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ID Category Severity Type Date Submitted Last Update
0000708 [1003.1(2013)/Issue7+TC1] System Interfaces Editorial Enhancement Request 2013-06-07 21:23 2013-06-13 15:35
Reporter dalias View Status public  
Assigned To
Priority normal Resolution Open  
Status New  
Name Rich Felker
Organization musl libc
User Reference
Section XSH 2.9.1 Thread-Safety
Page Number unknown
Line Number unknown
Interp Status ---
Final Accepted Text
Summary 0000708: Make mblen, mbtowc, and wctomb thread-safe for alignment with C11
Description Per C11 7.1.4 paragraph 5,

"Unless explicitly stated otherwise in the detailed descriptions that follow, library functions shall prevent data races as follows: A library function shall not directly or indirectly access objects accessible by threads other than the current thread unless the objects are accessed directly or indirectly via the function's arguments. A library function shall not directly or indirectly modify objects accessible by threads other than the current thread unless the objects are accessed directly or indirectly via the function's non-const arguments. Implementations may share their own internal objects between threads if the objects are not visible to users and are protected against data races."

7.22.7 (Multibyte/wide character conversion functions) does not specify that these functions are not required to avoid data races with other calls. The only time they would even potentially be subject to data races is for state-dependent encodings, which are all but obsolete; for single-byte or modern multi-byte (i.e. UTF-8) encodings, these functions are pure.

Note that 7.29.6.3 (Restartable multibyte/wide character conversion functions) does make exceptions that the "r" versions of these functions are not required to avoid data races when the state argument is NULL.
Desired Action Remove mblen, mbtowc, and wctomb from the list of functions which are not required to be thread-safe.
Tags C11
Attached Files

- Relationships

-  Notes
(0001647)
geoffclare (manager)
2013-06-08 09:12

It seems odd that C11 would have different thread-safety requirements
for mbrlen, mbrtowc, and wcrtomb with a null state argument than for
mblen, mbtowc, and wctomb. We should query this with the C committee,
as it may well be unintentional.
(0001648)
dalias (reporter)
2013-06-08 12:14

I think there's a very good reason for the discrepancy: the restartable versions can store a partially-decoded character in the mbstate_t object, so even for state-independent encodings, there is state which would need to be protected against data races. The non-restartable versions, on the other hand, are pure except in the case of state-dependent encodings, which are mostly a relic of the past and which were never supported on most POSIX systems, since these encodings are mostly incompatible with POSIX filesystem semantics. Only implementations supporting such encodings (which might not even exist - can anyone confirm?) would incur the burden of avoiding data races. Note that these functions give applications access to information on whether the locale's encoding is state-dependent, so a portable application could use the restartable interfaces when the locale is state-dependent, and the non-restartable ones otherwise.

As to the motivation behind my request for this change, I have spent a good deal of time investigating the performance bottlenecks in character-at-a-time multibyte processing, and it turns out that there is a fundamental bottleneck in the restartable interfaces due to their interface requirements for handling the ps argument and partially-decoded characters. For applications which don't need partial-character processing capability, I believe it would make sense to encourage a transition to the non-restartable interfaces, but of course this is problematic if the non-restartable interfaces are not thread-safe. In my experiments, I found the non-restartable interfaces capable of reaching roughly a 50% performance advantage over the restartable ones; this difference would of course become even more extreme if the core decoding algorithms were further optimized.
(0001651)
nick (manager)
2013-06-13 15:35

This will be raised as a potential defect with the C committee, and any decision on how to proceed should be made there first.

- Issue History
Date Modified Username Field Change
2013-06-07 21:23 dalias New Issue
2013-06-07 21:23 dalias Name => Rich Felker
2013-06-07 21:23 dalias Organization => musl libc
2013-06-07 21:23 dalias Section => XSH 2.9.1 Thread-Safety
2013-06-07 21:23 dalias Page Number => unknown
2013-06-07 21:23 dalias Line Number => unknown
2013-06-08 09:12 geoffclare Note Added: 0001647
2013-06-08 09:13 geoffclare Tag Attached: C11
2013-06-08 12:14 dalias Note Added: 0001648
2013-06-13 15:35 nick Note Added: 0001651
2013-12-03 21:09 torvald Issue Monitored: torvald


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