#warning directive (C/C++)

The #warning directive emits a user-specified warning message at compile time. It doesn't stop compilation. This directive is available starting in C23 and C++23.

Syntax

#warning token-string

Remarks

The warning message is the token-string parameter. The token-string parameter isn't subject to macro expansion and can be optionally enclosed in quotes.

Use this directive to inform the developer of a nonfatal issue or to communicate other important information during compilation.

The following example shows how to use the #warning directive:

#if defined(_LEGACY_FEATURE_FLAG)
#warning "_LEGACY_FEATURE is deprecated and should not be used."
#endif

See also

Preprocessor directives
#error directive