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Format attribute

As explained in Modern C, Third Edition

A format attribute supplies a compiler with the positions of a formatted-output specification and its variable arguments in a function call.

A format attribute tells the compiler which function parameter contains a printf-like format specification and where the variable arguments begin. For snprintf, positions 3 and 4 identify those parameters, enabling checks for nonliteral formats and mismatched argument types. section 10.2.3

Annotating snprintf

#if __has_c_attribute(__gnu__::__format__) [[__gnu__::__format__(__printf__, 3, 4)]] #endif int snprintf(char *buf, size_t size, const char *frmt, ...); section 10.2.3

Keep these portability and preprocessing rules in mind:

  • Use compiler-specific prefixes such as gnu or clang; implementations may accept attributes from another compiler family. section 10.2.3
  • Guard optional attributes with __has_c_attribute and provide a fallback that evaluates the test to zero when unavailable. section 10.2.3
  • Remember that attribute identifiers are subject to macro replacement and are not keywords. section 10.2.3

How Modern C, Third Edition covers it

How deep it goes
Explains parameter positions, supported compiler families, feature detection, and macro hazards without detailing compiler implementation internals.
Examples
Uses an annotated snprintf declaration and conditional __gnu__::__format__ syntax.
What you can do after
Diagnose format-string and argument-type mismatches by annotating variadic functions with compiler-supported metadata.
How it connects
Connects compiler attributes with variadic functions, printf format strings, preprocessing, and C23 feature detection.
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