ( expression ) ;

The parser converts it to an internal form. Upon execution a loop is established. Within the loop the statement is executed. Then the expression is evaluated and if it evaluates to true, flow of control resumes at the start of the loop. Otherwise the loop is terminated and flow of control resumes after the do-while statement.

The for statement

The for statement has the form:

for ( [ expression ]; [ expression ]; [ expression ] ) statement

The parser converts it to an internal form. Upon execution the first expression is evaluated (if present). Then, a loop is established. Within the loop: If the second expression is present, it is evaluated and if it is false the loop is terminated. Next the statement is executed. Finally, the third expression is evaluated (if present) and flow of control resumes at the start of the loop.

    The forall statement

The forall statement has the form:

forall ( expression [ ,expression ] in expression ) statement

The parser converts it to an internal form. In doing so the first and second expressions are required to be lvalues (that is, capable of being assigned to). Upon execution the first expression is evaluated and that storage location is noted. If the second expression is present the same is done for it. The third expression is then evaluated and the result noted; it must evaluate to an array, a set, a struct, a string, or NULL; we will call this the aggregate. If this is NULL, the forall statement is finished and flow of control continues after the statement; otherwise, a loop is established.

Within the loop, an element is selected from the noted aggregate. The value of that element is assigned to the location given by the first expression. If the second expression was present, it is assigned the key used to access that element. Then the statement is executed. Finally, flow of control resumes at the start of the loop.

Each arrival at the start of the loop will select a different element from the aggregate. If no as yet unselected elements are left, the loop terminates. The order of selection is predictable for arrays and strings, namely first to last. But for structs and sets it is unpredictable. Also, while changing the values of the structure members is acceptable, adding or deleting keys, or adding or deleting set elements during the loop will have an unpredictable effect on the progress of the loop.

Note in particular the interpretation of the value and key for a set. For consistency with the access method and the behavior of structs and arrays, the values are all 1 and the elements are regarded as the keys. As a special case, when the second expression is omitted, the first is set to each "key" in turn, that is, the elements of the set.

When a forall loop is applied to a string (which is not a true aggregate), the "sub-elements" will be successive one character sub-strings.

Note that although the sequence of choice of elements from a set or struct is at first examination unpredictable, it will be the same in a second forall loop applied without the structure or set being modified in the interim.

    The switch, case, and default statements

These statements have the form:

        switch ( expression ) compound-statement
        case expression :
        default :

The parser converts the switch statement to an internal form. As it is parsing the compound statement, it notes any case and default statements it finds at the top level of the compound statement. When a case statement is parsed the expression is evaluated immediately by the parser. As noted previously for parser evaluated expressions, it may perform arbitrary actions, but it is important to be aware that it is resolved to a particular value just once by the parser. As the case and default statements are seen their position and the associated expressions are noted in a table.

Upon execution, the switch statement’s expression is evaluated. This value is looked up in the table created by the parser. If a matching case statement is found, flow of control immediately moves to immediately after that case statement. On no match, if there is a default statement flow of control immediately moves to just after that. If there is no matching case and no default statement, flow of control continues just after the entire switch statement.

    The break and continue statements

The break and continue statements have the form:

        break ;
        continue ;

The parser converts these to an internal form. Upon execution of a break statement the execution engine will cause the nearest enclosing loop (a while, do, for or forall) or switch statement within the same scope to terminate. Flow of control will resume immediately after the affected statement. Note that a break statement without a surrounding loop or switch in the same function or module is illegal.

Upon execution of a continue statement the execution engine will cause the nearest enclosing loop to move to the next iteration. For while and do loops this means the test. For for loops it means the step, then the test. For forall loops it means the next element of the aggregate.

    The return statement

The return statement has the form:

return [ expression ] ;

The parser converts this to an internal form. Upon execution, the execution engine evaluates the expression if it is present. If it is not, the value NULL is substituted. Then the current function terminates with that value as its apparent value in any expression it is embedded in. It is an error for there to be no enclosing function.

    The try statement

The try statement has the form:

try statement onerror statement

The parser converts this to an internal form. Upon execution, the first statement is executed. If this statement executes normally flow continues after the try statement; the second statement is ignored. But if an error occurs during the execution of the first statement control is passed immediately to the second statement.

