Is type-punning through a union unspecified in C99, and has it become specified in C11?

by

The behavior of type punning with union changed from C89 to C99. The behavior in C99 is the same as C11.

As Wug noted in his answer, type punning is allowed in C99 / C11. An unspecified value that could be a trap is read when the union members are of different size.

The footnote was added in C99 after Clive D.W. Feather Defect Report #257:

Finally, one of the changes from C90 to C99 was to remove any restriction on accessing one member of a union when the last store was to a different one. The rationale was that the behaviour would then depend on the representations of the values. Since this point is often misunderstood, it might well be worth making it clear in the Standard.

[…]

To address the issue about “type punning”, attach a new footnote 78a to the words “named member” in 6.5.2.3#3:
78a If the member used to access the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called “type punning”). This might be a trap representation.

The wording of Clive D.W. Feather was accepted for a Technical Corrigendum in the answer by the C Committee for Defect Report #283.

More Related Contents:

  1. Is there a meaningful distinction between freestanding and hosted implementations?
  2. What is the default C -std standard version for the current GCC (especially on Ubuntu)?
  3. Anonymous union within struct not in c99?
  4. Is int main() { } (without “void”) valid and portable in ISO C?
  5. Is there any option to switch between C99 and C11 C standards in Visual Studio?
  6. Dynamic array allocation on stack in C
  7. Purpose of Unions in C and C++
  8. Passing a multidimensional variable length array to a function
  9. Tentative definitions in C and linking
  10. Enabling VLAs (variable length arrays) in MS Visual C++?
  11. When are anonymous structs and unions useful in C11?
  12. What is the difference between C, C99, ANSI C and GNU C?
  13. A question about union in C – store as one type and read as another – is it implementation defined?
  14. Get warning when a variable is shadowed
  15. Reading a character with scanf_s
  16. Does &((struct name *)NULL -> b) cause undefined behaviour in C11?
  17. Why didn’t gcc (or glibc) implement _s functions?
  18. union ‘punning’ structs w/ “common initial sequence”: Why does C (99+), but not C++, stipulate a ‘visible declaration of the union type’?
  19. Literal string initializer for a character array
  20. Create statically-linked binary that uses getaddrinfo?
  21. union for uint32_t and uint8_t[4] undefined behavior? [duplicate]
  22. Does Microsoft visual studio 2010 support c99?
  23. Operator precedence table for the C programming language
  24. How to wrap printf() into a function or macro?
  25. Does any C library implement C11 threads for GNU/Linux?
  26. Why was mixing declarations and code forbidden up until C99?
  27. What’s the difference between “int” and “int_fast16_t”?
  28. What is wrong with using turbo C?
  29. ftell at a position past 2GB
  30. Questions regarding C++ non-POD unions

Leave a Comment