Sunday, 6 October 2019
c - Declaring anonymous struct in for loop, clang fails to compile
Answer
Answer
Code declaring anonymous structs in a for loop worked fine in gcc with -std=c99/gnu99
for (struct {int foo; int bar;} i = {0}; i.foo < 10; i.foo++);
However when I switch to clang instead I got the error:
error: declaration of non-local variable in 'for' loop
Why is this an error? Why would it allow some types (e.g. "int") but not others (e.g. struct {int foo;}) ? This seems inconsistent. Does clang fail to implement c99 correctly or is that code invalid c99 and gcc just happens to support it?
Does anyone know of a way to declare more than one type of variable in a for loop that is supported by clang? (This is useful for macros.)
EDIT:
Since people asked why this is useful I will paste some example code:
#define TREE_EACH(head, node, field, iterator) for ( \
/* initialize */ \
struct { \
node* cur; \
node* stack[((head)->th_root == 0? 0: (head)->th_root->field.avl_height) + 1]; \
uint32_t stack_size; \
} iterator = {.cur = (head)->th_root, .stack_size = 0}; \
/* while */ \
iterator.cur != 0; \
/* iterate */ \
(iterator.stack_size += (iterator.cur->field.avl_right != 0) \
? (iterator.stack[iterator.stack_size] = avl_right, 1) \
: 0), \
(iterator.cur = (iterator.cur->field.avl_left == 0) \
? iterator.cur->field.avl_left \
: (iterator.stack_size > 0? (iterator.stack_size--, iterator.stack[iterator.stack_size]): 0)) \
)
This is a really convenient macro that I wrote which iterates over an AVL tree in depth-first order on the stack. Since declaring anonymous structs in the for loop is not allowed though I have to make the macro less intuitive to use. I could not possible out-source the declaration to the rest of the tree since it uses a variable length array.
Answer
I'm respectfully unconvinced by the previous answers. It builds successfully with gcc (with -Wall -pedantic), only not with clang nor Visual Studio.
Microsoft have acknowledged as a bug an extremely similar issue with Visual Studio at this Microsoft Connect bug item.
6.8.5 is saying that declarations of identifiers inside the for-init-expression cannot be typedef, extern or static (the only storage class specifiers other than auto and register).
The storage class specifier auto in C99 is default and is implicit. The struct type and identifier i are then auto (have scope within that code block). Surely the code is then valid? I don't see how 6.8.5 is forbidding the declaration of a type.
I suggest that gcc is correct, and it's a bug with the implementation by clang and Visual Studio.
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