如何指定不应该在C中编译的{} init来gcc？

Question

with gcc using -std=gnu99 , the following code compiles:

void f()
{
  struct X data = {};
  // do something with data
}

Is this valid C ? Is this a gnu extension ?

How can I tell gcc to not accept this kind of init ?

I want to ensure compatibility with other compilers (like visual 2015 for example)

Answer 1

No, the empty initializer is not standard C. It is a gcc extension. See this for a detailed description.

By specifying -std=gnu99 , you allowed the GNU extensions to be used. You can force the compiler to only allow the standard conforming code, by specifying -std=cXX option.

From the gcc online manual ( emphasis mine )

-std=

The compiler can accept several base standards, such as 'c90' or 'c++98', and GNU dialects of those standards, such as 'gnu90' or 'gnu++98'. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that do not contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the asm and typeof keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a ?: expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style '//' comments, while -std=gnu99 -Wpedantic does not.

Answer 2

The -pedantic option will cause a warning to be displayed in this case, and -Werror will cause all warnings to be treated as errors.

For example:

x1.c: In function ‘f’:
x1.c:11:19: error: ISO C forbids empty initializer braces [-Werror=pedantic]
   struct X data = {};

Answer 3

If you want to reject code containing GNU-specific extensions, use -std=c99 -pedantic-errors ( -pedantic will issue diagnostics for non-standard extensions, but it won't necessarily reject the code outright). However, if you want to guarantee ISO conformance, be aware that this isn't a 100% solution. From the gcc man page:

Some users try to use -pedantic to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all---only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.

A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from -pedantic . We don't have plans to support such a feature in the near future.