Note that "during the execution" applies to any depth of function calls, even to other modules or the parsing of sub-modules. When an error occurs both the parser and execution engine unwind as necessary until an error catcher (that is, a try statement) is found.

Errors can occur almost anywhere and for a variety of reasons. They can be explicitly generated with the fail function (described below), they can be generated as a side-effect of execution (such as division by zero), and they can be generated by the parser due to syntax or semantic errors in the parsed source. For whatever reason an error is generated, a message (a string) is always associated with it.

When any otherwise uncaught error occurs during the execution of the first statement, two things are done:

Firstly, the string associated with the failure is assigned to the variable error. The assignment is made as if by a simple assignment statement within the scope of the try statement.

Secondly, flow of control is passed to the statement following the onerror keyword.

Once the second statement finishes execution, flow of control continues as if the whole try statement had executed normally.

The handling of errors which are not caught by any try statement is implementation dependent. A typical action is to prepend the file and line number on which the error occurred to the error string, print this, and exit.

    The critsect statement

The critsect, or "critical section" statement has the form:

critsect statement

The parser converts this to an internal form. Upon execution, the statement is executed indivisibly with respect to other threads. Thus:

        critsect x = x + 1;

will increment x by 1, even if another thread is doing similar increments. Without the use of the critsect statement we could encounter a situation where both threads read the current value of x (say 2) at the same time, then both added 1 and stored the result 3, rather than one thread incrementing the value to 3, then the other to 4.

The indivisibility bestowed by a critsect statement applies as long as the code it dominates is executing, including all functions that code calls. Even operations that block (such as the waitfor statement) will be affected. The indivisibility will be revoked once the critsect statement completes, either through completing normally, or through an error being thrown by the code it is dominating.

    The waitfor statement

The waitfor statement has the form:

waitfor ( expression ; expression ) statement

The parser converts this to an internal form. Upon execution, a critical section is established that extends for the entire scope of the waitfor statement (except for the special exception explained below). Within the scope of this critical section, the waitfor statement repeatedly evaluates the first expression until it is true (that is, neither 0 nor NULL). Once the first expression evaluates to true, control passes to the statement (still within the scope of the critical section). After executing statement the critical section is released and the waitfor statement is finished.

However, each time the first expression evalutes to a false value, the second expression is evaluated and the object that it evaluates to is noted. Then, indivisibly, the current thread sleeps waiting for that object to be signaled (by a call to the wakeup() function), and the critical section is suppressed (thus allowing other thread to run). The thread will remain asleep until it is woken up by a call to wakeup() with the given object as an argument. Each time this occurs, the critical section is again enforced and the process repeats with the evaluation and testing of the first expression. While the thread is asleep it consumes no significant CPU time.

    The null statement

The null statement has the form:

;

The parser may convert this to an internal form. Upon execution it will do nothing.

    Declaration statements

There are two types of declaration statements:

declaration
        storage-class declaration-list ;
        storage-class identifier function-body

storage-class extern static auto

declaration-list identifier [ = expression ] declaration-list , identifier [ = expression ]

That is, a comma separated list of identifiers, each with an optional initialisation, terminated by a semicolon.

The storage class keyword establishes which scope the variables in the list are established in. Note that declaring the same identifier at different scope levels is permissible and that they are different variables.

A declaration with no initialisation first checks if the variable already exists at the given scope. If it does, it is left unmodified. In particular, any value it currently has is undisturbed. If it does not exist it is established and is given the value NULL.

A declaration with an initialisation establishes the variable in the given scope and gives it the given value even if it already exists and even if it has some other value.

Note that initial values are parser evaluated expressions. That is they are evaluated immediately by the parser, but may take arbitrary actions apart from that.

    Abbreviated function declarations

As seen above there are two forms of declaration. The second:

storage-class identifier function-body

is the normal way to declare simple functions, and is a shorthand for:

storage-class identifier = [ func function-body ] ;

E.g.:    static sum(a, b) { return a + b; }

is a shorthand for:

   static sum = [func (a, b) { return a + b; }];

SEE ALSO

ici(1), icinet(1), icioo(1), iciops(1), icisyn(1), icitypes(1), iciex(1)
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