| Title | mps_root_create_table is hard to use without punning |
| Status | closed |
| Priority | optional |
| Assigned user | Gareth Rees |
| Organization | Ravenbrook |
| Description | The functions mps_root_create_table and mps_root_create_table_masked are hard to use without type-punning. For example, qs.c contains the lines: die(mps_root_create_table(&actroot, arena, mps_rank_ambig(), 0, (mps_addr_t *)&activationStack, sizeof(QSCell)/sizeof(mps_addr_t)), "RootCreateTable"); (where activationStack has type struct QSCellStruct *) which fails to compile under gcc 4.4.3 [2], with the error: qs.c:354: error: dereferencing type-punned pointer will break strict-aliasing rules qs.c: In function 'scan1': In order to get this to work without puns, you need to write it like this: mps_addr_t base; base = &activationStack; die(mps_root_create_table(&actroot, arena, mps_rank_ambig(), 0, base, 1), "RootCreateTable"); which seems difficult to get right. Also, the manual entry for mps_root_create_table_masked has a buggy example! |
| Analysis | It would be better if the 'base' argument to mps_root_create_table and mps_root_create_table_masked were just an mps_addr_t. Then it would be easy to get right, as any pointer can safely be cast to an mps_addr_t. (If we are concerned to maintain API compatibility, we would have to deprecate these two functions and make two more.) Further evidence in support of this is that MMQA test function/96.c [3] makes the same mistake. But without breaking backward compatibility I think the best thing to do is to put a warning in the manual. |
| How found | customer |
| Evidence | [1] <https://info.ravenbrook.com/mail/2013/05/06/15-36-27/0/>[2] < https://info.ravenbrook.com/mail/2013/05/06/16-50-25/0/>[3] //info.ravenbrook.com/project/mps/master/test/function/96.c |
| Observed in | 1.110.0 |
| Created by | Gareth Rees |
| Created on | 2013-05-06 18:37:51 |
| Last modified by | Gareth Rees |
| Last modified on | 2014-04-04 11:49:19 |
| History | 2013-05-06 GDR Created. |
| Change | Effect | Date | User | Description |
|---|---|---|---|---|
| 185223 | closed | 2014-04-04 11:49:19 | Gareth Rees | Explain how to call mps_root_create_table() safely (avoiding type punning). Fix example for mps_root_create_table_masked(). Use the recommended approach in qs.c. |