Difference in array allocation in function call (gcc and cl.exe)

Question

When I use a GCC compiler, i could dynamically allocate the array size in a function using the function input arguments, but the same fails when I compile it using Windows cl.exe compiler. Why.?

void func1(int array_ele)
{
    int arr[array_ele];
    for (int i=0; i<array_ele; i++)
        arr[i] = i;
    /* Rest of the code */
}

int main(int argc, char* argv[])
{
    int a = 10;
    func1(a);
    /* Rest of the code */
}

@arrowd: no, it's because VS doesn't implement C99 (VLAs). – Mat Apr 13 '16 at 05:41 — Mat, Apr 13 '16 at 05:41
I'd go as far as saying that VS does not have a C compiler. – Antti Haapala -- Слава Україні Apr 13 '16 at 07:08 — Antti Haapala -- Слава Україні, Apr 13 '16 at 07:08

Some programmer dude · Accepted Answer · 2016-04-13T05:47:15.740

Variable-length arrays was introduced in the C99 standard, and the Microsoft C compiler haven't supported C99 until recently, and its support is still incomplete even in the latest 2015 edition (for example it still doesn't support VLAs).

GCC before version 5 by default used "gnu90" as C-dialect to use, which is an updated version of the C89 standard with GCC extensions, and one of those extensions is VLAs. From version 5 and later the default is "gnu11" which is the latest C11 standard with GCC extensions.

score 3 · Answer 2 · edited May 23 '17 at 11:50

int arr[array_ele]; is a variable-length array declaration. It is standard C (since C99). However, cl.exe is not a standard-compliant C compiler, and Microsoft has basically stated that they do not intend to produce a standard-compliant C compiler (one could assume malicious intent and say they do not want to help writing portable code), focusing instead on implementing C++ features. More discussion about Visual Studio support for C standards at Visual Studio support for new C / C++ standards?

Some features of the (by now and then obsoleted) C99 are supported on VS2015 (including library support, and variable declarations anywhere within a block etc); and some standard library functions are implemented: C++11/14/17 Features In VS 2015 RTM says that:

Visual Studio 2015 fully implements the C99 Standard Library, with the exception of any library features that depend on compiler features not yet supported by the Visual C++ compiler (for example, <tgmath.h> is not implemented).

However as far as I know, much of the C99 - let alone C11 - compiler features remain unimplemented.

You have noted that *implements the C99 standard library* and *support of the C99 standard* means something different? I guess so. — tofro, Apr 13 '16 at 06:17
@tofro Yeah I italicized part of the quote, however I couldn't even find anything definitive about *what* C99 compiler features are actually supported by VS2015. — Antti Haapala -- Слава Україні, Apr 13 '16 at 06:33

score 2 · Answer 3 · answered Apr 13 '16 at 05:42

2

int arr[array_ele];

is supported in C99 and later. This is called a variable length array. gcc supports it. Microsoft Visual Studio compilers don't seem to support it. See https://msdn.microsoft.com/en-us/library/zb1574zs.aspx.

answered Apr 13 '16 at 05:42

R Sahu

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Difference in array allocation in function call (gcc and cl.exe)

3 Answers3