Why aren t variable-length arrays part of the C standard

Question

I haven t used C very much in the last few years  When I read this question today I came across some C syntax which I wasn t familiar with   Apparently in C99 the following syntax is valid   void foo int n        int values n     Declare a variable length array     This seems like a pretty useful feature  Was there ever a discussion about adding it to the C   standard  and if so  why it was omitted   Some potential reasons    Hairy for compiler vendors to implement Incompatible with some other part of the standard Functionality can be emulated with other C   constructs   The C   standard states that array size must be a constant expression  8 3 4 1    Yes  of course I realize that in the toy example one could use std  vector lt int gt  values m    but this allocates memory from the heap and not the stack  And if I want a multidimensional array like   void foo int x  int y  int z        int values x  y  z      Declare a variable length array     the vector version becomes pretty clumsy   void foo int x  int y  int z        vector lt  vector lt  vector lt int gt   gt   gt  values     Really painful expression here           The slices  rows and columns will also potentially be spread all over memory   Looking at the discussion at comp std c   it s clear that this question is pretty controversial with some very heavyweight names on both sides of the argument  It s certainly not obvious that a std  vector is always a better solution

User · Accepted Answer

There recently was a discussion about this kicked off in usenet: Why no VLAs in C++0x.

I agree with those people that seem to agree that having to create a potential large array on the stack, which usually has only little space available, isn't good. The argument is, if you know the size beforehand, you can use a static array. And if you don't know the size beforehand, you will write unsafe code.

C99 VLAs could provide a small benefit of being able to create small arrays without wasting space or calling constructors for unused elements, but they will introduce rather large changes to the type system (you need to be able to specify types depending on runtime values - this does not yet exist in current C++, except for new operator type-specifiers, but they are treated specially, so that the runtime-ness doesn't escape the scope of the new operator).

You can use std::vector, but it is not quite the same, as it uses dynamic memory, and making it use one's own stack-allocator isn't exactly easy (alignment is an issue, too). It also doesn't solve the same problem, because a vector is a resizable container, whereas VLAs are fixed-size. The C++ Dynamic Array proposal is intended to introduce a library based solution, as alternative to a language based VLA. However, it's not going to be part of C++0x, as far as I know.

User · Answer

Background  I have some experience implementing C and C   compilers    Variable-length arrays in C99 were basically a misstep  In order to support VLAs  C99 had to make the following concessions to common sense    sizeof x is no longer always a compile-time constant  the compiler must sometimes generate code to evaluate a sizeof-expression at runtime  Allowing two-dimensional VLAs  int A x  y   required a new syntax for declaring functions that take 2D VLAs as parameters  void foo int n  int A        Less importantly in the C   world  but extremely important for C s target audience of embedded-systems programmers  declaring a VLA means chomping an arbitrarily large chunk of your stack  This is a guaranteed stack-overflow and crash   Anytime you declare int A n   you re implicitly asserting that you have 2GB of stack to spare  After all  if you know  n is definitely less than 1000 here   then you would just declare int A 1000   Substituting the 32-bit integer n for 1000 is an admission that you have no idea what the behavior of your program ought to be     Okay  so let s move to talking about C   now  In C    we have the same strong distinction between  type system  and  value system  that C89 does    but we ve really started to rely on it in ways that C has not  For example   template lt typename T gt  struct S          int A n   S lt decltype A  gt  s      equivalently  S lt int n  gt  s    If n weren t a compile-time constant  i e   if A were of variably modified type   then what on earth would be the type of S  Would S s type also be determined only at runtime   What about this   template lt typename T gt  bool myfunc T amp  t1  T amp  t2           int A1 n1   A2 n2   myfunc A1  A2     The compiler must generate code for some instantiation of myfunc  What should that code look like  How can we statically generate that code  if we don t know the type of A1 at compile time   Worse  what if it turns out at runtime that n1    n2  so that  std  is same lt decltype A1   decltype A2  gt     In that case  the call to myfunc shouldn t even compile  because template type deduction should fail  How could we possibly emulate that behavior at runtime   Basically  C   is moving in the direction of pushing more and more decisions into compile-time  template code generation  constexpr function evaluation  and so on  Meanwhile  C99 was busy pushing traditionally compile-time decisions  e g  sizeof  into the runtime  With this in mind  does it really even make sense to expend any effort trying to integrate C99-style VLAs into C     As every other answerer has already pointed out  C   provides lots of heap-allocation mechanisms  std  unique ptr lt int   gt  A   new int n   or std  vector lt int gt  A n   being the obvious ones  when you really want to convey the idea  I have no idea how much RAM I might need   And C   provides a nifty exception-handling model for dealing with the inevitable situation that the amount of RAM you need is greater than the amount of RAM you have  But hopefully this answer gives you a good idea of why C99-style VLAs were not a good fit for C       and not really even a good fit for C99        For more on the topic  see N3810  Alternatives for Array Extensions   Bjarne Stroustrup s October 2013 paper on VLAs  Bjarne s POV is very different from mine  N3810 focuses more on finding a good C  ish syntax for the things  and on discouraging the use of raw arrays in C    whereas I focused more on the implications for metaprogramming and the typesystem  I don t know if he considers the metaprogramming typesystem implications solved  solvable  or merely uninteresting     A good blog post that hits many of these same points is  Legitimate Use of Variable Length Arrays   Chris Wellons  2019-10-27

