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Headline

Graph of paths through a program that may be executed

Citation

(https://en.wikipedia.org/wiki/Control_flow_graph, 12 November 2015)

In a control flow graph each node in the graph represents a basic block, i.e., a straight-line piece of code without any jumps or jump targets; jump targets start a block, and jumps end a block. Directed edges are used to represent jumps in the control flow.

Illustration

Consider the following implementation of Factorial in Language:C 

int factorial(int n) {
  int r = 1;
start:
  if (n<=0) 
    goto end;
  r = r * n;
  n = n - 1;
  goto start;
end:
  return r;
}

The following version assign block ids (A)-(D) to all lines that are gotos or targets thereof. 

int factorial(int n) {
(A)   int r = 1;
    start:
(B)   if (n==0)
        goto end;
(C)   r = r * n;
(C)   n = n - 1;
      goto start;
    end:
(D)   return r;
}

By taking into account jumps and branching due to selection, we derive the following edges:

  • (A) -> (B)
  • (B) -> (D)
  • (B) -> (C)
  • (C) -> (B)
See also the illustration of spaghetti code where a C program is annotated with some edges of a control-flow graph.

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