为什么 gcc/clang 处理代码略有不同？ （给出的例子）

Question

So I was messing around with C code and I noticed something about the way that gcc and clang handle code.

If I declare an array in file scope using a variable size, clang compiles with no problem but gcc throws an error. My guess is that it has to do with which compiler flags are/are not enabled by default in gcc/clang, but I would be glad if someone could tell me exactly why this thing is happening and maybe suggest some online resource where I can learn more about this functionality.

Here is an arbitrary example of the code that throws the error -

typedef struct node{
    int data;
    struct node *next;
    struct node *prev;
}node;

const int N = 1000;
node *table[N]; // This is where the error is

int main() {
    return 0;

running clang example.c with no other flags compiles fine running gcc example.c with no other flags throws an error -

example.c:9:7: error: variably modified 'table' at file scope
    9 | node *table[N];
      |       ^~~~

Answer 1

N looks like a constant, but it is not really. The const means « I swear that I won't change the value of this variable » (or the compiler will remind me.); But it does not mean that it could not change by another mean I don't see in this compilation unit. thus this is not exactly a constant.

gcc seems to be very strict with the standard, but using the option -pedantic makes clang emit a warning.

$ clang -o prog_c prog_c.c -pedantic
prog_c.c:12:14: warning: variable length array folded to constant array as an extension [-Wgnu-folding-constant]
double table[N];
             ^
1 warning generated.

$ clang -o prog_c prog_c.c -pedantic -std=c90
prog_c.c:12:13: warning: variable length arrays are a C99 feature [-Wvla-extension]
double table[N];
            ^
prog_c.c:12:8: warning: size of static array must be an integer constant expression [-Wpedantic]
double table[N];
       ^
2 warnings generated.

$ clang -o prog_c prog_c.c -pedantic -std=c99
prog_c.c:12:8: warning: size of static array must be an integer constant expression [-Wpedantic]
double table[N];
       ^
1 warning generated.

Answer 2

Depending on the version of gcc you are using, it defaults to C90 (with some extensions), and variable-length arrays are not a part of C90.

If you specify -std=c99 or -std=c11 , it should support VLAs.

However, per the C language definition, VLAs cannot have static storage duration (as an array declared at file scope does). So clang must be treating N as a constant expression (much like C++ does) and the array is fixed-length, not variable-length. So far I haven't found anything in the clang documentation that would indicate that, though.

Run both compilers with the options -std=c11 -pedantic and see if you don't get the same errors.