Diagnostic directives


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


[edit] Syntax

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

[edit] 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.

[edit] Notes

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

[edit] Example

#if __STDC__ != 1
#  error "Not a standard compliant compiler"
#if __STDC_VERSION__ >= 202311L
#  warning "Using #warning as a standard feature"
#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 !!

[edit] References

  • 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

[edit] See also

C++ documentation for Diagnostic directives