CommonMistakes.txt

This is an overview of common mistakes and how to resolve them.

Error: every spawn() should be followed by a ssync() statement before returning from the function
Solution: insert appropriate ssync() statements
Signature: correct single-threaded execution but segmentation fault on multi-threaded execution
Example:
1. void f( int n ) {
2.         if( n > 0 )
3.              spawn( f, n-1 );
4.}
Fix: insert ssync() between lines 3 and 4.

Error: there should be a ssync() statement between a spawn() with chandle (return value) and use of the return value
Solution: place ssync() statement appropriately
Signature: correct single-threaded execution but segmentation fault on multi-threaded execution
Example:
1. int f( int n ) {
2.      chandle<int> r;
3.      int rr;
4.      spawn( f, r, n-1 );
5.      rr = r;
6.      ssync();
7.      return rr;
8. }
Fix: interchange lines 5 and 6.

Error: every Swan program should contain a run() call, or use my_main() as the entry point
Solution: introduce either at an appropriate point
Signature: segmentation fault
Example:
1. int main() {
2.     f(10);
3.     return 0;
4.}
Fix: replace line 2 with run(f, 10);

Error: every chandle<> may be used at most once between any two ssync() statements, or before the first ssync() in a procedure body
Solution: introduce multiple chandle<>'s
Signature: erroneous results, sometimes a hang
Example:
1. void f( int n ) {
2.     chandle<int> r;
3.     spawn(f, r, n-1 );
4.     spawn(f, r, n-2 );
5.     ssync();
6.     return r;
}
Fix:
1. void f( int n ) {
2.     chandle<int> r, s;
3.     spawn(f, r, n-1 );
4.     spawn(f, s, n-2 );
5.     ssync();
6.     return s;
}