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Diagnostic directives

From cppreference.net
C
C language
Preprocessor

Shows the given error message and renders the program ill-formed , or given warning message without affect the validity of the program (since C23) .

Contents

Syntax

#error diagnostic-message (1)
#warning diagnostic-message (2) (since C23)

Explanation

1) After encountering the #error directive, an implementation displays the message diagnostic-message and renders the program ill-formed (the compilation stops).
2) Same as (1) , except that the validity of the program is not affected and the compilation continues.

diagnostic-message can consist of several words not necessarily in quotes.

Notes

Before its standardization in C23, #warning has been provided by many compilers in all modes as a conforming extension.

Example

Run this code 
#if __STDC__ != 1
#  error "Not a standard compliant compiler"
#endif
#if __STDC_VERSION__ >= 202311L
#  warning "Using #warning as a standard feature"
#endif
#include <stdio.h>
int main (void)
{
    printf("The compiler used conforms to the ISO C Standard !!");
}

Possible output: 

The compiler used conforms to the ISO C Standard !!

References

  • C23 standard (ISO/IEC 9899:2024):
  • 6.10.5 Error directive (p: TBD)
  • C17 standard (ISO/IEC 9899:2018):
  • 6.10.5 Error directive (p: 126-127)
  • C11 standard (ISO/IEC 9899:2011):
  • 6.10.5 Error directive (p: 174)
  • C99 standard (ISO/IEC 9899:1999):
  • 6.10.5 Error directive (p: 159)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • 3.8.5 Error directive

See also

C++ documentation for Diagnostic directives