source: doc/proposals/concurrency/text/cforall.tex @ dcfc4b3

% ======================================================================
% ======================================================================
\chapter{Cforall crash course}
% ======================================================================
% ======================================================================

This thesis presents the design for a set of concurrency features in \CFA. Since it is a new dialect of C, the following is a quick introduction to the language, specifically tailored to the features needed to support concurrency.

\CFA is a extension of ISO-C and therefore supports all of the same paradigms as C. It is a non-object oriented system language, meaning most of the major abstractions have either no runtime overhead or can be opt-out easily. Like C, the basics of \CFA revolve around structures and routines, which are thin abstractions over machine code. The vast majority of the code produced by the \CFA translator respects memory-layouts and calling-conventions laid out by C. Interestingly, while \CFA is not an object-oriented language, lacking the concept of a received (e.g.: this), it does have some notion of objects\footnote{C defines the term objects as : [Where to I get the C11 reference manual?]}, most importantly construction and destruction of objects. Most of the following pieces of code can be found on the \CFA website \cite{www-cfa}

\section{References}

Like \CC, \CFA introduces references as an alternative to pointers. In regards to concurrency, the semantics difference between pointers and references are not particularly relevant but since this document uses mostly references here is a quick overview of the semantics :
\begin{cfacode}
int x, *p1 = &x, **p2 = &p1, ***p3 = &p2,
&r1 = x,    &&r2 = r1,   &&&r3 = r2;
***p3 = 3;                                                      //change x
r3    = 3;                                                      //change x, ***r3
**p3  = ...;                                            //change p1
*p3   = ...;                                            //change p2
int y, z, & ar[3] = {x, y, z};          //initialize array of references
typeof( ar[1]) p;                                       //is int, i.e., the type of referenced object
typeof(&ar[1]) q;                                       //is int &, i.e., the type of reference
sizeof( ar[1]) == sizeof(int);          //is true, i.e., the size of referenced object
sizeof(&ar[1]) == sizeof(int *);        //is true, i.e., the size of a reference
\end{cfacode}
The important thing to take away from this code snippet is that references offer a handle to an object much like pointers but which is automatically derefferenced when convinient.

\section{Overloading}

Another important feature of \CFA is function overloading as in Java and \CC, where routine with the same name are selected based on the numbers and type of the arguments. As well, \CFA uses the return type as part of the selection criteria, as in Ada\cite{Ada}. For routines with multiple parameters and returns, the selection is complex.
\begin{cfacode}
//selection based on type and number of parameters
void f(void);                   //(1)
void f(char);                   //(2)
void f(int, double);    //(3)
f();                                    //select (1)
f('a');                                 //select (2)
f(3, 5.2);                              //select (3)

//selection based on  type and number of returns
char   f(int);                  //(1)
double f(int);                  //(2)
char   c = f(3);                //select (1)
double d = f(4);                //select (2)
\end{cfacode}
This feature is particularly important for concurrency since the runtime system relies on creating different types to represent concurrency objects. Therefore, overloading is necessary to prevent the need for long prefixes and other naming conventions that prevent name clashes. As seen in chapter \ref{basics}, routines main is an example that benefits from overloading.

\section{Operators}
Overloading also extends to operators. The syntax for denoting operator-overloading is to name a routine with the symbol of the operator and question marks where the arguments of the operation would be, like so :
\begin{cfacode}
int ++? (int op);                       //unary prefix increment
int ?++ (int op);                       //unary postfix increment
int ?+? (int op1, int op2);             //binary plus
int ?<=?(int op1, int op2);             //binary less than
int ?=? (int & op1, int op2);           //binary assignment
int ?+=?(int & op1, int op2);           //binary plus-assignment

struct S {int i, j;};
S ?+?(S op1, S op2) {                           //add two structures
        return (S){op1.i + op2.i, op1.j + op2.j};
}
S s1 = {1, 2}, s2 = {2, 3}, s3;
s3 = s1 + s2;                                           //compute sum: s3 == {2, 5}
\end{cfacode}
While concurrency does not use operator overloading directly, this feature is more important as an introduction for the syntax of constructors.

\section{Constructors/Destructors}
Object life-time is often a challenge in concurrency. \CFA uses the approach of giving concurrent meaning to object life-time as a mean of synchronization and/or mutual exclusion. Since \CFA relies heavily on the life time of objects, constructors and destructors are a core feature required for concurrency and parallelism. \CFA uses the following syntax for constructors and destructors :
\begin{cfacode}
struct S {
        size_t size;
        int * ia;
};
void ?{}(S & s, int asize) {    //constructor operator
        s.size = asize;                         //initialize fields
        s.ia = calloc(size, sizeof(S));
}
void ^?{}(S & s) {                              //destructor operator
        free(ia);                                       //de-initialization fields
}
int main() {
        S x = {10}, y = {100};          //implict calls: ?{}(x, 10), ?{}(y, 100)
        ...                                                     //use x and y
        ^x{};  ^y{};                            //explicit calls to de-initialize
        x{20};  y{200};                         //explicit calls to reinitialize
        ...                                                     //reuse x and y
}                                                               //implict calls: ^?{}(y), ^?{}(x)
\end{cfacode}
The language guarantees that every object and all their fields are constructed. Like \CC, construction of an object is automatically done on allocation and destruction of the object is done on deallocation. Allocation and deallocation can occur on the stack or on the heap.
\begin{cfacode}
{
        struct S s = {10};      //allocation, call constructor
        ...
}                                               //deallocation, call destructor
struct S * s = new();   //allocation, call constructor
...
delete(s);                              //deallocation, call destructor
\end{cfacode}
Note that like \CC, \CFA introduces \code{new} and \code{delete}, which behave like \code{malloc} and \code{free} in addition to constructing and destructing objects, after calling \code{malloc} and before calling \code{free} respectively.

For more information on \CFA see \cite{cforall-ug,rob-thesis,www-cfa}.