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__has_c_attribute

As explained in Modern C, Third Edition

__has_c_attribute is a C preprocessor feature-test operator that reports whether an attribute is supported.

__has_c_attribute is a C23 preprocessor feature-test operator. Use it in a preprocessor conditional to test whether an attribute is supported before applying that attribute. section 10.2.3

Guarding a format attribute

The conditional applies the GNU format attribute only when the implementation reports support. The attribute tells the compiler that snprintf uses a printf-style format at argument 3 and begins its variable arguments at argument 4, enabling format warnings. section 10.2.3

#if __has_c_attribute(__gnu__::__format__)
  [[__gnu__::__format__(__printf__, 3, 4)]]
#endif
  int snprintf(char *buf, size_t size, const char *frmt, ...);

Inspect a conditional declaration using __has_c_attribute and the double-underscore form of the prefixed format attribute. section 10.2.3

 #ifndef __has_c_attribute
 # define __has_c_attribute(X) 0
 #endif

Inspect a fallback definition that makes an unavailable __has_c_attribute operator evaluate to zero. section 10.2.3

How Modern C, Third Edition covers it

How deep it goes
Defines conditional use and a fallback query, while stopping short of specifying attribute implementation details.
Examples
Demonstrates guarding __gnu__::__format__ and defining __has_c_attribute as zero when the operator is unavailable.
What it assumes
Assumes familiarity with C attributes and preprocessor conditionals, while showing how to test for the operator itself.
What you can do after
Apply it via preprocessor conditionals to select supported attributes and provide a zero-valued fallback.
How it connects
Connects C23 attributes with preprocessor conditionals, feature testing, macros, and compiler portability.
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