Question 409715 in Debian Question posted in
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Debian – C braced initializer for -non- array

Please consider the following absurd but compiling code:

int main(int argc, char *argv[]) {
    void *a, *b, *c, *d;

    void *the_good_array[] = { a, b, c, d };
    void *the_bad_array    = { a, b, c, d };

    return 0;
}

I accidentally produced something similar and, to my astonishment, it fully compiles.

  • Why does it compile?
  • What is { a, b, c, d } supposed to mean as an initializer for a non array? Without the braces, I would understand it’s an accidental use of the , operator resulting in the value of the last element (d). But, braces? What are they supposed to mean here?

If possible, please refer to the C standard when answering.

Edit 1:
Compiles with both:

gcc -std=iso9899:1990 /tmp/bad_array.c     # gcc (Debian 12.2.0-14)`
clang -std=iso9899:1990 /tmp/bad_array.c   # Debian clang version 14.0.6

Edit 2:
Apparently

int i = { 0 };

is perfectly valid C.

I would like to know why but don’t have the standard to read it.

Tags:

2

Answers


  1. 0

    What is { a, b, c, d } supposed to mean as an initializer for a non array?

    Having multiple intializer elements for single object is a constraint violation. N2176 (C17 standard draft) §6.7.9 Initialization:

    Constraints

    1. No initializer shall attempt to provide a value for an object not contained within the entity being
      initialized.

    In this case it’s depends on compiler what happens.

    Why does it compile?

    C compilers take backwards compatibility quite seriosly, and will accept a lot of code that would be considered invalid nowadays.

    But, braces? What are they supposed to mean here?

    Braces are allowed allowed even around single expression:

    Syntax

      • initializer:
        • assignment-expression
        • { initializer-list }
        • { initializer-list , }
      • initializer-list:
        • designationopt initializer
        • initializer-list , designationopt initializer
          …

    With rules above, initializer can after a few steps be expanded to { assignment-expression }.

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  2. 0

    Any initializer may have braces.
    You can just omit them. From https://port70.net/~nsz/c/c11/n1570.html#6.7.9p11 :

    The initializer for a scalar shall be a single expression, optionally enclosed in braces.

    Example:

    int i = {1};

    The compiler informs you that:

    <source>:5:35: warning: excess elements in scalar initializer [-Wexcess-initializers]
    5 |     void *the_bad_array    = { a, b, c, d };

    Looks like the compiler chooses to ignore the "excess", taking only a.

    Why does it compile?

    The short answer: the compiler was created by people in the way that compiles it. The GCC compiler writers chose to handle excess elements in scalar initializer in this particular way.

    The language-lawyer answer: the code is invalid. It breaks the "shall" from the link above:

    The initializer for a scalar shall be a single expression

    From https://port70.net/~nsz/c/c11/n1570.html#4p2 we know that:

    If a ”shall” or ”shall not” requirement that appears outside of a constraint or runtime- constraint is violated, the behavior is undefined.

    The behavior is not defined, which means that anything can happen. Compiling is "anything".

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