OpenMP pragmas can be applied only to loops in OpenMP canonical form. For a loop to be in a canonical form, it must have the following properties:

• It must be a for loop
• The iterator variable is of int, pointer, or iterator type
• The iterator variable remains unmodified throughout the body of the iteration
• The iterator variable changes only within the for clause and the change is iteration invariant
• The limit on the iterator variable is an iteration invariant
• The body of the iteration does not transfer control outside itself

Example of loops not in canonical form:

// Not a for loop
while(*p != 0) {
   p++;
}

// Modifies the iterator variable in the loop body
for (int i = 0; i < n; i++) {
   if (b[i] == 0) {
      i++;
   } else {
      …
   }
}

// Iterator variable changes unpredictably
for (int i = 0; i < n; i+= inc) {
   …
   if (b[i] == 0) {
      inc = 1;
   } else {
      inc = 2;
   }
}

// The limit on the iterator variable changes
for (int i = 0; i < n; i++) {
   …
   if (b[i] == 0) {
      n++;
   }
}

// Transfer of control outside of the loop
for (int i = 0; i < n; i++) {
    if (b[i] ==0) break;
    …
}