User · Answer

Use std  vector for this  For example   std  vector lt int gt  values  values resize n     The memory will be allocated on the heap  but this holds only a small performance drawback  Furthermore  it is wise not to allocate large datablocks on the stack  as it is rather limited in size

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C99 allows VLA  And it puts some restrictions on how to declare VLA  For details  refer to 6 7 5 2 of the standard  C   disallows VLA  But g   allows it

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In my own work  I ve realized that every time I ve wanted something like variable-length automatic arrays or alloca    I didn t really care that the memory was physically located on the cpu stack  just that it came from some stack allocator that didn t incur slow trips to the general heap   So I have a per-thread object that owns some memory from which it can push pop variable sized buffers   On some platforms I allow this to grow via mmu   Other platforms have a fixed size  usually accompanied by a fixed size cpu stack as well because no mmu    One platform I work with  a handheld game console  has precious little cpu stack anyway because it resides in scarce  fast memory   I m not saying that pushing variable-sized buffers onto the cpu stack is never needed   Honestly I was surprised back when I discovered this wasn t standard  as it certainly seems like the concept fits into the language well enough   For me though  the requirements  variable size  and  must be physically located on the cpu stack  have never come up together   It s been about speed  so I made my own sort of  parallel stack for data buffers

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There are situations where allocating heap memory is very expensive compared to the operations performed  An example is matrix math  If you work with smallish matrices say 5 to 10 elements  and do a lot of arithmetics the malloc overhead will be really significant  At the same time making the size a compile time constant does seem very wasteful and inflexible   I think that C   is so unsafe in itself that the argument to  try to not add more unsafe features  is not very strong  On the other hand  as C   is arguably the most runtime efficient programming language features which makes it more so are always useful  People who write performance critical programs will to a large extent use C    and they need as much performance as possible  Moving stuff from heap to stack is one such possibility  Reducing the number of heap blocks is another  Allowing VLAs as object members would one way to achieve this  I m working on such a suggestion  It is a bit complicated to implement  admittedly  but it seems quite doable

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Seems it will be available in C  14  https   en wikipedia org wiki C 2B 2B14 Runtime-sized one dimensional arrays Update  It did not make it into C  14

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This was considered for inclusion in C   1x  but was dropped  this is a correction to what I said earlier    It would be less useful in C   anyway since we already have std  vector to fill this role

User · Answer

I have a solution that actually worked for me   I did not want to allocate memory because of fragmentation on a routine that needed to run many times   The answer is extremely dangerous  so use it at your own risk  but it takes advantage of assembly to reserve space on the stack   My example below uses a character array  obviously other sized variable would require more memory    void varTest int iSz        char  varArray        asm           sub esp  iSz          Create space on the stack for the variable array here         mov varArray  esp     save the end of it to our pointer               Use the array called varArray here             asm           add esp  iSz          Variable array is no longer accessible after this point            The dangers here are many but I ll explain a few  1  Changing the variable size half way through would kill the stack position 2  Overstepping the array bounds would destroy other variables and possible code 3  This does not work in a 64 bit build    need different assembly for that one  but a macro might solve that problem   4  Compiler specific  may have trouble moving between compilers    I haven t tried so I really don t know

User · Answer

You could always use alloca   to allocate memory on the stack at runtime  if you wished   void foo  int n        int  values    int   alloca sizeof int    n       Being allocated on the stack implies that it will automatically be freed when the stack unwinds   Quick note  As mentioned in the Mac OS X man page for alloca 3    The alloca   function is machine and compiler dependent  its use is dis-couraged    Just so you know

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Arrays like this are part of C99  but not part of standard C    as others have said  a vector is always a much better solution  which is probably why variable sized arrays are not in the C   standatrd  or in the proposed C  0x standard    BTW  for questions on   why  the C   standard is the way it is  the moderated Usenet newsgroup comp std c   is the place to go to

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If you know the value at compile time you can do the following   template  lt int X gt  void foo void       int values X        Edit  You can create an a vector that uses a stack allocator  alloca   since the allocator is a template parameter

[c++] Why aren't variable-length arrays part of the C++ standard?

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